Polyphenolic bioactives as an emerging group of nutraceuticals for promotion of gut health: A review

Polyphenols, although not classified as nutrients, provide health benefits beyond the mere maintenance of normal health in humans and animals alike. Traditionally, much of the focus in polyphenol research has been their antioxidant property. Of late, polyphenols are receiving greater interest for their potential gut-health promoting effects. The gut microbiota modifies the ingested polyphenols to produce ‘microbial derivatives’ that are postulated to mediate beneficial effects in the host. Microbial modifications convert the large variety of polyphenols from the diet to fewer end-products. Polyphenol-derived intermediate- and end-products can affect the intestinal ecology as significant constituents are not fully absorbed and are metabolized in the liver and excreted back into the gut through the biliary secretions. Some of these accumulated metabolites may exert prebiotic-like effects on the gut microflora, leading to stimulation of bacterial growth, and in the process, promoting certain probiotic bacteria. The unabsorbed dietary phenolics and their metabolites exert antimicrobial or bacteriostatic activities by selectively inhibiting the growth of pathogenic bacteria, while at the same time stimulating the growth of commensal bacteria. This, in turn, leads to an altered microbiota composition in the gut lumen. Eventually, health benefits to the host come directly from the action of the polyphenolic compounds and metabolites on the tissue and organ systems, and indirectly through the positive impact of microbial fermentation. This review thus aims to discuss the benefits of polyphenols from varied sources with a focus on their gut metabolism and the ensuing health promoting effects.