Staunch the Age Related Decline into Depression, Dementia, Diabetes, Obesity, Cancer, Long Covid, and Other Diseases with a Prebiotic, Probiotic, Postbiotic Triple Play

1. Oxidative Stress and Gut Dysbiosis

Aging reflects the accumulated damage over a lifetime wrought by oxidative stress. This stress arises when energy needs increase and reactive oxygen species (ROS) generated within mitochondria remain unquenched due to insufficient onboard antioxidants. Psychological stress induces oxidative stress by increasing circulating cortisol and norepinephrine, which generate mitochondrial ROS [1]. Mental stress is also linked with gut dysbiosis [2], which upregulates oxidative stress [3]. Excess ROS compromise mitochondrial efficiency and gut microbial diversity. Gut microbes themselves impact ROS generation.

Gut dysbiosis occurs when the gut microbiome is unbalanced, i.e., gut microbiota are not diverse and SCFA producing bacteria are in short supply. SCFAs are the end products of fermentation of dietary fibers by anaerobic intestinal bacteria and exert multiple beneficial effects on energy metabolism [4]

They are the primary energy substrate for colonic epithelial cells. Propionate and butyrate comprise 25% and 15% respectively of these SCFAs [5]. Acetate, which comprises ~60%, promotes obesity by stimulating insulin secretion and hyperphagia [6,7]. Propionate and butyrate stimulate secretion of GLP-1, which suppresses appetite and insulin secretion [8,9].

On the other hand, oxidative stress enhances acetate dependent lipogenesis, i.e., promotes obesity [10]. Persistent low grade oxidative stress is tightly linked to excitatory glutamate neurotransmission [11]. Glutamate producing gut bacteria outperform their GABA producing counterparts and create an imbalance in excitatory and inhibitory neurotransmission in the ANS [12]. The aging process and low grade chronic inflammation (increased ROS) are linked with upregulation of kynurenine and a shift in tryptophan metabolism from serotonin synthesis (decreased serotonergic inhibitory neurotransmission) to the kynurenine pathway [13], increasing KTR.

2. Altered Tryptophan Metabolism (ATM) and KTR

Tryptophan, an essential amino acid, from diet or synthesized by intestinal bacteria can follow one of three major metabolic pathways: 1) intestinal bacterial indole synthesis, 2) the kynurenine pathway in immune and epithelial cells, or 3) the serotonin pathway (90% of total body serotonin) in enteroendocrine aka enterochromaffin cells and initiation of vagal afferent signals [14]

During ATM tryptophan pivots away from the serotonin pathway and synthesis of serotonin and melatonin to the kynurenine pathway (see Figure 2). Inhibitory parasympathetic signals are suppressed due to the increase in excitatory glutamate activity. This pivot down-regulates bacterial indole synthesis with loss of indole induced glucagon-like peptide 1 (GLP-1). Benefits of GLP-1 include appetite suppression, stimulation of insulin [15], and decrease in fasting blood sugar [16], curbing obesity and T2DM [17]. Many of the same bacteria that produce SCFAs, e.g., Bifidobacteria and Lactobacilli, also synthesize indoles from tryptophan [18]. Although the end product NAD+ (see Figure 2) assists dysfunctional mitochondria in ATP production, what drives the ATM pivot is not exactly clear. However, IFN-γ, upregulated in females, is a cofactor for many enzymes in the kynurenine pathway (see Figure 2) and may drive the pivot [19] (see Figure 2). Tryptophan depletion lowers HRV (and increases KTR) [20]. Increased tryptophan intake (eggs) increases HRV, which appears to be driven by the subsequent increase in serotonin [21].

KTR, an indicator of rate-limiting IDO/TDO activity, is positively correlated with cardiovascular disease mortality [22,23], depression, bipolar disorder, schizoprenia, [24]

Alzheimer’s disease, fronto-temporal dementia, [25]

Parkinson’s disease [26], and neurological disease in general [27].

Increased KTR has also been reported in cancer [28], autoimmune disease, including RA [29], and SLE [30]. Infectious diseases are also linked to an elevated KTR [31] with a ratio that reflects severity [32,33]. This includes SARS CoV2 [34]. SARS CoV2 induced loss of ACE2 receptor bearing intestinal epithelial cells decreases absorption of the essential amino acid tryptophan [35] with additional negative influence on KTR and prognosis.

But there is another non enzymatic pathway from tryptophan that involves TGF-β.

3. IFN-γ and TGF-β

IFN-γ and TGF-β are polarizing cytokines (reciprocal relationship) [38] and counterbalance each other [39]. IFN-γ is pro-inflammatory and TGF-β is anti-inflammatory. When an imbalance arises, autoimmune disease/IFN-γ and cancer/TGF-β, two immunological opposites [40], can develop. These counterbalancing cytokines are in turn immuno-modulated by the gut microbiome. This is demonstrated by the utility of fecal microbiota transplant in both cancer and autoimmune disease.

Elevated IFN-γ characterizes parasitic infestations. In such patients this cytokine was positively associated with a good prognosis in Covid-19 [41]. Low baseline IFN-γ response could predict hospitalization [42] and post discharge fibrosis in COVID-19 patients [41]. On the other hand its reciprocal, TGF-β, was positively associated with Covid-19 severity [43] and fibrosis [44]. Even outside the TME TGF-β promotes fibrosis, counterbalanced by IFN-γ. These cytokines are directly linked to the KTR and IDO. IDO, the enzyme, works to restrain excessive or inappropriate immune activation in the TME [45]

However, IDO is not only an enzyme induced by IFN-γ (increased KTR) but also an intracellular signal transducer induced by TGF-β (TME) [46,47,48]

Pleiotropism is the expression of different traits by the same gene. IFN-γ can pivot from pro-inflammatory and anti-proliferative to tumor promoter and TGF-β can pivot from tumor suppressor to tumor promoter. What triggers the pleiotropic switch from tumor suppressor to tumor promoter for either IFN-γ or TGF-β is not clear, but may be related to the TME milieu, where TGF-β appears to dominate [49]. In an imbalanced (elevated TGF-β/IFN-γ) TGF-β may trigger fibrosis and the TME via paracrine transmission.

IFN-γ is generally considered pro-inflammatory but anti-proliferative. But in the TME it can induce PD-1 expression linked to metastasis (see Figure 3) [50]. TGF-β is generally considered anti inflammatory and a tumor suppressor, but in the TME it becomes a tumor promoter, triggering cancer associated fibroblasts (CAF), vascular endothelial growth factor (VEGF), and epithelial/endothelial mesenchymal transformation (EMT), possibly mediated by methylation of its epigenome. The switch seems to occur in the TME. The relative concentrations of IFN-γ and TGF-β [51] or local hypoxia [52] may instigate this. Interestingly tumors treated with low-dose IFN-γ acquired metastatic properties while tumors infused with high dose IFN-γ regressed [51].

Perhaps TGF-β concentration in the TME can trigger a pleiotropic switch in low dose IFN-γ but at a higher dose IFN-γ can modulate its reciprocal in the TME. Cancer cells can also produce TGF-β. Pleiotropic IFN-γ is linked with metastatic behavior via upregulation of PD-1 [53]. Angiotensin II stimulates the TGF-β signaling pathway [54]. This may in part explain the predilection for and severity of Covid-19 in males with comorbidities and for recurrent cancer in those previously in remission (see Figure 3). On the other hand, females are robust producers of type I interferon [55]. Type 1 IFNs (IFN-α and IFN-β) are first responders to any invading pathogen and trigger release of interferon-stimulated genes for synthesis of IFN-γ.

Its reciprocal, TGF-β, is vital to the maintenance of tolerogenesis and avoidance of autoimmunity. If TGF-β/IFN-γ is low, self recognition and tolerance may be compromised (autoimmunity) [56]. If TGF-β/IFN-γ is high, tumor associated antigens may be tolerated (cancer) [57]. An increased TGF-β/IFN-γ is also a risk factor for tissue fibrosis [58,59,60]

4. HRV and Inflammatory Cytokines, Cancer, Disease, Obesity

The cytokine triad of TNF-α [61,62,63], IL-6 [61,64], and IL-1β [61] are negatively linked to HRV and positively linked to CRP [64]. Cancer diagnosis and prognosis are linked to TNF-α [65], IL-1β [66,67], and IL-6. Low HRV can alert one to asymptomatic infection and inflammation [68,69], anxiety [70], depression [71,72], cognition and neurodegenerative disorders [73,74], psychosis spectrum disorders [75], cancer [76,77], cardiovascular disease [78,79], stroke [80,81], T2DM [82,83], severe Covid-19 [84], Long Covid [85,86,87], SLE [63,89], and RA [90]. Central aka visceral adiposity (WHR, WC) is negatively related to HRV and a much more sensitive indicator than BMI [91,92,93]. Peripheral obesity is not only not associated with a low HRV but is protective with elevated HRV [94].

5. Butyrate, D-mannose, and Probiotics

A.

Butyrate

Butyrate enhances mitochondrial function during oxidative stress [95] and rescues tryptophan [96]. Serotonin cannot cross the BBB, but tryptophan can, and by rescuing tryptophan, butyrate can increase brain serotonin (inhibitory neurotransmitter). Butyrate also suppresses IDO activity [97] and immuno-modulates IFN-γ and TGF-β [98]. Many of the same intestinal bacteria that produce butyrate possess glutamate dehydrogenase and can produce GABA [99] and the most recent research suggests that GABA can not only activate GABA receptors locally but may also enter circulation and cross the blood brain barrier (BBB) [100,101] to activate predominantly inhibitory signals. Some GABAergic nuclei in circumventricular organs are parasympathetic centers and don’t have a BBB.

The quantity of butyrate producing bacteria has generally been considered the gold standard for evaluating the health of the gut microbiome and the gut-brain axis [102], but neurotransmission by GABA, produced by these same bifidobacteria and lactobacilli, seems unacknowledged [103,104]. Butyrate producing gut microbiota [105], gut biodiversity, and production of SCFAs [106] are associated with elevated HRV. Unfortunately Bacteroidetes species (and butyrate) decline with age [107]. Butyrate alleviates obesity and related comorbidities [108,109,110]. But not all SCFAs have beneficial effects on human health. Acetate not only promotes obesity [7,111] but can also be used by tumor cells as an energy substrate during oxidative stress [112]. Postbiotic butyrate bypasses the negative effects [6] of Bacteroides produced acetate [113].

• B. D-mannose

Prebiotic D-mannose improves the gut microbiome by increasing its biodiversity and butyrate producers, preventing gut dysbiosis [114,115]. D-mannose prevents diet induced obesity [116], positively associated with CRP and negatively associated with HRV [117]. Central adiposity is adverse and linked to elevated CRP, while peripheral adiposity is favorable and not so linked [118,119]. D-mannose not only downregulates gut dysbiosis by enhancing intestinal barrier integrity [114,115] but also suppresses the adipokine and cytokine triad (TNF-α, IL-6, IL-1β) [120,121,122], linked to cancer [123,124], cardiovascular disease [125], stroke [126], obesity [127], diabetes [128], neurodegenerative disease [129], and autoimmune disease [130,131].

D-mannose suppresses these autoimmune diseases like T1DM, asthma, and SLE [132] by suppressing IFN-γ [39,115]. D-mannose can suppress ERK (extracellular signal regulated kinase) signaling pathways (see Figure 3) [133] integral to TGF-β induced organ fibrosis [134], transformation of fibroblasts into CAFs [135], epithelial/endothelial mesenchymal transformation (EMT) [136], and VEGF synthesis [137]. D-mannose inhibits programmed cell death protein-1 (PD-1) (see Figure 3) [138], upregulated in Covid-19 [139]. This pathway to tumorigenesis is separate but complementary to that induced by TGF-β [140] (see Figure 3). Probiotics also increase HRV and have proven efficacious in LC [142,142]. They engage the prebiotic d-mannose and enhance the butyrate surge.

6. Other Considerations

There are other determinants of a low HRV, e.g., micronutrient deficiencies, especially vitamin D, which not only increases HRV [143] but also enhances gut biodiversity and growth of SCFA producers [144,145]. Imbalances, especially Ca:Mg [146], of many other micronutrient deficiencies, including B-12, C, E, magnesium, iron, zinc are associated with low HRV but the evidence is not definitive [147,148,149]. Vitamin B6 goes unnoticed, yet its active form (pyridoxal phosphate) is a required cofactor for conversion of glutamate to GABA and of tryptophan to serotonin and serotonin to melatonin. HRV reflects the state of our personal health and can be utilized to follow the efficacy of any changes in our lifestyle. KTR is negatively linked to HRV, but not yet commercially available [36,79]. CRP is also negatively correlated with HRV [146] and positively correlated with oxidative stress [149], smoking, heavy metal exposure [150], aromatic hydrocarbon exposure [151]. HRV is negatively linked to the western diet and sedentary lifestyle [152].