1. Introduction
Cardiovascular diseases (CVD) are the leading cause of death and disability worldwide [
1]. CVD risk increases exponentially with age, largely due to pathophysiological changes in the vasculature, namely calcification, loss of elastin, increased collagen deposition, and increased vascular diameter [
2]. The major consequence of vascular aging is increased arterial stiffness, which decreases compliance and the ability of the vessel to adapt to stressors [
2]. The gold standard measurement of arterial stiffness is carotid-femoral pulse-wave velocity (cfPWV) which has been shown to be a strong predictor of future CVD events [
3]. Recent data suggests that the combination of the vessel structural property (i.e., carotid intima–media thickness (cIMT)) with the functional property (i.e., PWV) into a vascular aging index (VAI) may improve the prediction of CVD risk [
4]. Many mechanisms have been proposed to explain the vascular aging process including oxidative stress, chronic inflammation, and the cellular mechanisms driven by mammalian target of rapamycin signaling pathway [
5]. In addition, it is well known that hypertension leads to early vascular aging [
6], as well as certain lifestyle factors including psychological stress and diet [
7,
8,
9]. Given that the U.S. population over 65 years is expected to nearly double by 2060, and that age is a major risk factor for CVD, there is a need to elucidate the mechanisms that affect the vascular aging process [
10].
According to the 2018 National Health Interview Survey, males have a higher prevalence of heart disease, coronary heart disease, hypertension, and stroke (12.6%, 7.4%, 26.1%, and 3.1%) when compared to age-matched females (10.1%, 4.1%, 23.5%, and 2.6) [
1]. Sex hormones, mainly estrogens, have been used to explain such differences, given that the post-menopausal period is associated with increased CVD prevalence and heart failure development in females [
11]. One of the mechanisms proposed includes estrogen-mediated β-adrenergic vasodilation [
12]. The hypothesis that estrogen is cardioprotective is supported by the finding that pre-menopausal arteries have significantly less endothelial inflammation in the setting of enhanced vasodilatory capacities [
13]. This in turn prevents oxidative damage, which is a significant mediator of arterial stiffening and thus vascular aging [
14]. Any protective effects of estrogen pre-menopause appear to be eliminated post-menopause whereby post-menopausal females experience a rapid increase in blood pressure, calcification scores, and central arteriole stiffness [
14].
In addition to intrinsic phenotypic sex differences, it is now recognized that extrinsic or psychosocial factors also contribute to the sex differences in CVD risk and vascular aging [
14]. For example, females are at a two-fold greater risk of experiencing depression compared to males and depression is a known CVD risk factor [
15]. Adolescent females may experience more interpersonal stress [
16]. In addition, childcare responsibilities may impact the prevalence of depression and anxiety in females more than males [
17]. The effects of depression on vascular aging have been largely unexplored. Given the sex and gender differences in CVD risk factors, CVD outcomes, and vascular aging, it has been suggested that future studies should incorporate sex disaggregated data for traditional and non-traditional risk factors to help inform precision medicine [
18].
Diet constitutes a modifiable lifestyle factor with potential consequences for vascular aging. According to the AHA’s Life’s Essential 8, a diet rich in fruit and vegetables, complex carbohydrates, fish, beans, fiber, moderate in meat, and minimal in processed sugary foods can prevent endothelial dysfunction and arterial stiffness [
7,
19]. While these recommendations offer broad, healthy dietary patterns, there has been an impetus for studies focusing on specific micro- and macronutrients within those patterns for promoting healthy vascular aging [
20]. One relatively well-studied dietary component is sodium. Reducing sodium intake has been shown to consistently lower arterial stiffness in randomized controlled and crossover trials with normotensive and hypertensive middle-to-older aged males and females [
21]. Limitations of these clinical trials include the small sample sizes and limited racial and ethnic diversity [
22]. Moreover, a recent trial in individuals with heart failure (HF) showed that reducing dietary sodium did not improve clinical events compared to those HF patients who did not reduce dietary sodium [
23]. Indeed, additional epidemiological data is needed exploring the associations between dietary sodium and vascular aging in the context of covariates important for CVD development: sex, race and ethnicity, hypertension status, and other dietary culprits, such as high fructose corn syrup (HFCS). Given that dietary potassium may also play a vital role in modulating CVD risk, exploring the effects of potassium intake on vascular aging is also warranted [
24,
25].
Dietary sugar is a disaccharide of glucose and fructose, but an important distinction exists in the fructose component in North American countries compared to some of the European countries (i.e., Scandinavia). Where sugar-sweetened processed foods contain HFCS, the fructose component is typically in excess to glucose by about 50% [
26,
27]. In fact, since the advent of HFCS in the 1970′s, this has been the primary component of sugar used by the food industry in North America [
28]. Indeed, pre-clinical work showed that even short-term moderate increase in dietary fructose causes salt-sensitive blood pressure, diastolic dysfunction and increased aortic PWV in rats [
29,
30,
31,
32]. Additionally, other rodent models showed that increased ingestion of fructose and salt during early age (i.e., equivalent to adolescence in humans), contributes to hypertension, vascular stiffness and decrease in glomerular filtration rate (GFR) later in life even after the exposure to fructose and salt had been removed and later re-introduced to rats once they were older (i.e., equivalent to mid-life in humans) [
33,
34]. Likewise, in a systematic review and meta-analysis of six human cohort studies, sugar-sweetened beverages were significantly associated with an increased risk of hypertension, although vascular aging variables were not measured [
35].
In the present investigation we included the participants from the CARDIA (Coronary Artery Risk Development in Young Adults) study. To our knowledge, few studies have explored the role of diet on vascular aging within the CARDIA cohort. Gao et al. [
36] found that among CARDIA participants, carbohydrate intake was negatively correlated with the risk of coronary artery calcium progression, a marker of atherosclerosis. Duffey et al. [
37] showed that sugar-sweetened beverage consumption was positively associated with waist circumference, triglycerides, LDL cholesterol, and hypertension. The objective of the present retrospective observational study was to assess how dietary behaviors, specifically fructose and sodium, and endured psychological stress in young adult males and females impact the VAI and CVD risk by mid-life. We found that baseline psychological stress was positively associated with vascular aging in females. While in males, sodium intake positively predicted vascular aging and potassium intake inversely predicted vascular aging. Fructose consumption was a significant predictor of CVD risk while having high blood pressure at baseline was a significant predictor of stroke risk.
4. Discussion
We designed this retrospective observational study to assess how dietary behaviors and psychological stress in young adult White and Black males and females impact vascular aging and CVD risk by mid-life. We found the predictors of VAI at mid-life to be sex-specific whereby dietary sodium, potassium and aerobic activity were significant predictors in males, while depression scores during young adulthood were a significant predictor in females. Moreover, fructose consumption in mid-life was found to be a significant predictor of CVD 15 years later. Likewise, BP > 130/80 mmHg during young adulthood, irrespective of hypertension diagnosis, was found to be associated with an outcome of stroke 35 years later. Lastly, BMI measured during young adulthood was a significant predictor of CVD, stroke, and death.
Our finding that the predictors of vascular aging at mid-life are sex-specific supports the importance of stratifying data by sex, especially data that includes pre-menopausal females. VAI was found to be higher in males compared to females (15.00 ± 2.74 vs. 14.39 ± 2.33, p <0.001, respectively;
Table 1). We found that the fructose and sodium intake when the CARDIA cohort was in their adolescent age were not significantly correlated with VAI at mid-life, but that psychological stress was a significant predictor of VAI in females only. Given that depression scores did not differ between males and females (
Table 1), this reflects a direct, persistent effect on vascular aging in females only. Similar to other risk factors of vascular aging, psychological stress causes endothelial dysfunction and vascular inflammation [
9]. It is of note that significant associations between VAI and depression scores at year 20 were not observed in our analysis, suggesting that chronic psychological stress, rather than acute, has a negative impact on vascular aging in females. It will be important to investigate the persistent associations between psychological stress and VAI in this aging cohort, especially given the transition to menopause that is occurring concurrently at the time of this writing. When considering acute predictors of VAI, we found that in males only, dietary sodium was directly correlated with VAI (B-weight = 0.145, p = 0.003), while dietary potassium appears to be protective (B-weight = -0.160, p < 0.001). Additionally, having at least 1 hour of aerobic activity (run, bike or racquet sport) was also negatively correlated with VAI (B-weight = - 0.085, p = 0.007) in males. This suggests that in mid-life age, decreasing dietary sodium, while increasing dietary potassium and aerobic activity for males appears to confer protection against vascular aging. It is tempting to speculate that perhaps at least one of the reasons why we did not observe such associations between sodium, potassium and aerobic activity with VAI in females is because their contributions are overpowered by estrogen-dependent β-adrenergic mediated vasodilation [
44].
In our multivariable Cox regression analyses, only fructose consumption at year 20 and BMI at baseline predicted CVD (primary outcome). Even after we adjusted for potassium intake (Model 2) and consistent aerobic activity (Model 3), these associations still persisted (
Table 3). A number of epidemiological studies and subsequent meta-analyses have shown that sugar sweetened beverages increase the risk of hypertension and CVD morbidity and mortality [
35,
45]. However, few studies have yet to explore the relative impact of various types of sugars on CVD risk. To our knowledge, this is the first study to demonstrate in a longitudinal cohort that fructose, not sucrose, predicts CVD risk. Here we corroborated the preclinical data which demonstrated that dietary fructose contributes to vascular stiffness both in adolescence and adulthood [
29,
30,
33]. In addition to vascular changes, high fructose consumption has been shown to reduce plasma insulin and leptin levels and increase ghrelin concentrations [
46], which may contribute to obesity and thus a pro-inflammatory state. While our findings suggest that fructose is an independent predictor of CVD, it may also contribute to obesity, which is agreeable with our finding that BMI is another significant predictor of CVD morbidity in the CARDIA cohort. In our analysis for the secondary outcome, stroke, we found that both BMI and having BP > 130/80 mmHg, in adolescence were significant predictors in all three models. We chose to assess actual measurements of BP, independent of hypertension diagnosis, because we believe that this is a more clinically relevant scenario contributing to vascular aging. Persistent shear stress caused by elevated BP is known to cause local and functional changes to vascular compliance [
47]. This highlights the importance of BP control, and the elevated risk in scenarios when such is not achieved even with medical therapy. Here we show that BP > 130/80 mmHg in adolescence doubles the risk of having a stroke 35 years later. BMI in adolescence was the only variable in our study to predict all three outcomes: CVD, stroke, and death. The link between obesity and CVD has been shown to be multi-faceted and includes interactions between environment, socioeconomic status, genetics, physical activity behaviors, and internal factors [
48]. A prolonged state of energy imbalance (greater energy intake compared to expenditure) leads to changes in the functions of adipose tissue, including increased pro-inflammatory adipokine production. The pro-inflammatory state causes atherogenesis and increased endothelial vasomotor tone, both of which can contribute to CVD risk [
49].
A limitation of the present study is its retrospective observational nature. Furthermore, dietary practices are derived from Diet History Questionnaires, which are associated with inherent social desirability bias. Compared to other dietary history methods (i.e., 24-hour recall or food records), the diet history method used in the CARDIA study may overestimate absolute intake of nutrients, but this should not interfere with analyses that employ comparisons of means among different subgroups within the CARDIA study and only becomes a potential problem when comparing intakes to populations outside of the CARDIA cohort. Every effort was made to keep the diet history questionnaire at all visits consistent. Regarding outliers and missing values, the Nutrition Working Group during visit 1 decided to include all out-of- range values in the participants’ records and to not impute any missing data on serving amounts or frequency.
Author Contributions
Conceptualization and design, D.K..; data analysis, D.K., M.O., A.B., D.P., E.F., A.V.; formal analysis, D.K., M.O.; writing—original draft preparation, M.O., D.K.; writing—review and editing, M.O., A.B., D.P., E.F., A.V, D.K.; D.K.; supervision, D.K.; project administration. All authors have read and agreed to the published version of the manuscript.