Beyond Cholesterol: A Comprehensive Integrative and Systems Medicine Reassessment of Lipid and Lipoprotein Biomarkers in ASCVD

1. Introduction

1.1. Evolution of the Lipid-Centric Paradigm

For more than six decades, the lipid-centric paradigm has served as the dominant conceptual framework for understanding atherosclerotic cardiovascular disease (ASCVD). Beginning with observations linking serum cholesterol to coronary heart disease and subsequently reinforced by epidemiological investigations, mechanistic studies, genetic discoveries, and randomized clinical trials, low-density lipoprotein cholesterol (LDL-C) became the principal therapeutic target for cardiovascular prevention[1].

Subsequent decades witnessed remarkable advances in lipid biology. The traditional emphasis on total cholesterol gradually expanded to include LDL-C, HDL-C, triglycerides, non-HDL cholesterol, apolipoprotein B (ApoB), LDL particle number (LDL-P), small dense LDL (sdLDL), oxidized LDL (oxLDL), and, more recently, lipoprotein(a) [Lp(a)]. Each successive biomarker was introduced to improve cardiovascular risk prediction, better characterize lipoprotein biology, or address recognized limitations of preceding markers[2].

This progression reflects substantial scientific progress. At the same time, it raises an important conceptual question: do increasingly sophisticated lipid biomarkers identify progressively more fundamental causes of ASCVD, or do they primarily characterize different biological levels within the disease process?

This distinction is clinically important. Cardiovascular events frequently occur in individuals with LDL-C concentrations considered acceptable by current standards, whereas many individuals with lifelong elevations in cholesterol remain free of clinical ASCVD. Furthermore, intensive LDL-C lowering substantially reduces cardiovascular events but does not eliminate residual cardiovascular risk[3,4,5]. These observations suggest that although lipid biomarkers are highly informative for risk stratification and therapeutic guidance, they may not fully explain disease initiation, progression, plaque vulnerability, or clinical events.

Accordingly, the central question addressed in this review is not whether lipid and lipoprotein biomarkers are clinically useful—they clearly are—but rather how each biomarker should be classified within an Integrative and Systems Medicine Framework for ASCVD. Specifically, we ask whether each biomarker is best understood as (1) a physiological molecule, (2) a risk biomarker, (3) an exposure biomarker, (4) an intermediate pathogenic mechanism, (5) a downstream manifestation of disease, or (6) a potential upstream causal contributor.

1.2. The Evolution of Lipid Biomarkers: Why Has the Field Continued to Introduce New Biomarkers?

Table 1. Evolution of major lipid biomarkers in ASCVD research and the limitations that prompted successive biomarker development.

Era	Dominant Biomarker	Why Introduced	Limitation
1960s	Total cholesterol	First epidemiologic marker	Poor specificity
1980s	LDL-C	Better risk correlation	Discordance between LDL-C and events
1990s	HDL-C	Protective association	Pharmacologic elevation failed to reduce events
2000s	Non-HDL-C	Includes remnant lipoproteins	Still concentration-based
2000s	ApoB	Reflects particle number	Does not distinguish particle biology
2000s	LDL-P	Better estimate of particle burden	Measures exposure rather than pathogenic mechanism
2010s	sdLDL	Captures particle quality	Strongly influenced by metabolic dysfunction
2010s	oxLDL	Reflects oxidative modification	Downstream consequence of oxidative stress
2010s–2020s	Lp(a)	Genetically determined inherited risk	Important causal contributor in selected populations but insufficient to explain most ASCVD

Table 1. Evolution of major lipid biomarkers in ASCVD research and the limitations that prompted successive biomarker development.

Era	Dominant Biomarker	Why Introduced	Limitation
1960s	Total cholesterol	First epidemiologic marker	Poor specificity
1980s	LDL-C	Better risk correlation	Discordance between LDL-C and events
1990s	HDL-C	Protective association	Pharmacologic elevation failed to reduce events
2000s	Non-HDL-C	Includes remnant lipoproteins	Still concentration-based
2000s	ApoB	Reflects particle number	Does not distinguish particle biology
2000s	LDL-P	Better estimate of particle burden	Measures exposure rather than pathogenic mechanism
2010s	sdLDL	Captures particle quality	Strongly influenced by metabolic dysfunction
2010s	oxLDL	Reflects oxidative modification	Downstream consequence of oxidative stress
2010s–2020s	Lp(a)	Genetically determined inherited risk	Important causal contributor in selected populations but insufficient to explain most ASCVD

Over the past six decades, each successive lipid biomarker has attempted to solve an important limitation of its predecessor. Total cholesterol lacked specificity; LDL-C improved discrimination; ApoB and LDL particle number sought to better quantify particle burden; small dense LDL and oxidized LDL emphasized particle quality and biological modification; and Lp(a) introduced inherited cardiovascular susceptibility into the lipid paradigm.

Nevertheless, the introduction of each new biomarker has improved cardiovascular risk assessment while also raising additional questions regarding the biological hierarchy and disease causation.

This historical progression raises three fundamental scientific questions:

• Why has the field repeatedly adopted new lipid biomarkers over the past six decades?
• What limitations of previous biomarkers prompted the introduction of each successive marker?
• Do these newer biomarkers represent progressively more fundamental causes of ASCVD, or are they increasingly refined indicators of intermediate biological mechanisms and disease manifestations?[6,7,8]

1.2.1. A Fundamental Observation

Over the past six decades, the dominant lipid biomarker has changed repeatedly, yet the underlying disease—ASCVD—has not. This raises an important conceptual question: Does the continual replacement of biomarkers reflect progressive identification of root causes, or does it reflect progressively improved characterization of different aspects of the disease process?

1.3. Prediction versus Causation

Prediction and causation represent fundamentally different scientific concepts.

A biomarker may predict disease without initiating disease.

Likewise, an intermediate pathogenic mechanism may participate directly in lesion progression while itself arising from more upstream biological disturbances.

Failure to distinguish these different biological levels has contributed to persistent controversy in cardiovascular medicine[9,10,11].

This distinction motivates the present review.

1.4. Objectives of This Review

This review differs from previous reviews in one important respect.

Rather than asking which lipid biomarker best predicts ASCVD, we ask a different question:

At what biological level does each biomarker operate?

Specifically, we evaluate each major lipid and lipoprotein biomarker according to evidence from:

• molecular biology
• physiology
• pathology
• epidemiology
• genetics
• randomized clinical trials

Existing reviews and clinical guidelines have primarily organized lipid biomarkers according to their ability to predict cardiovascular risk or guide lipid-lowering therapy. In contrast, the present review classifies these biomarkers within an Integrative and Systems Medicine Framework , distinguishing physiological functions, biomarkers, intermediate pathogenic mechanisms, and upstream causal contributors.

The objective is not to rank biomarkers according to predictive performance but to determine whether they primarily represent[12]:

• physiological molecules
• biomarkers
• intermediate mechanisms
• possible root drivers

This Integrative and Systems Medicine Framework perspective forms the conceptual basis for the synthesis presented later in this review[2].

Recent consensus statements from the European Atherosclerosis Society, the American College of Cardiology, and the National Lipid Association have continued to organize lipid biomarkers primarily according to cardiovascular risk prediction and therapeutic decision-making, further highlighting the rationale for the Integrative and Systems Medicine Framework proposed in this review.

2. The Evolution of Lipid Biomarkers: Progressive Refinement or Progressive Clarification?

The lipid-centric paradigm has not remained static. Over the past six decades, the dominant lipid marker used in ASCVD risk assessment has repeatedly changed. This progression—from total cholesterol to LDL-C, HDL-C, triglycerides, non-HDL-C, ApoB, LDL particle number, small dense LDL, oxidized LDL, and Lp(a)—reflects a continuing attempt to improve risk prediction and biological interpretation.

However, this historical evolution also reveals a central conceptual problem: each new biomarker was introduced because previous markers were incomplete. The key question is whether each successive marker brought the field closer to identifying the root causes of ASCVD, or whether these markers primarily provided increasingly refined measurements of downstream biological processes.

2.1. Total Cholesterol

Historical rationale

Total cholesterol was the first widely adopted lipid biomarker in cardiovascular epidemiology. It became central to early population studies because it was measurable, reproducible, and associated with coronary heart disease risk across populations[14,15,16,17,18].

Biological basis

Total cholesterol represents the aggregate cholesterol content of multiple lipoprotein classes, including LDL, HDL, VLDL, IDL, and remnant particles. As such, it is not a biologically specific marker. It reflects total circulating cholesterol mass rather than the functional behavior, particle number, oxidation status, inflammatory modification, or vascular interaction of specific lipoprotein particles[19,20].

Supporting evidence

Large epidemiological studies demonstrated an association between elevated serum cholesterol and coronary heart disease risk. These findings helped establish cholesterol as an early population-level risk factor and contributed to the development of cholesterol-lowering strategies[16,17,18].

Limitations

The major limitation of total cholesterol is lack of specificity. Individuals with similar total cholesterol may have very different LDL-C, HDL-C, triglyceride, ApoB, and remnant lipoprotein profiles. Total cholesterol also does not distinguish between cholesterol carried in potentially atherogenic ApoB-containing particles and cholesterol carried in HDL particles[19,20,21].

Remaining controversies

Total cholesterol remains useful historically and epidemiologically but has largely been replaced by more specific lipid markers in clinical decision-making. Its decline as a primary marker illustrates the first major limitation of the lipid-centric paradigm: total cholesterol was predictive at the population level but insufficiently informative at the biological or individual level.

Summary

Total cholesterol is best interpreted as an early, nonspecific population biomarker of cardiovascular risk, not as a root cause of ASCVD.

2.2. LDL Cholesterol

Historical rationale

LDL-C replaced total cholesterol as the central lipid marker because LDL particles carry a substantial portion of plasma cholesterol and are consistently associated with ASCVD risk. LDL-C became the dominant therapeutic target following epidemiological studies, mechanistic research, familial hypercholesterolemia observations, and randomized trials showing event reduction with LDL-lowering therapy[1,2,22,23,24,25].

Biological basis

LDL-C measures the cholesterol mass contained within LDL particles, not the number of LDL particles, their size, residence time, oxidative status, glycation, inflammatory modification, or arterial-wall behavior. This distinction is central because cholesterol cargo and particle number can be discordant[8,26,27].

Mechanistic evidence

Mechanistic models emphasize LDL entry into the arterial wall, retention in the subendothelial space, modification, macrophage uptake, and foam-cell formation. These mechanisms support the biological importance of LDL-containing particles in atherogenesis[3,5,28,29].

Clinical Evidence

Statins, ezetimibe, and PCSK9 inhibitors reduce LDL-C and lower cardiovascular event rates. This evidence strongly supports LDL-C as a clinically actionable risk marker and therapeutic target[30,31,32,33,34].

Strengths

LDL-C is widely available, inexpensive, standardized, and strongly supported by clinical trial evidence. It remains the foundation of most lipid-lowering guidelines[13,35].

Limitations

LDL-C does not fully explain ASCVD risk. Cardiovascular events occur in individuals with normal or low LDL-C, and residual risk persists despite intensive LDL-lowering therapy. LDL-C also performs less well when discordant with ApoB or LDL particle number, particularly in insulin resistance, metabolic syndrome, diabetes, and hypertriglyceridemia[8,27,36,37,38].

Summary

LDL-C is a strong clinical biomarker and therapeutic target, but it measures cholesterol cargo rather than root disease initiation. It is best classified as a major biomarker and mechanistic participant, not a complete root cause of ASCVD.

2.3. HDL Cholesterol

Historical rationale

HDL-C emerged as a major lipid biomarker after epidemiological studies showed an inverse association between HDL-C and coronary heart disease risk. This gave rise to the concept of HDL as “protective cholesterol”[21,39,40].

Biological basis

HDL participates in reverse cholesterol transport, endothelial function, antioxidant activity, anti-inflammatory signaling, and immune modulation. However, HDL-C measures cholesterol content within HDL particles, not HDL function[41,42,43].

Supporting evidence

Low HDL-C is consistently associated with increased cardiovascular risk in observational studies. This association contributed to the inclusion of HDL-C in risk calculators and clinical lipid panels[21,39,40].

Limitations

Pharmacologic attempts to raise HDL-C have generally failed to reduce cardiovascular events when LDL-C is adequately treated. Trials of niacin and CETP inhibitors showed that raising HDL-C concentration does not necessarily improve outcomes. This strongly suggests that HDL-C is not equivalent to HDL function[44,45,46,47,48].

Remaining controversies

The central unresolved issue is whether HDL-C is a causal protective factor or primarily a biomarker of metabolic health, insulin sensitivity, inflammation, and lipoprotein function. Current evidence increasingly favors HDL function over HDL-C concentration[41,42,43,49].

Summary

HDL-C is best interpreted as a biomarker associated with metabolic and vascular health. HDL biology may be protective, but HDL-C concentration itself is not a proven root driver.

2.4. Triglycerides

Historical rationale

Triglycerides were introduced as a cardiovascular risk marker because elevated triglycerides are common in insulin resistance, metabolic syndrome, obesity, diabetes, and hypertriglyceridemic states. They also reflect triglyceride-rich lipoproteins and remnant particles[50,51,52,53].

Biological basis

Plasma triglycerides primarily reflect VLDL, chylomicrons, and remnant lipoprotein metabolism. Elevated triglycerides often indicate impaired metabolic regulation, hepatic overproduction of VLDL, reduced lipoprotein lipase activity, insulin resistance, and increased remnant particle burden[52,53,54,55].

Supporting evidence

Observational and genetic studies support associations among triglycerides, remnant cholesterol, and ASCVD risk. Nonfasting triglycerides and remnant cholesterol have been linked to myocardial infarction and ischemic heart disease[51,52,53,54,55,56].

Limitations

Triglycerides are highly variable, influenced by diet, fasting status, insulin sensitivity, alcohol intake, hepatic metabolism, and genetic factors. They may represent an upstream metabolic disturbance more than a direct causal lipid species[50,51,52,53,54,55].

Remaining Controversies

The key question is whether triglycerides themselves are causal or whether they mark the atherogenic burden of remnant particles and the metabolic dysfunction that produces them.

Summary

Triglycerides are best interpreted as markers of metabolic dysfunction and triglyceride-rich lipoprotein/remnant metabolism. They are closer to upstream metabolic disturbance than LDL-C but still are not, by themselves, root causes of ASCVD.

2.5. Non-HDL Cholesterol

Historical rationale

Non-HDL-C was introduced to capture all cholesterol carried by potentially atherogenic ApoB-containing particles, including LDL, VLDL, IDL, and remnant lipoproteins. It was designed to overcome limitations of LDL-C, especially in patients with elevated triglycerides[57,58,59,60].

Biological basis

Non-HDL-C is calculated as total cholesterol minus HDL-C. It reflects the cholesterol content of all non-HDL particles but still measures cholesterol mass rather than particle number or particle function[57,58,59,60].

Supporting evidence

Non-HDL-C predicts ASCVD risk and may outperform LDL-C in hypertriglyceridemic or insulin-resistant populations because it better captures remnant cholesterol burden[58,59,60,61].

Limitations

Like LDL-C, non-HDL-C remains concentration-based. It does not distinguish among particle number, particle size, oxidative status, inflammatory modification, or vascular retention[36,61].

Remaining controversies

Non-HDL-C is clinically useful but remains a composite biomarker. It improves risk prediction but does not identify the upstream processes that generate atherogenic lipoprotein patterns.

Summary

Non-HDL-C is a broader biomarker of atherogenic cholesterol exposure, but it remains a downstream concentration marker rather than a root cause.

2.6. Apolipoprotein B

Historical rationale

ApoB was introduced because each atherogenic lipoprotein particle generally contains one ApoB molecule. ApoB therefore provides an estimate of the number of circulating atherogenic particles, addressing the discordance between LDL-C concentration and particle burden[62,63,64,65,66].

Biological basis

ApoB is present on LDL, VLDL, IDL, chylomicron remnants, and Lp(a). ApoB measurement reflects particle number rather than cholesterol mass. This is important because many small cholesterol-depleted LDL particles may produce a high ApoB despite relatively normal LDL-C[62,63,64,65,66,67].

Supporting evidence

Large epidemiological and clinical analyses suggest that ApoB may predict ASCVD risk better than LDL-C or non-HDL-C in some populations, particularly when these markers are discordant[63,64,65,66,67].

Limitations

ApoB improves quantification of atherogenic particle burden, but it does not explain why particles become retained, oxidized, glycosylated, inflammatory, or pathogenic. It measures exposure, not root biological initiation[66,67,68].

Remaining controversies

The main controversy is not whether ApoB is clinically useful—it is—but whether ApoB should be interpreted as a causal root driver or as a superior exposure biomarker of atherogenic particle burden.

Summary

ApoB is one of the strongest lipid-related risk markers, but within Integrative and Systems Medicine Framework it is best classified as an exposure biomarker and mechanistic participant, not a primary root driver.

2.7. LDL Particle Number

Historical rationale

LDL particle number was introduced to further resolve discordance between LDL-C and ASCVD risk. Two individuals with identical LDL-C may have very different numbers of LDL particles, especially when LDL particles are small and cholesterol-depleted[8,69,70,71].

Biological basis

LDL-P estimates the number of circulating LDL particles, commonly measured by nuclear magnetic resonance spectroscopy or related methods. It provides a particle-based measure of LDL burden[8,69,70,71].

Supporting evidence

Studies show that LDL-P may better predict ASCVD risk than LDL-C when the two are discordant, especially in insulin resistance, diabetes, and metabolic syndrome[8,69,70,71,72].

Limitations

LDL-P measures the quantity of LDL particles but not their biological state. It does not measure oxidation, glycation, immune activation, endothelial permeability, arterial retention, or repair capacity[69,70,71,73].

Remaining controversies

LDL-P is useful for refined risk assessment but remains a particle burden marker. It improves measurement but does not necessarily identify disease initiation.

Summary

LDL-P is best classified as an advanced exposure biomarker. It improves upon LDL-C in discordant cases but still primarily reflects particle burden rather than root causation.

2.8. Small Dense LDL

Historical rationale

Small dense LDL was introduced to capture qualitative differences in LDL particles. It became clinically important because small dense LDL is commonly associated with insulin resistance, hypertriglyceridemia, low HDL-C, and metabolic syndrome[74,75,76,77].

Biological basis

Small dense LDL particles are smaller, more numerous, often cholesterol-depleted, and more susceptible to oxidative modification. Their presence usually reflects altered triglyceride-rich lipoprotein metabolism and insulin resistance[74,75,76,77,78].

Supporting evidence

Small dense LDL has been associated with increased ASCVD risk in observational studies. However, its independent contribution is difficult to separate from triglycerides, HDL-C, insulin resistance, ApoB, and LDL-P[75,76,77,78,79].

Limitations

Small dense LDL is strongly influenced by metabolic state. It may be more accurately interpreted as a marker of impaired metabolic regulation than as a primary initiating factor[77,78,79].

Remaining controversies

The major question is whether small dense LDL is independently causal or whether it represents a downstream manifestation of insulin resistance and triglyceride-rich lipoprotein remodeling.

Summary

Small dense LDL is best classified as a mechanistic biomarker reflecting metabolic dysfunction and altered lipoprotein remodeling, not a root cause.

2.9. Oxidized LDL

Historical rationale

Oxidized LDL was introduced because native LDL does not fully explain foam-cell formation and inflammatory activation. Oxidative modification of LDL provided a mechanistic bridge between lipoproteins, oxidative stress, macrophage uptake, and plaque inflammation[80,81,82,83,84].

Biological basis

Oxidized LDL is generated when LDL particles undergo oxidative modification in an environment of oxidative stress, inflammation, endothelial dysfunction, or impaired antioxidant capacity. OxLDL can promote macrophage uptake, foam-cell formation, endothelial activation, immune responses, and plaque progression[80,81,82,83,84,85].

Supporting evidence

Experimental studies strongly support oxLDL as a mechanistic mediator in atherosclerosis. Observational studies also associate oxLDL markers with cardiovascular risk and plaque burden[81,82,83,84,85,86].

Limitations

Oxidized LDL is not simply a lipid marker; it is a marker of oxidative stress acting on lipoproteins. Thus, it may be downstream of more fundamental processes such as mitochondrial dysfunction, inflammation, redox imbalance, endothelial injury, or impaired antioxidant defenses[83,84,85,86,87].

Remaining controversies

The key question is whether oxidized LDL is a root cause or a downstream mediator generated by upstream oxidative and inflammatory conditions.

Summary

OxLDL is best interpreted as an intermediate mechanism and downstream manifestation of oxidative stress. It may directly contribute to plaque biology, but it does not explain the upstream origin of oxidative stress itself.

2.10. Lipoprotein(a)

Historical rationale

Lp(a) emerged as a distinct lipid-related biomarker because it is largely genetically determined and independently associated with ASCVD and aortic valve stenosis. Unlike LDL-C or triglycerides, Lp(a) is minimally affected by lifestyle and most conventional lipid-lowering therapies[7,88,89,90,91].

Genetics

Lp(a) levels are primarily determined by variation in the LPA gene, especially kringle IV type 2 repeat number. Genetic and Mendelian randomization studies support a causal association between elevated Lp(a) and cardiovascular disease[7,89,90,91,92,93].

Mechanisms

Lp(a) may promote ASCVD through several mechanisms, including LDL-like cholesterol delivery, oxidized phospholipid carriage, pro-inflammatory signaling, endothelial dysfunction, impaired fibrinolysis, and vascular calcification. These mechanisms distinguish Lp(a) from standard LDL particles[94,95,96,97].

Clinical Evidence

Epidemiological studies and meta-analyses consistently associate elevated Lp(a) with ASCVD risk. Unlike most lipid biomarkers, Lp(a) has strong genetic evidence supporting causal relevance[7,90,91,92,93].

Current therapies

Conventional statins do not substantially lower Lp(a). PCSK9 inhibitors produce modest reductions. New antisense oligonucleotide and siRNA therapies targeting LPA are under investigation and may clarify whether selective Lp(a) lowering reduces events[98,99,100,101].

Remaining uncertainty

Although Lp(a) has stronger root-level features than most other lipid biomarkers because of its genetic determination, it still does not explain most ASCVD. Its pathogenic impact likely depends on the broader vascular environment, including endothelial integrity, inflammation, oxidative stress, thrombosis, and repair capacity[7,92,93,94,95,96,97].

Summary

Lp(a) is the lipid-related biomarker most plausibly classified as a partial upstream causal contributor. However, it is best understood as a context-dependent risk amplifier rather than a universal root cause of ASCVD.

2.11. Summary of Chapter 2

The historical evolution of lipid biomarkers reveals a repeated pattern. Each new marker improved one aspect of measurement but exposed another layer of complexity.

Total cholesterol improved population screening but lacked specificity.

LDL-C improved risk discrimination but measured cholesterol cargo rather than particle number or biological behavior.

HDL-C appeared protective epidemiologically, but raising HDL-C pharmacologically failed to reliably reduce events.

Triglycerides and non-HDL-C captured metabolic and remnant lipoprotein risk but remained concentration-based markers.

ApoB and LDL-P improved particle burden assessment but still measured exposure rather than the upstream causes of particle retention, oxidation, inflammation, or vascular injury.

Small dense LDL and oxidized LDL moved closer to mechanism but also revealed dependence on metabolic dysfunction and oxidative stress.

Lp(a) introduced inherited risk and is the strongest partial exception, yet even Lp(a) appears to operate within a broader vascular and inflammatory context.

Thus, the progressive refinement of lipid biomarkers has improved cardiovascular risk assessment but has not, by itself, resolved the distinction between biomarkers, intermediate mechanisms, and root drivers.

3. Physiological Functions of Lipids and Lipoproteins

Before evaluating whether lipid and lipoprotein biomarkers represent root causes, intermediate mechanisms, or downstream manifestations of ASCVD, it is essential to first examine their normal physiological roles. Lipids and lipoproteins are not intrinsically pathological molecules. They are evolutionarily conserved transport, structural, metabolic, endocrine, immune, and reparative systems required for life. Disease may arise not simply from the presence of these molecules, but from dysregulation, modification, retention, or maladaptive interaction with an altered vascular and inflammatory environment.

3.1. Evolutionary and Biological Rationale

Cholesterol and lipoproteins evolved because hydrophobic lipids must be transported through an aqueous circulatory system. Cholesterol is required by virtually all mammalian cells, while triglycerides, phospholipids, fat-soluble vitamins, and lipid-soluble signaling molecules require organized transport systems. Lipoproteins solve this biochemical problem by packaging hydrophobic molecules into particles that can circulate in plasma, interact with receptors, deliver cargo, and participate in metabolic communication between tissues[22,102,103,104].

From this perspective, lipoproteins are best understood first as physiological transport systems. Their association with ASCVD must therefore be interpreted in the context of normal biological function, adaptive responses, and pathological modification.

3.2. Cholesterol as an Essential Structural Molecule

Cholesterol is a core structural component of mammalian cell membranes. It regulates membrane fluidity, permeability, thickness, curvature, and organization. It is also a major component of lipid rafts and membrane microdomains involved in signaling, receptor function, vesicular trafficking, and cellular communication[105,106,107,108].

These functions help explain why cholesterol synthesis and cellular cholesterol homeostasis are tightly regulated. The LDL receptor pathway, intracellular cholesterol trafficking, and feedback regulation of cholesterol synthesis represent highly conserved mechanisms designed to maintain cellular lipid balance[22,103,104].

Thus, cholesterol should not be conceptualized only as a pathological substance. It is an essential structural molecule whose pathological significance depends on biological context, compartmentalization, oxidation, transport, and vascular interaction.

3.3. Lipoproteins as Physiological Transport Systems

The major lipoprotein classes have distinct physiological roles.

Chylomicrons transport dietary triglycerides, cholesterol, and fat-soluble nutrients from the intestine to peripheral tissues and the liver.

Very-low-density lipoproteins (VLDL) transport endogenously synthesized triglycerides from the liver to peripheral tissues.

Intermediate-density lipoproteins (IDL) represent transitional particles generated during VLDL metabolism.

Low-density lipoproteins (LDL) transport cholesterol to peripheral tissues and interact with LDL receptors.

High-density lipoproteins (HDL) participate in reverse cholesterol transport, cholesterol efflux, antioxidant activity, anti-inflammatory signaling, and immune modulation[22,41,43,102,109,110].

This physiological diversity indicates that lipoproteins are not a single biological entity. They represent a coordinated transport network that integrates intestinal absorption, hepatic lipid export, peripheral fuel delivery, cholesterol homeostasis, immune defense, and tissue maintenance.

3.4. Lipoproteins in Energy Metabolism

Triglyceride-rich lipoproteins are central to systemic energy metabolism. Chylomicrons deliver dietary triglycerides after meals, while VLDL transports hepatic triglycerides during fasting and metabolic adaptation. Lipoprotein lipase hydrolyzes triglycerides in circulating lipoproteins, releasing fatty acids for uptake by muscle, heart, and adipose tissue[111,112,113].

This system enables fuel distribution between organs. During fasting, exercise, carbohydrate restriction, insulin resistance, or increased energy demand, triglyceride-rich lipoprotein metabolism may change substantially. Therefore, changes in triglycerides, VLDL, remnants, and related markers may reflect systemic metabolic state rather than isolated lipid pathology.

In this context, hypertriglyceridemia often reflects altered energy metabolism, hepatic lipid handling, insulin signaling, and adipose tissue function. It may therefore be closer to a marker of metabolic dysfunction than a primary initiating cause of ASCVD.

3.5. Lipoproteins and Reverse Cholesterol Transport

HDL is involved in reverse cholesterol transport, a process by which excess cholesterol is removed from peripheral cells, including macrophages, and returned to the liver for excretion or recycling. Cholesterol efflux from macrophages is mediated by transporters such as ABCA1 and ABCG1 and is influenced by ApoA-I and HDL particle function[41,43,113,114].

Importantly, HDL-C concentration does not necessarily measure HDL function. Cholesterol efflux capacity and HDL particle functionality may provide more biologically meaningful information than HDL-C concentration alone[41,43].

This distinction is important for the broader argument of this review. A biomarker measuring lipid content may not adequately reflect the physiological function of the lipoprotein system it is intended to represent.

3.6. Lipids and Steroid Hormone Synthesis

Cholesterol is the biochemical precursor for steroid hormone synthesis. Steroidogenic tissues use cholesterol to generate pregnenolone, which is then converted into glucocorticoids, mineralocorticoids, and sex steroids, including cortisol, aldosterone, progesterone, testosterone, estradiol, and DHEA[115,116].

Cholesterol delivery to steroidogenic tissues occurs through multiple pathways, including uptake of lipoprotein-derived cholesterol and intracellular cholesterol trafficking. The steroidogenic acute regulatory protein (StAR) mediates the movement of cholesterol into mitochondria, where steroidogenesis begins[115,116,117].

Thus, cholesterol is not merely a cardiovascular risk marker. It is a necessary substrate for endocrine physiology, stress adaptation, reproductive biology, and electrolyte homeostasis.

3.7. Lipoproteins and Fat-Soluble Vitamin Transport

Lipoproteins participate in the absorption, circulation, and tissue distribution of fat-soluble vitamins and lipid-soluble nutrients. Chylomicrons transport dietary vitamins A, D, E, and K from the intestine. VLDL, LDL, and HDL also participate in the redistribution of tocopherols, carotenoids, retinoids, vitamin K forms, and other lipid-soluble compounds[118,119,120].

Vitamin E transport illustrates this principle. Alpha-tocopherol is transported in lipoproteins and redistributed among tissues through lipoprotein metabolism. Similarly, carotenoids and retinoids are transported in association with lipoprotein pathways after intestinal absorption[118,119,120,121].

These functions reinforce the concept that lipoproteins are nutrient-transport systems. Their physiological role extends far beyond cholesterol carriage.

3.8. Lipoproteins in Innate Immunity and Host Defense

Lipoproteins also participate in innate immunity. LDL, HDL, VLDL, and chylomicrons can bind and neutralize microbial toxins, including lipopolysaccharide (LPS) from Gram-negative bacteria and lipoteichoic acid from Gram-positive bacteria[122,123,124,125,126,127].

During infection and inflammation, lipid and lipoprotein metabolism changes substantially. The acute-phase response can alter HDL composition, reduce reverse cholesterol transport, increase triglyceride-rich lipoproteins, and modify apolipoprotein profiles[122,123].

Experimental and clinical studies suggest that lipoproteins can buffer endotoxin toxicity, modulate inflammatory responses, and influence host survival during infection[124,125,126,127]. This immune function provides an important counterbalance to the simplified view that ApoB-containing lipoproteins are only pathogenic.

The same biological property may be protective in one context and potentially maladaptive in another. For example, lipoprotein binding of microbial products may protect against acute toxicity, while chronic inflammatory modification of lipoproteins may contribute to vascular injury.

3.9. Lipoproteins and Oxidative Homeostasis

HDL carries enzymes and proteins involved in antioxidant and anti-inflammatory functions, including paraoxonase-1 (PON1), platelet-activating factor acetylhydrolase, ApoA-I, and other HDL-associated proteins. These systems may reduce lipid oxidation, modulate endothelial activation, and influence inflammatory signaling[128,129,130,131].

However, HDL can become dysfunctional during systemic inflammation, oxidative stress, diabetes, chronic kidney disease, and other pathological states. In such settings, HDL-C concentration may remain measurable while HDL function declines[128,129,130,131,132].

This again illustrates a recurring principle: concentration-based lipid biomarkers do not necessarily capture biological function.

3.10. Lipoproteins in Tissue Maintenance and Repair

Cellular repair, proliferation, and regeneration require membrane synthesis, lipid remodeling, cholesterol trafficking, and energy supply. Cholesterol and phospholipids are needed for membrane biogenesis, vesicular trafficking, myelin integrity, and cellular signaling. Lipoproteins provide one mechanism by which lipid substrates are distributed among tissues[22,103,106,107,108,133,134].

Although excessive or modified lipoproteins may participate in atherosclerotic lesion development, lipoprotein transport itself is necessary for tissue maintenance. Therefore, elevated lipoproteins in certain physiological or stress states may represent adaptive responses to increased metabolic, inflammatory, endocrine, or reparative demand.

This does not imply that elevated lipoproteins are always benign. Rather, it suggests that their interpretation requires biological context.

3.11. Physiological Adaptation of Lipoprotein Metabolism

Lipoprotein concentrations and composition change in response to infection, inflammation, pregnancy, fasting, dietary composition, insulin status, thyroid function, hepatic function, renal function, aging, and pharmacologic interventions. These changes demonstrate that lipid biomarkers are dynamic physiological variables rather than fixed pathological entities[122,123,135,136,137].

For example, inflammation can lower HDL-C and alter HDL function, while insulin resistance can raise triglycerides, increase VLDL secretion, promote small dense LDL formation, and increase ApoB-containing particle burden. These patterns are often interpreted as lipid abnormalities, but they may also reflect upstream metabolic and inflammatory regulation.

Thus, lipid biomarkers should be interpreted not only as risk markers but also as readouts of systemic physiology.

3.12. Summary of Chapter 3

Lipids and lipoproteins perform indispensable physiological functions. Cholesterol supports membrane structure, lipid rafts, myelin biology, intracellular signaling, and steroid hormone synthesis. Lipoproteins transport triglycerides, cholesterol, phospholipids, fat-soluble vitamins, carotenoids, tocopherols, and other lipid-soluble molecules. They participate in energy metabolism, reverse cholesterol transport, innate immunity, endotoxin neutralization, antioxidant defense, tissue maintenance, and repair.

This physiological foundation is essential for interpreting the lipid-ASCVD relationship. Lipoproteins are not inherently pathological; rather, they become clinically relevant when their concentrations, composition, modification, retention, or tissue interactions occur in a pathological vascular, metabolic, oxidative, inflammatory, or reparative environment.

Therefore, before classifying lipid biomarkers as root causes of ASCVD, one must distinguish normal physiological function from pathological modification and downstream disease participation. This distinction provides the foundation for the next section, which examines how lipoproteins participate mechanistically in atherosclerosis.

4. Lipoproteins in Atherosclerosis: Mechanistic Participation or Root Causation?

The preceding chapters reviewed the evolution of lipid biomarkers and the physiological functions of lipids and lipoproteins. The present chapter turns to the mechanistic evidence linking lipoproteins to atherosclerosis. This literature is extensive and biologically compelling. ApoBcontaining lipoproteins can enter the arterial wall, bind to extracellular matrix proteoglycans, undergo oxidative and enzymatic modification, promote macrophage uptake, contribute to foam-cell formation, and participate in inflammatory plaque progression.

However, mechanistic participation is not equivalent to root causation. A molecule may participate in disease propagation without being the primary initiating driver. Likewise, a biological process may be necessary for plaque development but insufficient to explain why the pathological environment arises in the first place. Therefore, this chapter reviews major lipoprotein-related mechanisms in ASCVD and asks where each mechanism belongs within Integrative and Systems Medicine Framework: root driver, biological process, intermediate mechanism, or downstream manifestation.

4.1. Endothelial Dysfunction and Increased Vascular Permeability

The healthy endothelium is a dynamic regulatory interface that controls vascular tone, permeability, thrombosis, leukocyte adhesion, inflammation, and arterial homeostasis. Endothelial dysfunction is one of the earliest detectable abnormalities in atherosclerosis and is influenced by disturbed blood flow, hypertension, hyperglycemia, smoking, oxidative stress, inflammation, insulin resistance, and other systemic factors[138,139,140,141,142].

Endothelial dysfunction may increase vascular permeability and facilitate the entry of ApoB-containing lipoproteins into the intimal space. It may also promote expression of adhesion molecules, monocyte recruitment, platelet activation, and local inflammatory signaling[3,5,138,139,140,141,142]. These effects create a vascular environment in which lipoproteins are more likely to be retained, modified, and incorporated into developing lesions.

Importantly, endothelial dysfunction can occur independently of elevated LDL-C. Hypertension, diabetes, smoking, chronic inflammation, environmental toxicants, oxidative stress, and disturbed shear stress can all impair endothelial function. This suggests that endothelial dysfunction may represent an upstream vascular process that permits or amplifies lipoprotein-mediated plaque development rather than merely a consequence of lipid excess.

Interpretation Within the Integrative and Systems Medicine Framework

Endothelial dysfunction is best classified as an upstream biological process and potential root-level contributor to ASCVD. It is not itself a lipid biomarker. Rather, it modifies the vascular environment in which lipoproteins become pathogenic.

Key question

Does endothelial dysfunction arise because lipoproteins are elevated, or do elevated or modified lipoproteins become pathogenic because endothelial integrity is impaired?

4.2. Lipoprotein Entry and Subendothelial Retention

The response-to-retention hypothesis proposes that atherogenesis begins when ApoB-containing lipoproteins enter the arterial intima and bind to extracellular matrix proteoglycans. Retained lipoproteins remain in the arterial wall longer, increasing the likelihood of oxidative, enzymatic, and inflammatory modification[29,143,144,145].

This model helps explain why atherosclerosis develops preferentially at specific arterial sites, particularly regions of disturbed flow and increased endothelial permeability. Lipoprotein retention is also supported by animal models, human pathology, and molecular studies showing interactions between ApoB and arterial proteoglycans[29,143,144,145,146,147].

Nevertheless, retention itself may require a permissive arterial environment. Endothelial permeability, extracellular matrix composition, local inflammation, shear stress, and vascular repair processes influence whether lipoproteins become trapped. Therefore, although retention is central to plaque initiation in many models, it does not fully explain the upstream causes of endothelial dysfunction, arterial susceptibility, or site-specific plaque formation.

Interpretation Within the Integrative and Systems Medicine Framework

Lipoprotein retention is best classified as a major intermediate mechanism. It is likely important for lesion development but does not fully explain why the arterial wall becomes susceptible to retention.

Key question

Is lipoprotein retention the initiating event, or is it an early mechanism enabled by upstream endothelial, hemodynamic, inflammatory, or matrix changes?

4.3. Oxidative Modification of LDL

Native LDL is not efficiently taken up by macrophage scavenger receptors. The discovery that chemically or biologically modified LDL could promote macrophage cholesterol accumulation led to the LDL modification hypothesis of atherogenesis[152,153,154,155]. Oxidized LDL can activate endothelial cells, promote monocyte adhesion, stimulate macrophage uptake, induce foam-cell formation, and contribute to inflammatory signaling within plaques[80,81,82,83,85,86,148].

Oxidized LDL and oxidized phospholipids have been identified in human atherosclerotic lesions and are associated with plaque inflammation and cardiovascular risk[83,84,86,87,148]. These findings provide strong mechanistic support for the role of oxidative modification in atherosclerosis.

However, oxidized LDL is generated under conditions of oxidative stress. Reactive oxygen species, myeloperoxidase activity, metal-catalyzed oxidation, mitochondrial dysfunction, inflammation, and impaired antioxidant defenses can all promote LDL oxidation[83,85,86,148]. Therefore, oxLDL may be more accurately interpreted as a downstream product of an oxidative vascular environment rather than a primary root cause.

Interpretation Within the Integrative and Systems Medicine Framework

Oxidized LDL is best classified as an intermediate pathogenic mechanism and biomarker of oxidative modification. It participates in plaque biology but depends on upstream oxidative and inflammatory conditions.

Key question

Does oxLDL initiate ASCVD, or does oxidative stress convert otherwise physiological LDL particles into pro-inflammatory mediators?

4.4. Macrophage Recruitment and Foam-Cell Formation

Foam cells are a histological hallmark of early atherosclerotic lesions. They arise when macrophages, and in some cases vascular smooth muscle cells, accumulate excess cholesterol and lipid droplets after uptake of modified lipoproteins[149,150,151,152].

Macrophage uptake of modified LDL occurs through scavenger receptors such as CD36, SR-A, and LOX-1. Unlike the LDL receptor, these pathways are not tightly downregulated by intracellular cholesterol levels, allowing progressive lipid accumulation[149,150,151,152,153,154]. Foam cells produce inflammatory cytokines, reactive oxygen species, proteases, and other mediators that contribute to lesion progression.

However, foam-cell formation requires several prior events: lipoprotein entry, retention, modification, monocyte recruitment, macrophage differentiation, and impaired cholesterol efflux. Foam cells are therefore unlikely to represent root causes. They are better understood as downstream cellular manifestations of altered lipid handling, inflammation, and impaired resolution.

Interpretation Within the Integrative and Systems Medicine Framework

Foam-cell formation is a downstream cellular manifestation and intermediate mechanism of plaque progression. It is not a root driver of ASCVD.

Key question

Do foam cells initiate disease, or do they represent the cellular consequence of retained and modified lipoproteins in an inflammatory arterial environment?

4.5. Chronic Vascular Inflammation

Atherosclerosis is now widely recognized as a chronic inflammatory disease of the arterial wall[5,155,156,157]. Lipoproteins and inflammation interact bidirectionally. Retained and modified lipoproteins activate endothelial cells, macrophages, dendritic cells, and smooth muscle cells, while inflammation promotes further lipid modification, endothelial dysfunction, protease activity, and plaque instability.

Clinical evidence has strengthened the inflammatory model. Elevated C-reactive protein predicts cardiovascular events, and anti-inflammatory therapies targeting IL-1β or colchicine pathways have reduced cardiovascular events in selected high-risk populations[158,159,160,161]. These findings demonstrate that inflammation is not merely a passive bystander.

Yet inflammation itself is not a single root cause. It may arise from infection, obesity, insulin resistance, smoking, autoimmune activation, environmental exposures, chronic stress, endothelial injury, metabolic dysfunction, or oxidized lipoproteins. Therefore, inflammation is both a mediator and amplifier of ASCVD, but its upstream origin varies across individuals.

Interpretation Within the Integrative and Systems Medicine Framework

Chronic vascular inflammation is a major biological process and intermediate mechanism. In some patients it may function as a root-level driver, but inflammation itself requires etiological specification.

Key question

Is inflammation the root cause of ASCVD, or is it a common downstream response to multiple upstream insults?

4.6. Immune Activation and Adaptive Responses

Atherosclerosis involves innate and adaptive immune responses. Monocytes, macrophages, dendritic cells, T cells, B cells, neutrophils, and mast cells have all been implicated in lesion development and plaque complications[162,163,164,165,166].

Modified LDL can act as an immunogenic stimulus. Oxidized LDL epitopes may be recognized by innate immune receptors and autoantibodies, linking lipid modification with immune activation[80,85,163,164,165,166,167]. T-cell subsets, cytokines, and antigen-presenting cells contribute to plaque inflammation, while regulatory immune pathways may modulate lesion progression.

Despite strong mechanistic evidence, immune activation is not specific to lipid pathology. Immune responses may be triggered by modified lipoproteins, microbial products, damaged endothelial cells, necrotic debris, oxidative stress, or systemic inflammatory states.

Interpretation Within the Integrative and Systems Medicine Framework

Immune activation is an intermediate biological mechanism. It may amplify plaque progression but does not, by itself, define the primary root cause.

Key question

Does immune activation arise because lipoproteins are inherently pathogenic, or because lipoproteins become modified and immunogenic in a disturbed vascular environment?

4.7. Smooth Muscle Cell Phenotypic Switching and Plaque Progression

Vascular smooth muscle cells (VSMCs) play complex roles in atherosclerosis. They contribute to extracellular matrix production and fibrous cap formation, which can stabilize plaques. However, VSMCs can also undergo phenotypic switching, migrate into the intima, become macrophage-like, accumulate lipids, and contribute to necrotic core formation[168,169,170,171].

This dual role complicates simplistic interpretations of plaque biology. VSMCs are not merely pathological. Their reparative functions help maintain plaque stability, while dysregulated phenotypic switching can contribute to plaque progression.

Lipoproteins influence VSMC behavior through cholesterol loading, oxidative stress, inflammatory signaling, and extracellular matrix interactions. However, VSMC dysfunction also reflects broader vascular injury, inflammation, mechanical stress, and altered repair responses.

Interpretation Within the Integrative and Systems Medicine Framework

VSMC phenotypic switching is an intermediate mechanism and repair-response alteration. It is central to plaque progression and stability but is not primarily a lipid biomarker or isolated root driver.

Key question

Does VSMC remodeling represent lipid-driven pathology, or an altered vascular repair response occurring in a chronically injured arterial wall?

4.8. Necrotic Core Formation and Defective Efferocytosis

Advanced plaques contain necrotic cores formed by the accumulation of dead macrophages, lipid debris, extracellular cholesterol, and inflammatory mediators. Defective efferocytosis—the impaired clearance of apoptotic cells—contributes to necrotic core expansion and plaque vulnerability[172,173,174,175].

Modified lipoproteins, oxidative stress, endoplasmic reticulum stress, inflammation, and impaired macrophage function can all contribute to cell death and defective clearance. This mechanism links lipid accumulation with failed resolution and impaired tissue repair.

Necrotic core formation is clearly important for advanced plaque biology, but it occurs after multiple earlier events. It represents failure of resolution and repair rather than initial disease onset.

Interpretation Within the Integrative and Systems Medicine Framework

Necrotic core formation is a downstream pathological manifestation and advanced intermediate mechanism. It reflects failed clearance, unresolved inflammation, and impaired repair.

Key question

Is the necrotic core caused by lipid accumulation alone, or by the failure of macrophage survival, efferocytosis, and vascular repair systems?

4.9. Plaque Calcification

Vascular calcification is common in advanced atherosclerosis and is strongly associated with cardiovascular risk. Calcification involves osteogenic differentiation of vascular cells, inflammatory signaling, oxidative stress, phosphate metabolism, matrix vesicles, apoptosis, and impaired mineralization control[176,177,178,179].

Lipoproteins may contribute to calcification through inflammation, oxidized phospholipids, Lp(a), endothelial dysfunction, and foam-cell necrosis. Lp(a) has been linked to calcific aortic valve disease and may promote osteogenic signaling through oxidized phospholipid carriage[95,97,180].

However, calcification is not simply a lipid deposition process. It reflects a complex tissue remodeling response involving vascular cells, inflammation, mineral metabolism, oxidative stress, and repair pathways.

Interpretation Within the Integrative and Systems Medicine Framework

Plaque calcification is best classified as a downstream remodeling process and marker of chronic vascular injury. It may contribute to plaque behavior but is not a primary lipid root cause.

Key question

Does calcification represent lipid causation, or chronic vascular injury and maladaptive repair?

4.10. Plaque Rupture, Erosion, and Thrombosis

Acute cardiovascular events usually result from plaque rupture, plaque erosion, or thrombosis rather than from lipid levels alone. Vulnerable plaques often exhibit a large lipid-rich necrotic core, thin fibrous cap, inflammatory cell infiltration, protease activity, microcalcification, and impaired structural repair[181,182,183,184].

Thrombosis involves platelet activation, tissue factor exposure, coagulation cascade activation, endothelial injury, and impaired fibrinolysis. Lipoproteins may influence thrombosis indirectly through inflammation, endothelial dysfunction, oxidized phospholipids, and Lp(a)-related antifibrinolytic effects[93,94,95,185].

However, plaque rupture and thrombosis are terminal events in the disease cascade. They explain acute clinical events but not necessarily the initiating root causes of plaque development.

Interpretation Within the Integrative and Systems Medicine Framework

Plaque rupture, erosion, and thrombosis are late-stage mechanisms and clinical event triggers. They are not root causes of ASCVD but downstream consequences of plaque vulnerability.

Key question

Do lipid biomarkers predict acute events because they initiate disease, or because they correlate with plaque burden and vulnerability created by multiple interacting mechanisms?

4.11. Mechanistic Synthesis

The mechanistic evidence reviewed in this chapter strongly supports the conclusion that lipoproteins participate in atherosclerosis. ApoB-containing particles can enter the arterial wall, become retained, undergo modification, stimulate macrophage uptake, promote foam-cell formation, interact with immune and inflammatory pathways, and contribute to plaque progression. Lp(a) may additionally promote inflammation, thrombosis, and calcification through oxidized phospholipid carriage and apo(a)-related mechanisms.

However, most of these mechanisms depend on upstream biological conditions, including endothelial dysfunction, disturbed flow, oxidative stress, inflammation, metabolic dysfunction, impaired cholesterol efflux, defective efferocytosis, and altered vascular repair. Thus, the mechanistic evidence supports lipoproteins as important participants and mediators, but does not uniformly establish them as primary root causes.

5. Epidemiological Evidence: Risk Prediction, Association, and Residual Uncertainty

The preceding chapter reviewed mechanistic evidence showing that lipoproteins participate in atherosclerosis. Epidemiology addresses a different question: do lipid and lipoprotein biomarkers predict ASCVD events in human populations, and if so, does prediction imply root causation?

Large cohort studies have repeatedly demonstrated associations between lipid biomarkers and cardiovascular outcomes. However, epidemiology also reveals discordance, residual risk, population heterogeneity, and competing risk patterns that complicate simple causal interpretation. This chapter reviews major epidemiological evidence supporting the lipid paradigm, then examines its limitations.

5.1. Early Population Studies and the Cholesterol–ASCVD Association

Early observational studies established serum cholesterol as a population-level predictor of coronary heart disease. The Framingham Heart Study, Seven Countries Study, and MRFIT contributed substantially to the view that higher cholesterol was associated with higher coronary risk[16,17,18,186,187].

These studies were foundational because they demonstrated that cholesterol could function as a measurable risk marker across populations. They also helped justify later intervention trials and guideline-based cholesterol screening.

However, these early studies primarily evaluated total cholesterol, a nonspecific aggregate marker. Total cholesterol did not distinguish among LDL-C, HDL-C, triglycerides, remnant cholesterol, ApoB particle number, Lp(a), inflammatory status, metabolic dysfunction, or vascular integrity. Thus, while early epidemiology established cholesterol as a risk-associated biomarker, it did not establish total cholesterol as a root cause of ASCVD.

Interpretation Within the Integrative and Systems Medicine Framework

Early epidemiological studies support cholesterol as a population risk marker, but not as a biologically specific root cause.

5.2. LDL-C and Population Risk

LDL-C became the principal lipid biomarker because it showed stronger biological and clinical relevance than total cholesterol. Prospective studies and meta-analyses consistently demonstrate that higher LDL-C is associated with higher ASCVD risk, especially in middle-aged populations and in genetically high-risk states[2,21,188].

However, the relationship between LDL-C and events is probabilistic, not deterministic. Many individuals with elevated LDL-C do not develop clinical ASCVD, while many cardiovascular events occur among individuals with average or below-average LDL-C. This discordance has been observed in both primary-prevention cohorts and clinical populations[36,189,190].

LDL-C is therefore a useful risk marker but does not fully explain disease initiation, plaque vulnerability, inflammatory risk, thrombosis, or individual heterogeneity.

Interpretation Within the Integrative and Systems Medicine Framework

LDL-C is a strong epidemiological risk marker but remains an exposure biomarker rather than a complete causal explanation.

5.3. HDL-C: Strong Observational Association, Weak Causal Translation

Low HDL-C has been consistently associated with increased ASCVD risk in observational studies[39,40,89]. This association led to the concept of HDL-C as “protective cholesterol.”

However, epidemiology and genetics have challenged the causal interpretation of HDL-C concentration. Mendelian randomization studies found that genetically elevated HDL-C does not necessarily reduce myocardial infarction risk[49]. Furthermore, pharmacological trials that raised HDL-C did not consistently reduce ASCVD events when LDL-C was treated.

This creates an important epidemiological lesson: a biomarker may be strongly associated with risk without being causally protective. HDL-C may reflect metabolic health, insulin sensitivity, inflammation, triglyceride metabolism, or HDL function rather than acting as a root protective driver itself.

Interpretation Within the Integrative and Systems Medicine Framework

HDL-C is best classified as a risk-associated biomarker and physiological correlate, not a proven causal root factor.

5.4. Triglycerides and Remnant Cholesterol

Elevated triglycerides are associated with ASCVD risk, especially in metabolic syndrome, diabetes, obesity, and insulin resistance[50,51,52,53]. Nonfasting triglycerides and remnant cholesterol have shown strong associations with myocardial infarction, ischemic heart disease, and mortality[50,52,53,55].

Genetic evidence has strengthened the case that triglyceride-rich lipoproteins and remnant cholesterol may be causally involved in ASCVD. However, triglycerides themselves may be surrogate markers for remnant particle burden, ApoB particle number, hepatic VLDL overproduction, insulin resistance, and metabolic dysfunction[54,55,191].

Thus, triglyceride epidemiology points beyond lipids alone. It links ASCVD risk to systemic metabolic state.

Interpretation Within the Integrative and Systems Medicine Framework

Triglycerides and remnant cholesterol are epidemiological markers of atherogenic remnant burden and metabolic dysfunction. They may reflect more upstream metabolic disturbances rather than isolated root causes.

5.5. Non-HDL-C, ApoB, and Particle Burden

Non-HDL-C and ApoB were introduced because LDL-C alone does not fully capture all atherogenic ApoB-containing particles. Epidemiological studies and meta-analyses show that non-HDL-C and ApoB often outperform LDL-C in risk prediction, particularly in individuals with elevated triglycerides, diabetes, obesity, insulin resistance, or discordant LDL-C and particle burden[58,59,62,64,65].

ApoB is especially important because it approximates the number of atherogenic particles. When LDL-C and ApoB are discordant, ApoB often better predicts cardiovascular risk[59,61,65].

However, ApoB remains a particle-exposure biomarker. It does not explain why particles become retained, oxidized, glycosylated, inflammatory, or pathogenic within the arterial wall. Epidemiology supports ApoB as a superior risk marker but does not, by itself, establish ApoB as a root cause.

Interpretation Within the Integrative and Systems Medicine Framework

ApoB and non-HDL-C improve risk prediction by better capturing atherogenic particle burden, but they primarily measure exposure rather than root disease initiation.

5.6. LDL Particle Number and Discordance Studies

LDL particle number was developed to address discordance between LDL-C and LDL particle burden. Studies using nuclear magnetic resonance spectroscopy and other methods suggest that LDL-P may predict ASCVD events better than LDL-C when the two markers disagree[8,69,70,71].

Discordance studies are conceptually important for this review. They show that cholesterol mass and particle number are not equivalent. LDL-C may be normal while LDL-P is elevated, especially in insulin resistance and hypertriglyceridemia. This helps explain why some individuals with normal LDL-C remain at high risk.

However, LDL-P still measures particle burden. It does not capture endothelial function, oxidative modification, inflammatory state, vascular permeability, or repair capacity.

Interpretation Within the Integrative and Systems Medicine Framework

LDL-P is an advanced risk marker of particle burden. It improves exposure assessment but does not identify the root drivers of atherosclerosis.

5.7. Small Dense LDL and Metabolic Risk

Small dense LDL is associated with increased ASCVD risk in observational studies[236,237,238]. It is also strongly linked to insulin resistance, hypertriglyceridemia, low HDL-C, central obesity, and metabolic syndrome.

This epidemiological pattern suggests that small dense LDL is less an isolated lipid abnormality and more a marker of metabolic dysfunction. Its association with risk may reflect a broader metabolic phenotype characterized by hepatic VLDL overproduction, triglyceride enrichment of LDL, lipoprotein remodeling, increased ApoB particle number, and impaired insulin signaling.

Interpretation Within the Integrative and Systems Medicine Framework

Small dense LDL is best interpreted epidemiologically as a marker of metabolic dysfunction and altered lipoprotein remodeling rather than an independent root cause.

5.8. Oxidized LDL and Oxidative Risk

Oxidized LDL and oxidized phospholipids have been associated with coronary artery disease, plaque burden, and cardiovascular outcomes in observational studies[83,84,192]. These findings align with mechanistic evidence showing that LDL modification contributes to macrophage uptake, foam-cell formation, and vascular inflammation.

However, epidemiology cannot easily distinguish whether oxidized LDL is causal or whether it reflects oxidative stress, inflammation, endothelial dysfunction, and impaired antioxidant defenses. OxLDL may therefore be better understood as a biomarker of oxidative modification and an intermediate mechanism rather than a primary initiating factor.

Interpretation Within the Integrative and Systems Medicine Framework

OxLDL is an epidemiological marker of oxidative vascular biology and a mechanistic mediator, but it likely reflects upstream oxidative and inflammatory conditions.

5.9. Lipoprotein(a): Epidemiology of an Inherited Risk Factor

Lp(a) differs from most lipid biomarkers because its circulating concentration is largely genetically determined. Large prospective studies and meta-analyses show that elevated Lp(a) is associated with increased risk of coronary heart disease, stroke, peripheral arterial disease, and calcific aortic valve disease[88,89,90,91,93].

Because Lp(a) is relatively stable over life and strongly genetically influenced, epidemiological and genetic studies provide stronger causal support than for many other lipid biomarkers. Lp(a) therefore represents the clearest lipid-related example of a possible partial upstream causal contributor.

Nevertheless, elevated Lp(a) does not explain most ASCVD. Many individuals with ASCVD do not have markedly elevated Lp(a), and not all individuals with elevated Lp(a) develop premature disease. Its pathogenic impact appears to depend on additional vascular, inflammatory, thrombotic, oxidative, and reparative factors.

Interpretation Within the Integrative and Systems Medicine Framework

Lp(a) is the strongest lipid-related candidate for a partial root-level contributor, but it functions best as a genetically determined risk amplifier rather than a universal root cause.

5.10. Residual Risk Despite Lipid Risk Stratification

One of the most important epidemiological observations is residual cardiovascular risk. Even when LDL-C is reduced or guideline targets are achieved, substantial event risk remains[38,193,194].

Residual risk may be lipid-related, inflammatory, thrombotic, metabolic, genetic, environmental, or related to vascular repair and plaque vulnerability. Studies of residual inflammatory risk and residual cholesterol risk demonstrate that ASCVD risk cannot be fully explained by LDL-C alone[38,194,195].

This does not invalidate LDL-C as a risk marker or therapeutic target. Rather, it demonstrates that LDL-C is only one component of the Integrative and Systems Medicine Framework for ASCVD. Interpretation Within the Integrative and Systems Medicine Framework.

Interpretation Within the Integrative and Systems Medicine Framework

Residual risk supports the need to distinguish lipid exposure from upstream disease drivers and downstream clinical event mechanisms.

5.11. Epidemiological Discordance and the Limits of Single Biomarkers

Across populations, lipid biomarkers often show discordance. LDL-C, non-HDL-C, ApoB, LDL-P, triglycerides, HDL-C, oxLDL, and Lp(a) may provide different risk estimates in the same individual[8,59,61,65,69,70,71].

Discordance is particularly common in metabolic syndrome, diabetes, obesity, chronic inflammation, and hypertriglyceridemia. These observations suggest that no single lipid biomarker captures the full biology of ASCVD.

This is directly relevant to the central thesis of this review. If lipid biomarkers measure different layers of disease biology, then ranking them by predictive performance may be less informative than classifying them by biological level.

Interpretation Within the Integrative and Systems Medicine Framework

Discordance supports a multi-level interpretation of lipid biomarkers rather than a single lipid-root-cause model.

5.12. Summary of Chapter 5

Epidemiological evidence strongly supports the association of multiple lipid and lipoprotein biomarkers with ASCVD risk. Total cholesterol, LDL-C, non-HDL-C, ApoB, LDL-P, triglycerides, remnant cholesterol, small dense LDL, oxidized LDL, and Lp(a) all provide clinically relevant information.

However, epidemiology primarily establishes association and prediction. It does not automatically establish root causation. The epidemiological literature also reveals discordance, residual risk, population heterogeneity, and biomarker-specific limitations.

The major conclusion of this chapter is that lipid biomarkers are powerful risk markers, but their predictive value does not necessarily define their position in disease architecture. Epidemiology supports the importance of lipoproteins in ASCVD but also reinforces the need to distinguish between biomarkers, mechanisms, and root drivers.

6.1. Clinical Intervention Evidence: What Can Intervention Trials Tell Us About Causality?

Randomized controlled trials (RCTs) represent the highest level of evidence for evaluating the efficacy of therapeutic interventions. Unlike observational epidemiology, which primarily identifies associations, intervention studies test whether modifying a specific biological target alters clinical outcomes. Consequently, lipid-lowering trials have become one of the principal pillars supporting the lipid hypothesis of atherosclerotic cardiovascular disease (ASCVD). Over the past four decades, numerous RCTs have demonstrated that therapies targeting low-density lipoprotein cholesterol (LDL-C), apolipoprotein B (ApoB)-containing lipoproteins, triglyceride-rich lipoproteins, or more recently lipoprotein(a) [Lp(a)] can reduce cardiovascular events to varying degrees[2,31,32,33,34,196].

However, an important distinction must be made between therapeutic efficacy and proof of root causation. Demonstrating that modification of a biological pathway reduces disease risk does not necessarily establish that the pathway represents the primary initiating cause of disease. Throughout medicine, many successful therapies target intermediate mechanisms rather than the fundamental etiological drivers. Antihypertensive therapy lowers the risk of stroke but does not eliminate the upstream causes of hypertension. Insulin effectively controls hyperglycemia in diabetes mellitus but does not reverse the underlying pathophysiology of insulin resistance or β-cell dysfunction. Likewise, anti-inflammatory therapies reduce symptoms in autoimmune diseases without necessarily correcting the initiating immune dysregulation. Therefore, clinical benefit should not automatically be interpreted as proof that the therapeutic target represents the earliest or sole causal factor in disease pathogenesis[9,10].

This principle is particularly relevant to ASCVD. Lipid-lowering therapies consistently reduce cardiovascular risk, yet they do so to differing extents, with varying effects on lipid biomarkers, inflammatory pathways, plaque stabilization, endothelial function, thrombosis, and vascular remodeling. Furthermore, substantial residual cardiovascular risk persists despite intensive lipid lowering, even when LDL-C concentrations are reduced to historically unprecedented levels[38,159,197,198]. These observations suggest that although lipid pathways play an important role in disease progression, they may represent only one component within the broader Integrative and Systems Medicine Framework.

Clinical intervention studies also differ in the biological questions they address. Early statin trials primarily evaluated whether reducing LDL-C lowered cardiovascular events. Subsequent studies investigated whether additional LDL-C reduction with ezetimibe or proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors produced incremental benefit. Other trials tested alternative lipid hypotheses, including raising high-density lipoprotein cholesterol (HDL-C) with niacin or cholesteryl ester transfer protein (CETP) inhibitors, lowering triglycerides with fibrates or omega-3 fatty acids, and more recently reducing Lp(a) with antisense oligonucleotides and small interfering RNA (siRNA)-based therapies. Collectively, these interventions provide a unique opportunity to examine which lipid pathways appear to be clinically actionable and which hypotheses have not been supported by randomized evidence[2,33,45,46,100,101,199,200].

Importantly, intervention trials evaluate modifiable therapeutic targets, not necessarily root biological drivers. A pathway may be therapeutically important because it lies at a critical point in disease progression, even if it is activated downstream of more fundamental abnormalities. For example, endothelial dysfunction, oxidative stress, chronic inflammation, insulin resistance, disturbed vascular repair, and environmental exposures may all influence the biological context in which ApoB-containing lipoproteins become retained, modified, and ultimately pathogenic. Consequently, successful intervention against lipoprotein-related pathways demonstrates that these pathways contribute to disease progression but does not, by itself, establish their biological roles within the Integrative and Systems Medicine Framework[3,5,28,29] .

Another important consideration is the distinction between relative risk reduction (RRR) and absolute risk reduction (ARR). Most major lipid-lowering trials report statistically significant reductions in relative risk, typically ranging from approximately 15% to 30%, depending on baseline risk, treatment intensity, and duration of follow-up[24,31,198]. While these reductions are clinically meaningful and have substantially influenced guideline recommendations, the corresponding absolute risk reductions are generally more modest, often between 1% and 5% over several years of treatment. Consequently, the number needed to treat (NNT) remains substantial for many patient populations. These observations do not diminish the value of lipid-lowering therapy but underscore that modification of lipid pathways alone does not eliminate the majority of cardiovascular events[24,31,198,201].

The persistence of residual cardiovascular risk provides additional insight within the Integrative and Systems Medicine Framework. Even among patients receiving intensive statin therapy, combination lipid-lowering therapy, or PCSK9 inhibition, cardiovascular events continue to occur at appreciable rates[38,159,197]. Residual risk may reflect persistent inflammation, thrombosis, endothelial dysfunction, insulin resistance, metabolic syndrome, hypertension, chronic kidney disease, genetic susceptibility, Lp(a), environmental exposures, impaired vascular repair, or combinations of these factors. Thus, residual risk supports a multifactorial model of ASCVD rather than a disease driven exclusively by any single lipid biomarker[38,159,197].

The objective of this chapter is therefore not simply to summarize lipid-lowering therapies, but to critically examine what intervention trials can legitimately tell us about disease causation. Specifically, this chapter addresses four fundamental questions:

• Which lipid-modifying interventions have consistently reduced cardiovascular events?
• Which lipid hypotheses have been supported or challenged by randomized clinical trials?
• What do successful intervention trials demonstrate regarding the biological role of lipoproteins in ASCVD?
• Equally importantly, what conclusions cannot be drawn from intervention studies alone?

Addressing these questions provides an essential bridge between the mechanistic and epidemiological evidence reviewed in the preceding chapters and the Integrative and Systems Medicine reassessment presented in the subsequent chapter. Rather than viewing intervention trials as definitive proof of a single causal pathway, they are interpreted here as experimental tests of specific components within a complex biological network. This perspective recognizes the unquestionable clinical importance of lipid modification while maintaining a clear distinction between therapeutic efficacy, mechanistic participation, and root causation.

6.2. Statins

Statins have been the cornerstone of lipid-lowering therapy for more than three decades and remain the most extensively studied pharmacological intervention for the prevention of atherosclerotic cardiovascular disease (ASCVD). Since the publication of the Scandinavian Simvastatin Survival Study (4S) in 1994, numerous randomized controlled trials have consistently demonstrated that statins reduce major adverse cardiovascular events (MACE), cardiovascular mortality, and, in selected populations, all-cause mortality[31,196,198,202,203,204]. Consequently, statins have become the foundation of contemporary lipid-management guidelines worldwide.

Mechanism of Action

Statins competitively inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in hepatic cholesterol biosynthesis. Reduced intracellular cholesterol stimulates hepatic expression of low-density lipoprotein (LDL) receptors through activation of sterol regulatory element-binding protein-2 (SREBP-2), thereby increasing clearance of circulating ApoB-containing lipoproteins and lowering plasma LDL-C concentrations[22,23].

Depending on the specific agent and dosage, statins typically reduce LDL-C by approximately 20%–60%, with corresponding reductions in ApoB and non-HDL cholesterol (9). Intensive statin therapy generally produces greater LDL-C reduction and larger cardiovascular benefit than moderate-intensity therapy[31].

Evidence from Landmark Randomized Trials

The Scandinavian Simvastatin Survival Study (4S) first demonstrated that simvastatin significantly reduced total mortality by 30% and major coronary events by 34% among patients with established coronary heart disease and hypercholesterolemia[196]. This landmark trial established LDL lowering as an effective therapeutic strategy for secondary prevention.

Subsequent trials extended these findings to broader patient populations. The West of Scotland Coronary Prevention Study (WOSCOPS) demonstrated significant reductions in first myocardial infarction and cardiovascular death among hypercholesterolemic men without previous myocardial infarction[25]. The Heart Protection Study (HPS) further showed that statin therapy reduced vascular events across a wide spectrum of baseline cholesterol concentrations and high-risk patient groups, suggesting that benefit extends beyond individuals with markedly elevated LDL-C[203].

More intensive LDL lowering was evaluated in the PROVE-IT TIMI-22 trial, in which high-dose atorvastatin produced greater reductions in cardiovascular events than standard-dose pravastatin following acute coronary syndrome[204]. These findings were subsequently confirmed by the Cholesterol Treatment Trialists' (CTT) Collaboration, which analyzed data from more than 170,000 participants and demonstrated that each 1 mmol/L (approximately 39 mg/dL) reduction in LDL-C was associated with an approximately 22% proportional reduction in major vascular events[31].

Collectively, these trials provide compelling evidence that statin therapy substantially lowers cardiovascular risk across diverse patient populations.

Beyond LDL Lowering: Pleiotropic Effects

Although LDL reduction is considered the principal mechanism underlying statin benefit, accumulating evidence suggests that statins exert additional biological effects independent of cholesterol lowering. Experimental and clinical studies have demonstrated improvements in endothelial function, reduced vascular inflammation, stabilization of atherosclerotic plaques, decreased oxidative stress, modulation of immune responses, and improved nitric oxide bioavailability[205,206,207,208].

Statins also reduce circulating concentrations of high-sensitivity C-reactive protein (hsCRP), an established marker of systemic inflammation. In the JUPITER trial, rosuvastatin significantly reduced cardiovascular events among apparently healthy individuals with elevated hsCRP despite relatively normal LDL-C concentrations, supporting the concept that anti-inflammatory effects may contribute to clinical benefit (14). Whether these so-called pleiotropic effects are entirely independent of LDL lowering remains debated, but they highlight the biological complexity of statin therapy.

Relative Risk Reduction Versus Absolute Benefit

Statin therapy consistently reduces the relative risk of major cardiovascular events by approximately 20%–30% depending on baseline risk, treatment intensity, and duration of follow-up[31]. However, absolute risk reductions are generally smaller and depend heavily on the patient's baseline cardiovascular risk. Consequently, the number needed to treat (NNT) varies substantially between primary and secondary prevention populations[201].

This distinction is clinically important. Relative risk reduction demonstrates biological efficacy, whereas absolute risk reduction reflects the practical magnitude of clinical benefit. Failure to distinguish between these measures may overestimate or underestimate therapeutic value depending on the clinical context.

Residual Cardiovascular Risk

Despite substantial reductions in LDL-C, cardiovascular events continue to occur in many patients receiving intensive statin therapy. Residual risk persists even when guideline-recommended LDL-C targets are achieved[2,38,159]. Several mechanisms have been proposed to explain this observation, including persistent inflammation, elevated Lp(a), insulin resistance, hypertension, chronic kidney disease, thrombosis, endothelial dysfunction, metabolic syndrome, and genetic susceptibility.

The persistence of residual risk suggests that LDL lowering alone does not fully address the complex biology of ASCVD. Rather, statin therapy modifies one important component of a multifactorial disease process.

Integrative Systems Medicine Interpretation

The statin literature provides some of the strongest clinical evidence supporting the importance of ApoB-containing lipoproteins in ASCVD progression. Randomized trials consistently demonstrate that lowering LDL-C reduces cardiovascular events, validating LDL-related pathways as clinically actionable therapeutic targets.

However, these trials do not establish LDL-C as the sole or primary root cause of ASCVD. Statins influence multiple biological pathways beyond cholesterol metabolism, including inflammation, endothelial function, oxidative stress, and plaque stability. Moreover, substantial residual cardiovascular risk remains despite intensive LDL reduction. Thus, statin trials demonstrate that LDL-related mechanisms occupy an important position within disease progression but do not determine where these mechanisms reside within the Integrative and Systems Medicine Framework.

From the perspective of the Integrative and Systems Medicine Framework, statins provide compelling evidence that ApoB-containing lipoproteins are major mechanistic participants and therapeutic targets. They do not, by themselves, establish that LDL-C represents the earliest initiating driver of atherosclerosis.

6.3. Ezetimibe

Mechanism of Action

Ezetimibe is a selective inhibitor of intestinal cholesterol absorption that acts by blocking the Niemann-Pick C1-like 1 (NPC1L1) transporter located on the brush border of enterocytes. Unlike statins, which reduce endogenous cholesterol synthesis, ezetimibe decreases intestinal cholesterol uptake, thereby reducing hepatic cholesterol stores and increasing LDL receptor expression. When combined with statins, ezetimibe provides complementary LDL-C lowering by targeting a different component of cholesterol homeostasis[209,210].

As monotherapy, ezetimibe lowers LDL-C by approximately 18–22%. When added to statin therapy, an additional 15–25% reduction in LDL-C can generally be achieved, depending on baseline lipid levels and statin intensity[211].

Clinical Evidence

The landmark IMPROVE-IT (Improved Reduction of Outcomes: Vytorin Efficacy International Trial) enrolled 18,144 patients hospitalized for acute coronary syndrome and randomized them to simvastatin plus ezetimibe or simvastatin alone[32].

During a median follow-up of six years, the combination therapy reduced median LDL-C from approximately 70 mg/dL to 54 mg/dL and produced a statistically significant reduction in the primary composite cardiovascular endpoint.

Compared with statin monotherapy:

• Relative risk reduction: approximately 6.4%
• Absolute risk reduction: approximately 2.0%
• Number needed to treat (NNT): approximately 50 over seven years

Importantly, the benefit was driven primarily by reductions in nonfatal myocardial infarction and ischemic stroke. Cardiovascular mortality and all-cause mortality were not significantly different between treatment groups[32].

These findings provided the first randomized evidence that LDL-C reduction beyond that achieved with statins alone could produce incremental cardiovascular benefit.

Interpretation of IMPROVE-IT

Several important conclusions emerged from IMPROVE-IT.

First, the trial supported the concept that "lower appears to be better" for LDL-C within the range studied. Patients achieving average LDL-C concentrations near 50–55 mg/dL experienced fewer cardiovascular events than those maintained around 70 mg/dL[32].

Second, because ezetimibe has little evidence of the broad pleiotropic actions attributed to statins, the observed benefit strengthened the argument that lowering ApoB-containing lipoprotein exposure itself contributes to cardiovascular risk reduction.

Third, however, the magnitude of benefit was modest. Although statistically significant, the absolute reduction in cardiovascular events remained relatively small despite prolonged follow-up. The majority of cardiovascular events still occurred despite intensive LDL lowering.

This distinction between statistical significance and biological completeness is important. IMPROVE-IT demonstrates that additional LDL reduction is clinically beneficial, but it does not demonstrate that LDL-C is the sole determinant of disease progression.

Residual Cardiovascular Risk

Even after combination therapy reduced LDL-C to approximately 54 mg/dL, substantial residual cardiovascular risk remained[32]. Many patients continued to experience myocardial infarction, stroke, hospitalization for unstable angina, or cardiovascular death.

Several explanations have been proposed for this residual risk, including:

• persistent vascular inflammation;
• elevated lipoprotein(a);
• thrombosis;
• endothelial dysfunction;
• insulin resistance and metabolic syndrome;
• hypertension;
• chronic kidney disease;
• oxidative stress;
• impaired vascular repair;
• genetic susceptibility.

Thus, while ezetimibe further reduces risk by lowering LDL-C, it does not eliminate the multifactorial biological processes underlying ASCVD.

Integrative Systems Medicine Interpretation

IMPROVE-IT represents an important experimental test of the lipid hypothesis. The study convincingly demonstrates that additional lowering of ApoB-containing lipoproteins beyond statin therapy produces additional cardiovascular benefit. This finding strongly supports LDL-related pathways as therapeutically modifiable contributors to ASCVD.

However, the trial does not establish LDL-C as the primary initiating cause of atherosclerosis. Rather, it demonstrates that reducing cholesterol exposure within ApoB-containing particles slows disease progression.

From the perspective of the Integrative and Systems Medicine Framework, IMPROVE-IT strengthens the classification of LDL-C as a major exposure biomarker and mechanistic participant. It confirms that modifying this pathway is clinically beneficial but does not determine whether LDL-related pathways occupy the highest level of biological causation. The persistence of substantial residual risk despite intensive LDL lowering suggests that additional upstream processes—including inflammation, endothelial dysfunction, metabolic dysregulation, oxidative stress, impaired vascular repair, and inherited susceptibility—continue to drive disease progression.

Consequently, the IMPROVE-IT trial is most appropriately interpreted as evidence that LDL reduction is an effective therapeutic strategy rather than definitive proof that LDL-C represents the universal root cause of ASCVD.

6.4. PCSK9 Inhibitors

Mechanism of Action

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a hepatic serine protease that regulates the degradation of low-density lipoprotein receptors (LDLR). PCSK9 binds to LDL receptors on hepatocytes and targets them for lysosomal degradation after endocytosis, thereby reducing receptor recycling and limiting hepatic clearance of circulating ApoB-containing lipoproteins[212,213].

Monoclonal antibodies against PCSK9, including evolocumab and alirocumab, inhibit PCSK9 binding to LDL receptors, allowing receptor recycling and markedly increasing LDL clearance. When added to maximally tolerated statin therapy, PCSK9 inhibitors typically reduce LDL-C by 50–65%, representing one of the most potent lipid-lowering strategies currently available[214].

The development of PCSK9 inhibitors was strongly supported by human genetics. Gain-of-function mutations in the PCSK9 gene cause severe hypercholesterolemia, whereas loss-of-function mutations are associated with lifelong reductions in LDL-C and substantially lower cardiovascular risk[215,216]. This genetic evidence provided one of the strongest rationales for therapeutic PCSK9 inhibition.

FOURIER Trial

The Further Cardiovascular Outcomes Research with PCSK9 Inhibition in Subjects with Elevated Risk (FOURIER) trial enrolled 27,564 patients with clinically established ASCVD receiving optimized statin therapy[33]. Participants were randomized to evolocumab or placebo and followed for a median of 2.2 years.

Evolocumab reduced median LDL-C from approximately 92 mg/dL to 30 mg/dL, corresponding to an average reduction of nearly 59%.

Compared with placebo:

• Relative risk reduction of the primary endpoint: 15%
• Relative risk reduction of the key secondary endpoint: 20%
• Absolute risk reduction: approximately 1.5%
• Number needed to treat (NNT): approximately 67 over 2.2 years

Importantly, FOURIER demonstrated that achieving LDL-C concentrations well below previously recommended therapeutic targets appeared safe during the duration of follow-up.

However, despite profound LDL lowering, cardiovascular events continued to occur in a substantial proportion of patients.

ODYSSEY OUTCOMES Trial

The ODYSSEY OUTCOMES trial evaluated alirocumab in 18,924 patients following recent acute coronary syndrome[34]. Patients receiving intensive statin therapy were randomized to alirocumab or placebo and followed for a median of 2.8 years.

Alirocumab significantly reduced LDL-C concentrations and lowered the incidence of major adverse cardiovascular events.

Compared with placebo:

• Relative risk reduction: approximately 15%
• Absolute risk reduction: approximately 1.6%
• Greater benefit was observed among patients with the highest baseline LDL-C concentrations.

Unlike FOURIER, ODYSSEY demonstrated a modest reduction in all-cause mortality in exploratory analyses, although cardiovascular mortality reduction did not reach statistical significance after adjustment for multiple comparisons[217].

Together, FOURIER and ODYSSEY firmly established PCSK9 inhibition as an effective strategy for reducing cardiovascular events in high-risk patients.

Lessons from Human Genetics

One of the most compelling aspects of PCSK9 biology is the remarkable concordance between human genetics and randomized clinical trials.

Individuals carrying naturally occurring loss-of-function variants in PCSK9 exhibit lifelong reductions in LDL-C accompanied by dramatically lower lifetime ASCVD risk (4,5). These observations support the concept that cumulative exposure to ApoB-containing lipoproteins contributes importantly to atherosclerotic disease.

However, genetic studies likewise demonstrate that reduced LDL exposure decreases risk rather than abolishing disease entirely. Even among individuals with genetically favorable lipid profiles, ASCVD continues to occur, indicating that additional biological determinants contribute to disease development.

Residual Cardiovascular Risk

The FOURIER and ODYSSEY trials demonstrated that cardiovascular risk persists despite achieving LDL-C concentrations near or below 30 mg/dL[33,217].

Residual events likely reflect multiple additional mechanisms, including:

• chronic vascular inflammation;
• elevated lipoprotein(a);
• thrombosis;
• endothelial dysfunction;
• insulin resistance;
• hypertension;
• diabetes mellitus;
• chronic kidney disease;
• oxidative stress;
• impaired vascular repair.

These observations indicate that profound LDL lowering substantially reduces—but does not eliminate—the biological processes responsible for ASCVD progression.

Integrative Systems Medicine Interpretation

PCSK9 inhibitor trials provide some of the strongest experimental evidence supporting the importance of ApoB-containing lipoproteins in ASCVD. Together with human genetic studies, FOURIER and ODYSSEY demonstrate that intensive lowering of LDL-C produces additional cardiovascular benefit beyond statin therapy.

These findings strongly support LDL-related pathways as major mechanistic contributors to disease progression and validate ApoB-containing lipoproteins as clinically actionable therapeutic targets.

Nevertheless, several important observations prevent these trials from establishing LDL-C as the universal root cause of ASCVD.

First, the absolute reduction in cardiovascular events remained relatively modest despite extraordinary reductions in LDL-C. Second, most cardiovascular events were not prevented by treatment. Third, persistent residual risk indicates that additional biological processes continue to drive disease despite near-complete suppression of circulating LDL-C.

Furthermore, PCSK9 inhibitors modify only one component of an intricate biological network. They do not directly address endothelial injury, chronic inflammation, mitochondrial dysfunction, insulin resistance, oxidative stress, thrombosis, vascular repair, environmental toxicants, or other upstream determinants of vascular homeostasis.

From the perspective of the Integrative and Systems Medicine Framework, the PCSK9 inhibitor trials strongly reinforce the conclusion that LDL-related pathways occupy a central position in atherosclerotic progression. However, they do not establish these pathways as the earliest initiating events in disease pathogenesis. Rather, they demonstrate that reducing cumulative ApoB-containing lipoprotein exposure effectively slows disease progression within a multifactorial biological system.

6.5. Inclisiran

Mechanism of Action

Inclisiran is the first clinically approved small interfering RNA (siRNA) therapy targeting proprotein convertase subtilisin/kexin type 9 (PCSK9). Unlike monoclonal antibodies, which neutralize circulating PCSK9 protein, inclisiran suppresses hepatic synthesis of PCSK9 messenger RNA through RNA interference. After subcutaneous administration, inclisiran is selectively taken up by hepatocytes via N-acetylgalactosamine (GalNAc) conjugation, where it incorporates into the RNA-induced silencing complex (RISC), resulting in prolonged degradation of PCSK9 mRNA[218,219].

This mechanism produces sustained reductions in circulating PCSK9 concentrations and increases hepatic LDL receptor recycling. Because the intracellular silencing effect persists for several months, inclisiran requires administration only twice yearly after the initial loading doses, representing a major advantage for long-term treatment adherence[220].

LDL-Cholesterol Lowering

The efficacy of inclisiran has been demonstrated in the ORION clinical trial program.

In the pivotal ORION-10 and ORION-11 trials, patients with established ASCVD or ASCVD risk equivalents receiving maximally tolerated statin therapy were randomized to inclisiran or placebo[221,222].

Inclisiran consistently reduced LDL-C by approximately 50% compared with placebo throughout 18 months of follow-up.

Additional reductions were observed in:

• ApoB
• non-HDL cholesterol
• total cholesterol
• lipoprotein(a) (modest reduction of approximately 15–25%)

Treatment was generally well tolerated, with injection-site reactions representing the most common adverse event.

Current Outcome Evidence

Unlike statins and PCSK9 monoclonal antibodies, inclisiran has not yet demonstrated definitive reductions in major adverse cardiovascular events (MACE) in completed randomized outcome trials.

The ORION-10 and ORION-11 studies were designed primarily as lipid-lowering efficacy trials rather than cardiovascular outcome trials. Although exploratory analyses suggested fewer cardiovascular events among inclisiran-treated patients, these studies were neither powered nor designed to establish clinical efficacy regarding hard cardiovascular endpoints[221,222].

The large randomized outcome trial ORION-4, enrolling approximately 15,000 patients with established ASCVD, is designed specifically to determine whether long-term inclisiran therapy reduces cardiovascular events beyond standard lipid-lowering therapy[223]. Results are eagerly awaited and will provide an important test of whether sustained RNA-based suppression of PCSK9 translates into clinical benefit comparable to monoclonal antibody therapy.

Advantages and Limitations

Inclisiran offers several potential advantages over existing PCSK9 inhibitors.

The twice-yearly dosing schedule may substantially improve long-term adherence compared with self-administered monoclonal antibody injections every two to four weeks. This simplified regimen may be particularly valuable for patients with poor medication compliance, a common limitation in chronic cardiovascular prevention.

Furthermore, inclisiran demonstrates remarkably consistent LDL-C reduction over prolonged intervals without significant fluctuations between doses.

However, important limitations remain.

Most importantly, long-term cardiovascular outcome data remain incomplete. At present, inclisiran has convincingly demonstrated biochemical efficacy but not yet definitive clinical efficacy comparable to that established by the FOURIER and ODYSSEY OUTCOMES trials.

Integrative Systems Medicine Interpretation

Inclisiran represents an important technological advance in lipid-lowering therapy rather than a fundamentally new biological concept. By suppressing hepatic production of PCSK9, it targets the same LDL receptor pathway previously validated by monoclonal antibodies.

Consequently, current evidence strongly supports inclisiran as an effective means of reducing circulating ApoB-containing lipoproteins. However, until randomized cardiovascular outcome trials are completed, its ability to reduce ASCVD events remains inferred rather than directly established.

From the perspective of the Integrative and Systems Medicine Framework, inclisiran presently reinforces the importance of LDL-related pathways as therapeutically modifiable mechanisms. It does not, however, provide additional evidence regarding the biological role of LDL-C within the Integrative and Systems Medicine Framework beyond that already established by statin and PCSK9 inhibitor trials.

Future outcome studies will clarify whether prolonged RNA interference against PCSK9 provides clinical benefits proportional to its profound LDL-C reduction. Even if positive, such findings would further validate LDL-related pathways as effective therapeutic targets but would not, by themselves, establish LDL-C as the universal initiating cause of ASCVD.

6.6. Niacin

Historical rationale

Niacin (nicotinic acid, vitamin B3) was one of the earliest lipid-modifying therapies shown to reduce cardiovascular events before the widespread use of statins. Unlike statins, niacin exerts broad effects on lipoprotein metabolism, lowering low-density lipoprotein cholesterol (LDL-C), triglycerides, lipoprotein(a) [Lp(a)], and apolipoprotein B (ApoB), while producing the largest pharmacological increase in high-density lipoprotein cholesterol (HDL-C) among currently available lipid therapies[224,225].

Because numerous epidemiological studies consistently demonstrated an inverse association between HDL-C concentrations and cardiovascular risk, niacin became the principal therapeutic strategy for testing the "HDL hypothesis"—the proposition that pharmacologically raising HDL-C would reduce ASCVD events[21,226].

Mechanism of Action

Niacin inhibits hepatic diacylglycerol acyltransferase-2 (DGAT2), decreasing triglyceride synthesis and very-low-density lipoprotein (VLDL) secretion. Reduced VLDL production subsequently lowers circulating LDL-C. Niacin also decreases hepatic clearance of ApoA-I, thereby increasing HDL particle concentrations and HDL-C levels[225,227].

Typical lipid effects include:

• LDL-C reduction: 10–20%
• Triglyceride reduction: 20–40%
• HDL-C increase: 15–35%
• Lp(a) reduction: 20–30%

These multiple lipid effects initially suggested that niacin might provide cardiovascular benefit beyond statins.

Early Clinical Evidence

The Coronary Drug Project (CDP) was the first large randomized trial to demonstrate cardiovascular benefit from niacin therapy. In men with previous myocardial infarction, niacin significantly reduced recurrent nonfatal myocardial infarction during active treatment. Long-term follow-up also demonstrated a modest reduction in all-cause mortality, despite discontinuation of therapy years earlier[228,229].

These findings established niacin as one of the earliest effective lipid-modifying therapies and contributed substantially to the development of lipid-based cardiovascular prevention.

The Statin Era: AIM-HIGH and HPS2-THRIVE

The role of niacin changed dramatically following the introduction of intensive statin therapy.

The AIM-HIGH trial enrolled patients with established cardiovascular disease, low HDL-C, and elevated triglycerides receiving intensive statin therapy[230]. Extended-release niacin significantly increased HDL-C and lowered triglycerides but failed to reduce the primary composite cardiovascular endpoint. The study was terminated early because of futility.

Similarly, the much larger HPS2-THRIVE trial randomized more than 25,000 high-risk patients receiving statins to extended-release niacin plus laropiprant or placebo[231]. Although niacin improved multiple lipid parameters, including HDL-C, LDL-C, and triglycerides, no significant reduction in major vascular events was observed. Moreover, treatment increased serious adverse events, including infections, gastrointestinal complications, new-onset diabetes, worsening glycemic control, bleeding, and myopathy.

These two landmark trials fundamentally altered the clinical role of niacin.

Implications for the HDL Hypothesis

The niacin experience represents one of the most informative natural experiments in cardiovascular medicine.

For decades, observational studies consistently showed that higher HDL-C concentrations were associated with lower cardiovascular risk[21,226]. However, randomized trials demonstrated that pharmacologically increasing HDL-C concentration did not reliably reduce cardiovascular events once LDL-C had been effectively lowered.

These findings strongly suggest that HDL-C concentration is not equivalent to HDL function.

Subsequent mechanistic studies have shown that HDL performs multiple biological activities—including reverse cholesterol transport, antioxidant defense, anti-inflammatory signaling, endothelial protection, and immune regulation—that are not adequately reflected by HDL-C concentration alone[43,232,233]. Consequently, HDL functionality appears biologically more important than HDL-C quantity.

The niacin trials therefore provide compelling evidence that modifying a biomarker does not necessarily modify the underlying biological process represented by that biomarker.

Integrative Systems Medicine Interpretation

From the perspective of the Integrative and Systems Medicine Framework, niacin occupies a unique position because it directly tests one of the central assumptions of the traditional lipid-centric paradigm.

The failure of AIM-HIGH and HPS2-THRIVE demonstrates that pharmacologically altering a biomarker associated with cardiovascular risk does not necessarily improve clinical outcomes. HDL-C clearly functions as a valuable epidemiological predictor of ASCVD, yet increasing HDL-C concentration alone proved insufficient to reduce cardiovascular events.

This distinction highlights an important conceptual principle developed throughout this review: risk prediction does not necessarily establish causation, and biomarker modification does not necessarily modify the underlying disease process.

Importantly, these findings should not be interpreted as evidence that HDL biology is unimportant. Rather, they suggest that HDL function—including cholesterol efflux capacity, antioxidant activity, endothelial protection, and immunomodulation—is considerably more relevant than HDL-C concentration itself.

From the perspective of the Integrative and Systems Medicine Framework, the niacin experience strongly supports classifying HDL-C as a biomarker of metabolic and vascular health rather than a universal therapeutic target or primary causal determinant of ASCVD.

6.7. Fibrates

Historical rationale

Fibrates were introduced as lipid-modifying agents primarily to lower triglyceride-rich lipoproteins and increase high-density lipoprotein cholesterol (HDL-C). Unlike statins, which principally target low-density lipoprotein cholesterol (LDL-C), fibrates were developed to address the dyslipidemia commonly observed in patients with obesity, metabolic syndrome, insulin resistance, and type 2 diabetes mellitus[234,235].

Elevated triglycerides and low HDL-C frequently coexist with increased numbers of small dense LDL particles, collectively referred to as atherogenic dyslipidemia. Because this lipid pattern is strongly associated with ASCVD, fibrates became an important therapeutic strategy for testing whether reducing triglycerides and modifying triglyceride-rich lipoprotein metabolism would improve cardiovascular outcomes.

Mechanism of Action

Fibrates are agonists of peroxisome proliferator-activated receptor-α (PPAR-α), a nuclear receptor that regulates genes involved in fatty acid oxidation, lipoprotein metabolism, and inflammation[236].

Activation of PPAR-α results in:

• increased lipoprotein lipase activity;
• enhanced clearance of triglyceride-rich lipoproteins;
• reduced hepatic VLDL production;
• increased fatty acid oxidation;
• modest increases in HDL-C through increased ApoA-I and ApoA-II synthesis.

Typical lipid effects include:

• triglyceride reduction: 30–50%
• HDL-C increase: 5–15%
• modest LDL-C reduction (variable depending on baseline triglycerides).

Early Clinical Trials

The Helsinki Heart Study first demonstrated that gemfibrozil significantly reduced coronary events in asymptomatic middle-aged men with dyslipidemia[237]. Similarly, the Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial (VA-HIT) showed that gemfibrozil reduced cardiovascular events among men with established coronary disease, relatively low HDL-C, and only modest elevations of LDL-C[238].

These early studies suggested that targeting triglyceride-rich lipoprotein metabolism could provide cardiovascular benefit independent of substantial LDL reduction.

Diabetes Trials

Interest subsequently focused on patients with diabetes mellitus and metabolic syndrome, in whom hypertriglyceridemia is especially common.

The FIELD (Fenofibrate Intervention and Event Lowering in Diabetes) trial enrolled nearly 10,000 patients with type 2 diabetes mellitus[239]. Although fenofibrate significantly lowered triglycerides and modestly increased HDL-C, the reduction in the primary coronary endpoint did not reach statistical significance. Nevertheless, reductions were observed in nonfatal myocardial infarction and several microvascular complications, including diabetic retinopathy.

The subsequent ACCORD Lipid Trial evaluated fenofibrate in combination with simvastatin among patients with type 2 diabetes mellitus[240]. Overall, combination therapy failed to significantly reduce the primary cardiovascular endpoint compared with statin therapy alone.

However, a prespecified subgroup analysis demonstrated benefit among patients with:

• triglycerides ≥204 mg/dL; and
• HDL-C ≤34 mg/dL.

This observation suggested that fibrates may be most effective in patients with pronounced atherogenic dyslipidemia rather than in unselected populations.

PROMINENT Trial

More recently, the PROMINENT trial evaluated pemafibrate, a selective PPAR-α modulator, in over 10,000 patients with type 2 diabetes mellitus, elevated triglycerides, and low HDL-C despite statin therapy[241].

Although pemafibrate substantially reduced triglycerides and remnant cholesterol, the trial failed to reduce major adverse cardiovascular events.

This negative study was particularly important because it challenged the assumption that lowering triglycerides alone necessarily translates into cardiovascular benefit.

Interpretation of the Fibrate Trials

Collectively, fibrate trials provide a more nuanced picture than statin trials.

Early studies suggested meaningful cardiovascular benefit, whereas more contemporary trials conducted in the era of intensive statin therapy have generally demonstrated limited or no overall benefit.

Nevertheless, an important and consistent observation has emerged.

Patients exhibiting the metabolic phenotype of:

• hypertriglyceridemia,
• low HDL-C,
• insulin resistance,
• obesity,
• metabolic syndrome,

appear to derive greater benefit than patients selected solely on the basis of LDL-C.

This finding suggests that fibrates may be treating a broader metabolic disorder rather than simply lowering triglycerides.

Integrative Systems Medicine Interpretation

The fibrate experience provides important insight into the biological significance of triglycerides.

Randomized trials indicate that lowering triglycerides alone does not uniformly reduce ASCVD risk. However, patients with pronounced metabolic dyslipidemia consistently appear to derive greater benefit from fibrate therapy.

These observations suggest that elevated triglycerides function primarily as markers of underlying metabolic dysfunction, including insulin resistance, hepatic overproduction of VLDL, adipose tissue dysfunction, and impaired energy metabolism.

From a disease architecture perspective, triglycerides are therefore better classified as biomarkers of disturbed metabolic physiology than as universal primary causal drivers of ASCVD.

The fibrate trials also reinforce an important conceptual distinction developed throughout this review. Therapeutic efficacy depends not only on modification of a lipid biomarker but also on the biological context in which that biomarker arises. Patients with insulin resistance and metabolic syndrome occupy different positions within the Integrative and Systems Medicine Framework than patients whose only abnormality is modest hypercholesterolemia.

Consequently, fibrates provide evidence supporting a systems-based interpretation of ASCVD. Their greatest benefit appears in patients with broader metabolic dysfunction, suggesting that the underlying metabolic environment may be more fundamental than triglyceride concentration itself.

6.8. Omega-3 Fatty Acid Trials

Historical rationale

Marine-derived omega-3 polyunsaturated fatty acids (PUFAs), particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have long been proposed to reduce cardiovascular risk. Early observational studies, including investigations of Greenland Inuit populations and subsequent epidemiological cohorts, suggested that higher fish consumption was associated with lower rates of coronary heart disease[242,243,244]. These observations led to the hypothesis that omega-3 fatty acids might reduce ASCVD through triglyceride lowering, anti-inflammatory effects, improved endothelial function, antithrombotic actions, plaque stabilization, and antiarrhythmic properties.

Unlike statins or PCSK9 inhibitors, omega-3 fatty acids were not developed primarily as LDL-lowering agents. Instead, they represent a broader metabolic intervention with multiple potential biological effects.

Mechanisms of Action

Omega-3 fatty acids reduce hepatic very-low-density lipoprotein (VLDL) synthesis and enhance fatty acid β-oxidation, resulting in reductions in circulating triglycerides[245]. Depending on formulation and dose, triglycerides are typically reduced by 20–35%, with little effect on LDL-C or HDL-C.

Experimental studies also suggest that EPA and DHA may:

• reduce production of pro-inflammatory eicosanoids;
• generate specialized pro-resolving mediators (resolvins, protectins, maresins);
• improve endothelial function;
• reduce oxidative stress;
• stabilize atherosclerotic plaques;
• decrease platelet activation;
• improve membrane fluidity and myocardial electrophysiology[246,247,248,249].

These pleiotropic actions distinguish omega-3 fatty acids from conventional lipid-lowering therapies.

Early Clinical Trials

Several early randomized trials suggested cardiovascular benefit.

The GISSI-Prevenzione trial demonstrated significant reductions in cardiovascular mortality and sudden cardiac death among patients receiving approximately 1 g/day omega-3 fatty acids following myocardial infarction[250].

Similarly, the JELIS (Japan EPA Lipid Intervention Study) reported that adding purified EPA (1.8 g/day) to statin therapy reduced major coronary events despite relatively modest triglyceride reductions[251].

These findings generated considerable enthusiasm for omega-3 supplementation in cardiovascular prevention.

REDUCE-IT

The REDUCE-IT trial fundamentally renewed interest in omega-3 therapy[252].

More than 8,000 statin-treated patients with established ASCVD or diabetes mellitus plus additional cardiovascular risk factors were randomized to receive icosapent ethyl (highly purified EPA, 4 g/day) or mineral oil placebo.

After a median follow-up of 4.9 years:

• triglycerides decreased by approximately 18%;
• primary cardiovascular endpoint reduced by 25% relative risk;
• absolute risk reduction approximately 4.8%;
• number needed to treat (NNT): approximately 21.

The magnitude of cardiovascular benefit substantially exceeded that expected from triglyceride reduction alone, suggesting additional biological mechanisms beyond lipid modification.

STRENGTH

In contrast, the STRENGTH trial produced markedly different results[253].

More than 13,000 high-risk patients received a mixed EPA/DHA formulation (4 g/day) or corn oil placebo.

Despite significant triglyceride reduction, STRENGTH demonstrated no reduction in major cardiovascular events and was terminated early because of futility.

The discordance between REDUCE-IT and STRENGTH has generated considerable debate.

Several explanations have been proposed:

• purified EPA versus EPA/DHA mixtures;
• differences in placebo selection (mineral oil versus corn oil);
• differences in achieved plasma EPA concentrations;
• biological differences between EPA and DHA;
• non-lipid pleiotropic effects of EPA.

These questions remain incompletely resolved.

Interpretation of Omega-3 Trials

The omega-3 literature illustrates both the strengths and limitations of intervention studies.

First, triglyceride reduction alone does not consistently predict cardiovascular benefit.

Second, omega-3 fatty acids likely influence multiple biological pathways beyond lipid metabolism, including inflammation, oxidative stress, endothelial biology, membrane function, thrombosis, and plaque stability.

Third, the marked benefit observed in REDUCE-IT cannot be fully explained by the relatively modest reduction in triglyceride concentration.

Consequently, omega-3 trials suggest that biological function may be more important than isolated biomarker modification.

Integrative Systems Medicine Interpretation

From the perspective of the Integrative and Systems Medicine Framework, omega-3 trials provide an important contrast with traditional lipid-lowering therapies.

Unlike statins or PCSK9 inhibitors, omega-3 fatty acids target multiple interconnected physiological systems simultaneously. Their effects extend beyond triglyceride metabolism to include inflammatory regulation, oxidative homeostasis, endothelial function, membrane biology, and specialized pro-resolving lipid mediators.

The divergent findings of REDUCE-IT and STRENGTH also emphasize that not all triglyceride-lowering interventions produce equivalent cardiovascular outcomes. This observation argues against a simplistic interpretation that triglyceride concentration alone determines cardiovascular risk.

Instead, omega-3 trials support a broader systems-based view in which triglycerides function primarily as biomarkers of underlying metabolic physiology, while omega-3 fatty acids exert therapeutic effects through multiple interacting biological mechanisms.

Accordingly, the cardiovascular benefits observed with purified EPA should not be interpreted solely as validation of the triglyceride hypothesis. Rather, they suggest that modulation of inflammation, oxidative stress, endothelial integrity, membrane biology, and vascular repair may contribute substantially to clinical benefit.

the the perspective of the Integrative and Systems Medicine Framework, omega-3 fatty acids illustrate an important principle: successful therapies may improve cardiovascular outcomes through coordinated effects on multiple biological pathways rather than by modifying a single lipid biomarker. This observation is highly consistent with a systems medicine model of ASCVD.

6.9. CETP Inhibitors

Historical rationale

The inverse association between high-density lipoprotein cholesterol (HDL-C) and cardiovascular risk observed in numerous epidemiological studies led to the long-standing "HDL hypothesis," which proposed that pharmacologically increasing HDL-C would reduce atherosclerotic cardiovascular disease (ASCVD)[21,226]. Cholesteryl ester transfer protein (CETP) inhibitors were developed specifically to test this hypothesis.

CETP facilitates the exchange of cholesteryl esters from HDL particles to apolipoprotein B (ApoB)-containing lipoproteins in exchange for triglycerides. Pharmacological inhibition of CETP markedly increases circulating HDL-C concentrations while producing modest reductions in low-density lipoprotein cholesterol (LDL-C). Because CETP inhibition favorably modifies two major lipid biomarkers simultaneously, these agents were initially expected to produce substantial reductions in cardiovascular events[254].

Mechanism of Action

CETP is a plasma glycoprotein that mediates lipid exchange between HDL and ApoB-containing lipoproteins. Inhibition of CETP reduces the transfer of cholesteryl esters from HDL to very-low-density lipoprotein (VLDL) and LDL particles, resulting in accumulation of cholesterol within HDL particles and modest reductions in LDL-C[255].

Typical lipid effects include:

• HDL-C increase: 60–140%
• LDL-C reduction: 15–40%, depending on the agent
• modest reductions in ApoB and non-HDL cholesterol

These dramatic improvements in lipid profiles made CETP inhibition one of the most promising lipid-targeted therapeutic strategies of the early 21st century.

Torcetrapib: A Major Setback

The first large cardiovascular outcomes trial evaluating CETP inhibition was ILLUMINATE, which investigated torcetrapib in combination with atorvastatin[256].

Although torcetrapib increased HDL-C by more than 70% and lowered LDL-C by approximately 25%, the trial was terminated prematurely because treatment significantly increased mortality and cardiovascular events.

Subsequent investigation demonstrated that torcetrapib possessed important off-target effects, including increased aldosterone production, elevated blood pressure, electrolyte abnormalities, and endothelial dysfunction[257]. Consequently, the adverse outcomes were attributed largely to molecule-specific toxicity rather than CETP inhibition itself.

Nevertheless, ILLUMINATE substantially weakened confidence in the HDL hypothesis.

Dalcetrapib and Evacetrapib

Subsequent CETP inhibitors were designed to avoid the off-target toxicity observed with torcetrapib.

The dal-OUTCOMES trial evaluated dalcetrapib following acute coronary syndrome[258]. Although HDL-C increased by approximately 30%, no reduction in cardiovascular events was observed.

Similarly, the ACCELERATE trial evaluated evacetrapib in more than 12,000 high-risk patients[259]. Evacetrapib increased HDL-C by more than 130% while lowering LDL-C by approximately 37%.

Despite these striking lipid changes, the trial demonstrated no reduction in major adverse cardiovascular events and was terminated early because of futility.

These findings represented one of the most important negative results in contemporary cardiovascular therapeutics.

Anacetrapib: A More Complex Result

The REVEAL trial evaluated anacetrapib in more than 30,000 patients with established ASCVD receiving intensive statin therapy[260].

Anacetrapib:

• increased HDL-C by approximately 104%;
• reduced non-HDL cholesterol by approximately 18%;
• reduced LDL-C by approximately 17%.

Unlike earlier CETP inhibitors, anacetrapib produced a modest but statistically significant reduction in major coronary events.

However, subsequent analyses suggested that the observed clinical benefit correlated more closely with reductions in ApoB-containing lipoproteins than with the extraordinary increase in HDL-C[66]. Because anacetrapib accumulated extensively in adipose tissue and possessed an exceptionally prolonged biological half-life, further development was discontinued despite positive trial results.

Lessons from CETP Inhibitor Trials

Collectively, CETP inhibitor trials represent one of the most informative series of experiments in cardiovascular medicine.

Three major observations emerge.

First, pharmacologically raising HDL-C concentration alone does not consistently reduce cardiovascular events.

Second, the magnitude of HDL-C increase bears little relationship to clinical benefit.

Third, when cardiovascular benefit does occur, it appears more closely associated with reductions in ApoB-containing lipoproteins than with increased HDL-C itself.

These findings fundamentally challenge the traditional interpretation of HDL-C as a direct therapeutic target.

HDL Function Versus HDL Concentration

The failure of CETP inhibitors has stimulated extensive investigation into HDL biology.

HDL particles possess numerous biological functions beyond cholesterol transport, including:

• reverse cholesterol transport;
• antioxidant activity;
• anti-inflammatory signaling;
• endothelial protection;
• nitric oxide preservation;
• immune regulation;
• modulation of thrombosis.

These functional properties vary substantially among HDL particles and are not adequately reflected by HDL-C concentration alone[43,233,261].

Consequently, HDL-C should be regarded primarily as a biomarker rather than a direct measure of HDL functionality.

Integrative Systems Medicine Interpretation

Among all lipid-modifying interventions, CETP inhibitors provide perhaps the clearest demonstration that changing a biomarker is not equivalent to changing the underlying disease biology.

Despite producing some of the largest increases in HDL-C ever achieved pharmacologically, most CETP inhibitors failed to reduce cardiovascular events. These findings strongly suggest that HDL-C concentration itself is not the primary determinant of HDL's biological functions.

the the perspective of the Integrative and Systems Medicine Framework, CETP inhibitor trials indicate that HDL-C occupies a fundamentally different position from LDL-C. Whereas LDL reduction consistently lowers cardiovascular risk, pharmacological elevation of HDL-C does not consistently improve outcomes.

This distinction supports classifying HDL-C primarily as a biomarker of vascular and metabolic health, rather than as a universal causal determinant or therapeutic target.

More broadly, the CETP experience reinforces one of the central conclusions of this review: biomarkers that strongly predict disease are not necessarily root causes of disease. Successful intervention depends on modifying biologically meaningful processes rather than simply changing biomarker concentrations.

Accordingly, CETP inhibitor trials provide compelling evidence that the Integrative and Systems Medicine Framework must distinguish among biomarker concentration, biological function, and root causation. This distinction extends beyond HDL biology and has important implications for interpreting all lipid biomarkers in ASCVD.

6.10. Lipoprotein(a)-Lowering Therapies: The Next Major Test of the Lipid Hypothesis

Background

Among all lipid biomarkers, lipoprotein(a) [Lp(a)] occupies a unique position. Unlike LDL-C, HDL-C, or triglycerides, circulating Lp(a) concentrations are determined predominantly by inherited variation in the LPA gene, with relatively little influence from diet, exercise, or conventional lipid-lowering therapies[7,91,92]. Mendelian randomization studies have consistently demonstrated that genetically elevated Lp(a) is associated with increased risk of coronary artery disease, ischemic stroke, calcific aortic valve disease, and peripheral arterial disease[262,263,264].

Structurally, Lp(a) consists of an LDL-like particle covalently linked to apolipoprotein(a), a highly polymorphic glycoprotein homologous to plasminogen[265]. In addition to carrying cholesterol, Lp(a) transports substantial quantities of oxidized phospholipids, promotes vascular inflammation, interferes with fibrinolysis, and contributes to vascular calcification[94,266,267].

These unique biological properties distinguish Lp(a) from conventional ApoB-containing lipoproteins and have made it one of the most promising emerging therapeutic targets in cardiovascular medicine.

Why Conventional Lipid Therapies Are Insufficient

Unlike LDL-C, Lp(a) is largely resistant to conventional lipid-lowering therapies.

Statins generally have little effect on Lp(a) and may even modestly increase circulating concentrations[268]. Ezetimibe has minimal influence, whereas PCSK9 inhibitors typically reduce Lp(a) by only 20–30%, substantially less than their effects on LDL-C[33,34]. Niacin lowers Lp(a) by approximately 20–30%; however, outcome trials failed to demonstrate additional cardiovascular benefit when niacin was added to contemporary statin therapy[45,46].

Consequently, no currently approved therapy has been specifically developed to target elevated Lp(a), highlighting an important unmet clinical need.

Antisense Oligonucleotide Therapy

The first major breakthrough in Lp(a)-targeted therapy came with antisense oligonucleotide (ASO) technology.

Pelacarsen (formerly AKCEA-APO(a)-LRx or TQJ230) is a GalNAc-conjugated antisense oligonucleotide that selectively inhibits hepatic synthesis of apolipoprotein(a), thereby reducing Lp(a) production at its source[269].

In a phase 2 randomized trial involving patients with established ASCVD and elevated Lp(a), pelacarsen produced dose-dependent reductions in circulating Lp(a) of up to 80%, representing an unprecedented degree of Lp(a) lowering[100].

The ongoing Lp(a) HORIZON trial is evaluating whether selective Lp(a) reduction translates into reductions in major adverse cardiovascular events. This study is widely regarded as one of the most important ongoing cardiovascular outcome trials because it directly tests the causal role of Lp(a).

Small Interfering RNA (siRNA) Therapies

RNA interference has provided a second major strategy for selectively lowering Lp(a).

Olpasiran

Olpasiran is a GalNAc-conjugated small interfering RNA (siRNA) that suppresses hepatic synthesis of apolipoprotein(a).

The OCEAN(a)-DOSE trial demonstrated reductions in Lp(a) exceeding 90%, with sustained suppression lasting many months following treatment[101].

Lepodisiran

Lepodisiran represents a newer generation of siRNA therapeutics.

In a first-in-human study, a single dose produced reductions approaching 95%, with suppression maintained for nearly one year in many participants[270].

These remarkable pharmacodynamic results suggest that durable Lp(a) suppression may become clinically feasible using infrequent dosing schedules.

Several additional siRNA agents, including zerlasiran (SLN360), are also under clinical investigation.

Current State of Clinical Evidence

Despite extraordinary reductions in circulating Lp(a), an important limitation remains.

To date, no completed randomized outcome trial has yet demonstrated that selective Lp(a) lowering reduces cardiovascular events.

Current evidence therefore establishes:

• remarkable biochemical efficacy;
• excellent target specificity;
• favorable safety profiles in early studies.

However, definitive proof that isolated Lp(a) reduction improves clinical outcomes is still pending.

Consequently, Lp(a)-lowering therapies presently occupy a unique position within cardiovascular therapeutics: they represent perhaps the most direct experimental test of whether a genetically determined lipid biomarker functions as an independent causal driver of ASCVD.

Integrative Systems Medicine Interpretation

Among all lipid-modifying therapies reviewed in this chapter, selective Lp(a)-lowering agents have the greatest potential to clarify disease causation.

If ongoing outcome trials demonstrate that isolated Lp(a) reduction substantially lowers cardiovascular events independent of LDL-C, this would provide compelling evidence supporting a direct causal role for Lp(a).

However, even a positive outcome would require careful interpretation.

Successful intervention would establish that Lp(a) participates causally in ASCVD progression and represents an effective therapeutic target. It would not, by itself, establish that Lp(a) is the universal initiating cause of atherosclerosis. Rather, it would demonstrate that one genetically determined upstream pathway contributes importantly to disease within susceptible biological environments.

Conversely, if cardiovascular benefit proves smaller than expected despite profound reductions in circulating Lp(a), this would suggest that Lp(a) functions primarily as one contributor among multiple interacting pathogenic mechanisms.

From the perspective of the Integrative and Systems Medicine Framework, Lp(a) remains the strongest candidate among lipid biomarkers for a partial upstream causal role. Nevertheless, its pathogenic effects likely depend upon interactions with endothelial dysfunction, inflammation, oxidative stress, thrombosis, vascular repair, metabolic dysfunction, and other biological determinants of vascular homeostasis.

Accordingly, ongoing Lp(a)-lowering outcome trials should be viewed not simply as tests of a new therapeutic agent, but as one of the most important experimental evaluations of the causal Integrative and Systems Medicine Framework for ASCVD.

6.11. What Clinical Intervention Trials Demonstrate

The accumulated evidence from randomized clinical trials provides compelling support for the clinical value of lipid-modifying therapies in the prevention of atherosclerotic cardiovascular disease (ASCVD). Collectively, studies involving statins, ezetimibe, PCSK9 inhibitors, and several emerging lipid-lowering therapies consistently demonstrate that modification of ApoB-containing lipoprotein pathways reduces cardiovascular events in appropriately selected patient populations[24,31,32,33,217]. These findings represent one of the strongest bodies of evidence supporting lipid-directed intervention in modern cardiovascular medicine.

First, intervention trials demonstrate that ApoB-containing lipoproteins are therapeutically modifiable determinants of ASCVD progression. Statins, ezetimibe, and PCSK9 inhibitors each reduce LDL-C through distinct biological mechanisms, yet all have demonstrated significant reductions in major adverse cardiovascular events. The consistency of these findings across multiple pharmacological classes strongly supports the conclusion that reducing cumulative exposure to circulating ApoB-containing lipoproteins slows progression of atherosclerotic disease[24,31,32,33,217].

Second, randomized trials demonstrate that greater reductions in LDL-C generally produce greater reductions in cardiovascular risk. Meta-analyses by the Cholesterol Treatment Trialists' (CTT) Collaboration have shown an approximately 20–25% proportional reduction in major vascular events for every 1 mmol/L (approximately 39 mg/dL) reduction in LDL-C[31]. Similarly, PCSK9 inhibitor trials demonstrated additional cardiovascular benefit when LDL-C concentrations were lowered substantially below targets previously considered optimal[33,217]. These observations support a dose-response relationship between cumulative LDL exposure and cardiovascular risk reduction.

Third, intervention studies establish that not all lipid biomarkers are biologically equivalent. Therapies designed to increase HDL-C, including niacin and most CETP inhibitors, largely failed to improve cardiovascular outcomes despite producing dramatic increases in HDL-C concentrations[94,264,265,266,267]. Likewise, fibrate therapy demonstrated benefit primarily in patients with hypertriglyceridemia and metabolic dyslipidemia rather than uniformly across all patient populations[33,34,268]. These findings indicate that the biological significance of individual lipid biomarkers depends on their underlying physiological context rather than simply their circulating concentrations.

Fourth, intervention trials demonstrate that lipoprotein biology is considerably more complex than individual lipid concentrations alone. Benefits observed with purified eicosapentaenoic acid (EPA) in REDUCE-IT exceeded those predicted by triglyceride reduction alone, suggesting that anti-inflammatory, membrane-stabilizing, endothelial, and other pleiotropic mechanisms contribute importantly to cardiovascular protection[230]. Similarly, statins exert multiple biological effects beyond LDL lowering, including improvements in endothelial function, plaque stabilization, and reduction of vascular inflammation[46,99,100]. These observations indicate that successful therapies frequently influence multiple biological pathways simultaneously.

Finally, emerging Lp(a)-lowering therapies demonstrate that selective targeting of genetically determined lipid pathways has become technically feasible[101,270]. Although definitive cardiovascular outcome data remain pending, antisense oligonucleotides and small interfering RNA (siRNA) therapies have achieved unprecedented reductions in circulating Lp(a), creating an important opportunity to directly test one of the strongest genetically supported hypotheses in cardiovascular biology.

Taken together, the intervention literature supports several important conclusions. First, lipid modification is an effective therapeutic strategy for reducing cardiovascular risk. Second, ApoB-containing lipoproteins play an important role in disease progression. Third, cumulative exposure to atherogenic lipoproteins contributes substantially to clinical risk. Finally, successful intervention often extends beyond simple lipid modification to include broader effects on vascular biology, inflammation, thrombosis, endothelial function, and plaque stability.

These conclusions represent major advances in cardiovascular medicine and have fundamentally improved the prevention and treatment of ASCVD.

Table 2. Summary of major lipid-modifying intervention trials and their implications for disease causation.

Intervention	Biomarker changed	Outcome benefit	Does it support t he lipid hypothesis?	Does it prove root causation?
Statins	LDL-C ↓	Yes	Strongly	No
Ezetimibe	LDL-C ↓	Modestly	Yes	No
PCSK9 inhibitors	LDL-C ↓↓↓	Yes	Strongly	No
Inclisiran	LDL-C ↓↓↓	Outcomes pending	Pending	No
Niacin	HDL ↑, TG ↓	No	Weakens HDL hypothesis	No
Fibrates	TG ↓	Mixed	Context-dependent	No
Omega-3 (EPA)	TG ↓	Mixed/positive	Suggests broader mechanisms	No
CETP inhibitors	HDL ↑↑	Mostly no	Refutes HDL-C hypothesis	No
Lp(a) therapies	Lp(a) ↓↓↓	Outcomes pending	Pending	Not yet

Table 2. Summary of major lipid-modifying intervention trials and their implications for disease causation.

Intervention	Biomarker changed	Outcome benefit	Does it support t he lipid hypothesis?	Does it prove root causation?
Statins	LDL-C ↓	Yes	Strongly	No
Ezetimibe	LDL-C ↓	Modestly	Yes	No
PCSK9 inhibitors	LDL-C ↓↓↓	Yes	Strongly	No
Inclisiran	LDL-C ↓↓↓	Outcomes pending	Pending	No
Niacin	HDL ↑, TG ↓	No	Weakens HDL hypothesis	No
Fibrates	TG ↓	Mixed	Context-dependent	No
Omega-3 (EPA)	TG ↓	Mixed/positive	Suggests broader mechanisms	No
CETP inhibitors	HDL ↑↑	Mostly no	Refutes HDL-C hypothesis	No
Lp(a) therapies	Lp(a) ↓↓↓	Outcomes pending	Pending	Not yet

6.12. What Clinical Intervention Trials Do Not Demonstrate

While randomized clinical trials provide compelling evidence supporting lipid-lowering therapy as an effective strategy for reducing cardiovascular risk, it is equally important to recognize the questions that these studies are not designed to answer. Failure to distinguish therapeutic efficacy from biological causation has contributed to considerable misunderstanding regarding the interpretation of lipid intervention trials.

First, intervention trials do not establish that low-density lipoprotein cholesterol (LDL-C) is the primary initiating cause of atherosclerosis. Randomized trials demonstrate that reducing LDL-C lowers cardiovascular event rates; however, they do not determine whether LDL-C occupies the highest level of disease causation or whether it functions within a broader pathogenic network. As emphasized by Hill's principles of causal inference, successful modification of a biological variable does not by itself identify that variable as the earliest or most fundamental cause of disease[9,10].

Second, lipid-lowering trials do not demonstrate that ASCVD is a single-factor disease. Despite substantial reductions in LDL-C achieved with intensive statin therapy, ezetimibe, or PCSK9 inhibitors, cardiovascular events continue to occur in a considerable proportion of treated patients[31,32,33,34]. The persistence of residual cardiovascular risk strongly suggests that multiple biological pathways—including chronic inflammation, endothelial dysfunction, thrombosis, insulin resistance, oxidative stress, vascular aging, chronic kidney disease, lipoprotein(a), and impaired vascular repair—continue to contribute to disease progression despite intensive lipid modification [5,38,159].

Third, intervention studies do not establish that all clinical benefit derives exclusively from lipid lowering itself. Several therapies exert important biological effects beyond changes in lipid concentrations. Statins improve endothelial function, reduce vascular inflammation, stabilize atherosclerotic plaques, and decrease oxidative stress[205,206,207]. Purified eicosapentaenoic acid (EPA) influences inflammatory resolution, membrane biology, oxidative stress, and thrombosis in addition to lowering triglycerides[200,248]. Consequently, improvements in cardiovascular outcomes may reflect coordinated effects on multiple biological systems rather than modification of a single lipid parameter.

Fourth, intervention trials do not demonstrate that all lipid biomarkers occupy equivalent positions within disease biology. Niacin and most CETP inhibitors markedly increased HDL-C without consistently reducing cardiovascular events[44,45,46,48,258]. Likewise, fibrate therapy has shown benefit primarily in patients with hypertriglyceridemia and metabolic syndrome rather than uniformly across all patient populations[239,240,241]. These findings indicate that changes in biomarker concentration do not necessarily correspond to changes in biological function or disease progression.

Fifth, current intervention studies do not fully resolve the causal role of lipoprotein(a). Although antisense oligonucleotide and small interfering RNA therapies produce unprecedented reductions in circulating Lp(a), definitive randomized outcome trials remain ongoing[100,101,269,271]. Consequently, current evidence strongly supports Lp(a) as an important inherited cardiovascular risk factor but does not yet establish the magnitude of clinical benefit achievable through selective Lp(a) reduction.

More broadly, randomized clinical trials are designed to evaluate whether modifying a therapeutic target improves clinical outcomes. They are not designed to reconstruct the complete biological sequence by which disease originates. The Integrative and Systems Medicine Framework for ASCVD extends beyond any single lipid pathway and encompasses interactions among genetic susceptibility, metabolic dysfunction, chronic inflammation, endothelial injury, oxidative stress, immune regulation, vascular repair, environmental exposures, aging, and numerous other physiological processes[3,5,155,272].

Accordingly, therapeutic efficacy should not be interpreted as synonymous with root causation. A biological pathway may represent an effective point for intervention while still functioning downstream of more fundamental disturbances. This principle is well recognized throughout medicine. Antihypertensive therapy reduces stroke without identifying the ultimate cause of hypertension. Insulin lowers blood glucose without reversing the underlying pathophysiology of type 2 diabetes mellitus. Similarly, antibiotics eradicate bacterial infections without correcting the environmental or host factors that predisposed to infection. Effective therapy therefore identifies an actionable component of disease biology but does not necessarily define the earliest pathogenic event.

Collectively, the intervention evidence reviewed in this chapter supports a more nuanced interpretation of the lipid hypothesis. Lipid modification clearly reduces cardiovascular risk and remains an essential component of contemporary ASCVD management. However, the available evidence does not establish that lipid abnormalities alone constitute the universal root cause of atherosclerosis. Instead, intervention trials support the concept that ApoB-containing lipoproteins participate importantly in disease progression while leaving open the broader question of how these pathways interact with upstream biological processes that initiate and sustain vascular injury.

This distinction provides the conceptual foundation for the disease architecture reassessment presented in the following chapter, in which lipid biomarkers are interpreted within a hierarchical systems framework that distinguishes root drivers, intermediate pathogenic mechanisms, biomarkers, and clinical manifestations.

7. An Integrative and Systems Medicine Framework for Root Cause Analysis of ASCVD

7.1. From the Lipid-Centric Paradigm to the Integrative and Systems Medicine Framework

The preceding chapters reviewed the evolution of lipid biomarkers, including total cholesterol, low-density lipoprotein cholesterol (LDL-C), non-high-density lipoprotein cholesterol (non-HDL-C), apolipoprotein B (ApoB), LDL particle number (LDL-P), small dense LDL (sdLDL), oxidized LDL (oxLDL), triglycerides, high-density lipoprotein cholesterol (HDL-C), and lipoprotein(a) [Lp(a)]. Mechanistic, epidemiological, genetic, and clinical intervention studies consistently demonstrate that these biomarkers are associated with ASCVD risk and, in several cases, represent clinically important therapeutic targets[2,7,13,31,143,273].

However, the collective evidence also reveals an important conceptual limitation of the traditional lipid-centric paradigm. Most discussions have focused primarily on whether a lipid biomarker is associated with disease, rather than where that biomarker resides within the Integrative and Systems Medicine Framework.

This distinction is fundamental. Biological variables may function as:

• Root drivers, initiating disease;
• Intermediate pathogenic mechanisms, propagating disease;
• Biomarkers, reflecting disease activity; or
• Clinical manifestations, representing downstream consequences.

Failure to distinguish these levels has contributed to longstanding confusion regarding the causal significance of lipid biomarkers.

Accordingly, we propose that lipid biomarkers should be interpreted within an Integrative and Systems Medicine Framework for Root Cause Analysis (RCA), in which biological variables are classified according to their biological roles and positions within the framework, rather than solely by their predictive performance or therapeutic responsiveness.

Table 2. Proposed Integrative and Systems Medicine Classification of Major Lipid and Lipoprotein Biomarkers.

Lipid / Lipoprotein Marker	Primary Physiological Role	Best Classified As	Role in Disease Architecture	Root Cause?
Total Cholesterol (TC)	Total circulating cholesterol	Biomarker	Population-level risk indicator	No
LDL-C	Cholesterol carried by LDL particles	Exposure biomarker	Major therapeutic target; reflects cholesterol exposure	No
Non-HDL-C	Total cholesterol in ApoB-containing lipoproteins	Exposure biomarker	Integrated measure of atherogenic cholesterol burden	No
Apolipoprotein B (ApoB)	Number of circulating atherogenic lipoprotein particles	Exposure biomarker	Best measure of cumulative ApoB particle exposure	No
LDL Particle Number (LDL-P)	Number of LDL particles	Exposure biomarker	Quantifies atherogenic particle burden	No
Small Dense LDL (sdLDL)	Highly atherogenic LDL subfraction	Intermediate pathogenic mechanism	Increased arterial retention and susceptibility to oxidation	No
Oxidized LDL (oxLDL)	Oxidatively modified LDL	Intermediate pathogenic mechanism	Promotes endothelial activation, foam-cell formation, and inflammation	No
Triglycerides (TG)	Transport of metabolic energy	Biomarker of metabolic dysfunction	Reflects insulin resistance and triglyceride-rich lipoprotein metabolism	No
HDL-C	Cholesterol within HDL particles	Biomarker of metabolic/vascular health	Marker of HDL metabolism; HDL-C concentration does not necessarily reflect HDL function	No
Lipoprotein(a) [Lp(a)]	Genetically determined ApoB-containing lipoprotein carrying apo(a)	Partial upstream causal contributor	Inherited risk factor with pro-inflammatory, pro-atherogenic, and pro-thrombotic properties	Partial

Table 2. Proposed Integrative and Systems Medicine Classification of Major Lipid and Lipoprotein Biomarkers.

Lipid / Lipoprotein Marker	Primary Physiological Role	Best Classified As	Role in Disease Architecture	Root Cause?
Total Cholesterol (TC)	Total circulating cholesterol	Biomarker	Population-level risk indicator	No
LDL-C	Cholesterol carried by LDL particles	Exposure biomarker	Major therapeutic target; reflects cholesterol exposure	No
Non-HDL-C	Total cholesterol in ApoB-containing lipoproteins	Exposure biomarker	Integrated measure of atherogenic cholesterol burden	No
Apolipoprotein B (ApoB)	Number of circulating atherogenic lipoprotein particles	Exposure biomarker	Best measure of cumulative ApoB particle exposure	No
LDL Particle Number (LDL-P)	Number of LDL particles	Exposure biomarker	Quantifies atherogenic particle burden	No
Small Dense LDL (sdLDL)	Highly atherogenic LDL subfraction	Intermediate pathogenic mechanism	Increased arterial retention and susceptibility to oxidation	No
Oxidized LDL (oxLDL)	Oxidatively modified LDL	Intermediate pathogenic mechanism	Promotes endothelial activation, foam-cell formation, and inflammation	No
Triglycerides (TG)	Transport of metabolic energy	Biomarker of metabolic dysfunction	Reflects insulin resistance and triglyceride-rich lipoprotein metabolism	No
HDL-C	Cholesterol within HDL particles	Biomarker of metabolic/vascular health	Marker of HDL metabolism; HDL-C concentration does not necessarily reflect HDL function	No
Lipoprotein(a) [Lp(a)]	Genetically determined ApoB-containing lipoprotein carrying apo(a)	Partial upstream causal contributor	Inherited risk factor with pro-inflammatory, pro-atherogenic, and pro-thrombotic properties	Partial

7.2. Reclassification of Lipid Biomarkers

Based on the evidence reviewed throughout this article, most lipid biomarkers are more appropriately classified as biomarkers of exposure or intermediate pathogenic mechanisms than as universal root causes of ASCVD.

LDL-C reflects the cholesterol content carried within LDL particles and remains an indispensable therapeutic target. However, LDL-C primarily quantifies lipoprotein exposure rather than directly measuring endothelial injury, oxidative modification, inflammation, or vascular repair.

ApoB and LDL-P more accurately estimate the number of circulating atherogenic particles and therefore provide superior measures of cumulative exposure to ApoB-containing lipoproteins. Nevertheless, they remain biomarkers of exposure rather than initiating biological events.

Small dense LDL and oxidized LDL occupy a different position. Both participate directly in plaque development and progression. Small dense LDL is more readily retained within the arterial wall and more susceptible to oxidative modification, while oxidized LDL actively promotes endothelial activation, macrophage recruitment, foam-cell formation, and vascular inflammation. These biomarkers therefore represent intermediate pathogenic mechanisms, but they are themselves generated downstream of metabolic and oxidative disturbances.

Triglycerides primarily reflect triglyceride-rich lipoprotein metabolism and systemic metabolic dysfunction. Similarly, HDL-C serves predominantly as a biomarker of metabolic and vascular health rather than a direct determinant of cardiovascular protection, as demonstrated by the failure of HDL-raising therapies to consistently improve cardiovascular outcomes[45,49,50,256,274].

Among currently recognized lipid biomarkers, Lp(a) occupies a unique position. Strong genetic evidence supports a direct contribution of elevated Lp(a) to ASCVD risk, and its biological effects extend beyond cholesterol transport to include carriage of oxidized phospholipids, promotion of inflammation, interference with fibrinolysis, and vascular calcification. Accordingly, Lp(a) may be regarded as a partial upstream causal contributor. Nevertheless, elevated Lp(a) alone is neither necessary nor sufficient for the development of most cases of ASCVD, indicating that it functions within a broader network of interacting biological processes rather than as a universal root cause.

7.3. Integrative and Systems Medicine Framework for ASCVD

The evidence reviewed throughout this article supports the Integrative and Systems Medicine Framework for ASCVD rather than a linear lipid-centric paradigm.

At the highest level are root drivers, including metabolic dysfunction, endothelial injury, chronic inflammation, oxidative stress, aging, hypertension, environmental exposures, and inherited susceptibility.

These upstream disturbances create the biological environment in which intermediate pathogenic mechanisms develop, including retention of ApoB-containing lipoproteins, oxidative modification of LDL, foam-cell formation, chronic vascular inflammation, extracellular matrix remodeling, and plaque progression.

Lipid biomarkers primarily quantify these biological processes rather than initiate them. LDL-C, ApoB, LDL-P, triglycerides, HDL-C, and non-HDL-C therefore function principally as measurable indicators of specific components of disease biology. Clinical manifestations—including coronary artery disease, myocardial infarction, ischemic stroke, peripheral arterial disease, and cardiovascular death—represent the final downstream consequences of prolonged interaction among these upstream and intermediate processes.

The Integrative and Systems Medicine Framework reconciles several observations that are difficult to explain within a purely lipid-centric paradigm. It explains why lowering LDL-C consistently reduces cardiovascular risk while substantial residual risk persists; why raising HDL-C does not reliably improve outcomes; why triglyceride lowering benefits only selected metabolic phenotypes; and why Lp(a), despite strong genetic evidence, acts primarily as a context-dependent risk amplifier rather than a universal initiating cause.

7.4. Implications

Reclassifying lipid biomarkers within the Integrative and Systems Medicine Framework has important implications for both research and clinical practice.

For research, it encourages investigation beyond increasingly refined lipid measurements toward identification of the upstream biological disturbances that create the vascular environment favorable for atherosclerosis.

For clinical practice, it reinforces the continued importance of lipid-lowering therapy while recognizing that optimal ASCVD prevention is unlikely to be achieved by lipid modification alone. Residual cardiovascular risk likely reflects persistent abnormalities in inflammation, endothelial function, metabolic regulation, oxidative stress, thrombosis, vascular repair, and other upstream physiological processes that warrant continued investigation.

Importantly, this framework does not diminish the value of lipid biomarkers. On the contrary, LDL-C, ApoB, and related biomarkers remain among the most valuable tools for cardiovascular risk assessment and therapeutic monitoring. Rather, this framework seeks to place these biomarkers within their appropriate biological context, distinguishing therapeutic targets from root causes.

Ultimately, the principal contribution of this review is not to dispute the importance of lipid biology, but to propose a more comprehensive interpretation of its role within the Integrative and Systems Medicine Framework for ASCVD. We suggest that most lipid and lipoprotein-related biomarkers are best understood as biomarkers or intermediate pathogenic mechanisms, whereas the root drivers of ASCVD reside further upstream within the complex physiological networks that govern vascular homeostasis.

8. Conclusions

This review examined the evolution of lipid biomarkers in atherosclerotic cardiovascular disease (ASCVD), tracing their progression from total cholesterol and low-density lipoprotein cholesterol (LDL-C) to increasingly sophisticated measures, including non-high-density lipoprotein cholesterol (non-HDL-C), apolipoprotein B (ApoB), LDL particle number (LDL-P), small dense LDL (sdLDL), oxidized LDL (oxLDL), triglycerides, high-density lipoprotein cholesterol (HDL-C), and lipoprotein(a) [Lp(a)]. Collectively, mechanistic studies, epidemiological investigations, Mendelian randomization analyses, and randomized clinical trials demonstrate that these biomarkers contribute substantially to cardiovascular risk assessment and remain indispensable components of contemporary ASCVD prevention and management[2,21,31,35,143,272].

However, the accumulated evidence also reveals an important conceptual limitation of the traditional lipid-centric paradigm. Much of the historical literature has focused on identifying progressively better lipid biomarkers without clearly distinguishing their positions within the Integrative and Systems Medicine Framework for ASCVD. As a result, biomarkers, pathogenic mechanisms, and root causes have often been interpreted interchangeably despite representing fundamentally different biological concepts.

The evidence reviewed in this article supports the Integrative and Systems Medicine Framework for ASCVD. Most lipid biomarkers—including LDL-C, ApoB, LDL-P, non-HDL-C, triglycerides, and HDL-C—primarily quantify exposure to, or reflect the activity of, underlying biological processes. Small dense LDL and oxidized LDL participate more directly in disease progression and are therefore better classified as intermediate pathogenic mechanisms. Among currently recognized lipid biomarkers, Lp(a) possesses the strongest evidence for a direct inherited causal contribution to ASCVD, although its pathogenic effects appear highly dependent upon biological context and do not account for the majority of disease.

Randomized intervention trials further reinforce this interpretation. Statins, ezetimibe, and PCSK9 inhibitors consistently reduce cardiovascular events, confirming that ApoB-containing lipoproteins are clinically important therapeutic targets. At the same time, the failure of HDL-raising therapies, the heterogeneous results of triglyceride-lowering interventions, and the persistence of substantial residual cardiovascular risk despite intensive LDL-C reduction collectively demonstrate that successful treatment of one pathway does not fully explain the complex biology of ASCVD.

Accordingly, we propose that future investigations move beyond a purely lipid-centric framework toward a disease architecture approach that distinguishes root drivers, intermediate pathogenic mechanisms, biomarkers, and clinical manifestations. Such a framework recognizes that therapeutic efficacy does not necessarily establish root causation and encourages more precise interpretation of biological evidence.

Importantly, this proposal does not diminish the importance of lipid biomarkers or lipid-lowering therapy. On the contrary, LDL-C, ApoB, and related biomarkers remain among the most valuable tools available for cardiovascular risk assessment and treatment monitoring. Rather, our proposal seeks to place these biomarkers within their appropriate biological context.

In conclusion, the evidence reviewed here suggests that most lipid and lipoprotein-related biomarkers are more appropriately classified as biomarkers of exposure or intermediate pathogenic mechanisms than as universal root causes of ASCVD. This distinction provides a more coherent framework for interpreting decades of lipid research, explains several longstanding inconsistencies in the literature, and may help guide future investigations toward identifying the upstream biological processes that initiate and sustain atherosclerotic disease.