The gut is home to trillions of microbes collectively known as the gut microbiota. The growth of these microbes ultimately depends upon food entering via the host’s diet. Microbes directly metabolise food components and mucus lining the GI tract 1, generated metabolites are further available to other microbes and the host 2. In this way trophic chains underlie the symbiosis between microbes and the host. Not surprisingly then many food components have been shown to alter the balance and metabolic activities of the gut microbiota. Basic macronutrients (i.e. protein, carbohydrates and fats) can alter the balance and activity of specific bacterial communities 3–5. Poorly digestible food components such as fibre pass through the entire intestines directly feeding bacteria. Soluble fibres in particular (e.g. FOS/inulin, GOS, MOS, etc.) are well known for their prebiotic effects (i.e. their ability to preferentially promote the growth of beneficial bacteria).
However less well recognised is the ability of other poorly absorbed plant phytochemicals such as polyphenols to modulate the gut microbiota. Polyphenols are a group of chemicals containing multiple phenol (C6H5OH) structural units. Phenolic compounds are abundant in many plant food stuffs highly revered for their health-promoting effects such as berries, green tea, cocoa and nuts 6. Common phenolic compounds include flavanols, flavonoids and anthrocyanins (for a full list see Wiki). Most polyphenols are poorly digested and have low bioavailability in humans although some can be partially absorbed 6. As such substantial quantities of polyphenols pass into the colon and are metabolised by the gut microbiota which can generate smaller absorbable molecules 6–8. Several studies have begun to show that phenolic compounds exert prebiotic effects. Consumption of high-flavanol cocoa versus low-flavanol cocoa was reported to increase Lactobacilli and Bifidobacteria while decreasing Clostridia levels in humans 9. These microbial changes were paralleled by significant reductions in plasma triacylglycerol and C-reactive protein (CRP) concentrations 9. Blueberries (rich in anthocyanins) have been shown to possess prebiotic effects in vitro and in rats 10,11. In humans consumption of a wild blueberry powder drink was shown to boost Bifidobacteria levels 12. Consumption of red wine polyphenols increased the levels of several bacterial genera including Bifidobacteria which correlated changes to blood markers in humans 13. Selenium-rich green tea increased Lactobacilli and Bifidobacteria while decreasing Clostridia and Bacteroides levels in rats 14. Green tea consumption has also been reported to increase Bifidobacteria levels in humans 15. In an animal model of obesity administration of a polyphenol-rich extract of pomegranate peel increased Bifidobacteria levels and alleviated tissue inflammation and hypercholesterolaemia 16. In addition to the prebiotic effects reviewed above plant phenolic compounds have also been reported to preferentially inhibit important pathogens (e.g. .
Prebiotics have never (knowingly) been trialled in ME/CFS however it is notable that in a small study consumption of a high-cocoa/polyphenol-rich chocolate improved fatigue and mood symptoms in CFS patients 24. Beneficial effects were attributed to the polyphenol content in this chocolate although it is not clear what the levels of other important nutrients were (e.g. magnesium). Polyphenols in cocoa do have some bioavailability and so could mediate some direct effects such as antioxidant activity 6. However much of the polyphenol content in cocoa may pass into the colon 9. As described above cocoa exhibits robust prebiotic activity in humans which correlates with systemic blood markers 9. Indeed pre and probiotics favourably modulate many facets of host immune, metabolic and neurological function. The prebiotic effects of cocoa have been found to correlate with blood CRP levels 9; CRP is a marker of inflammation and is often slightly increased in ME/CFS 25–28. Hence it seems logical to suppose the beneficial effects of cocoa reported in CFS patients may involve modulation of the gut microbiota.
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