CHAPTER 12: Non-extractable Polyphenols and the Gut Microbiome

Most polyphenols in the diet, both extractable and non-extractable, escape digestion in the human small intestine and enter the colon, where they are subject to catabolism by the complex microbiota. This results in the production of phenolic acids, which may then influence the function and health of colonic cells and/or be absorbed into the systemic blood, where they could be the main effectors of a high-polyphenol diet in reducing disease risk. There is increasing evidence of the greater health benefits of these phenolic acids in comparison to their parent polyphenols and they are more bioavailable. The role of the gut bacteria in promoting health has become more evident as we find out more about the microbiome and its associations with a range of chronic conditions from cardiovascular disease and cancer to autoimmune conditions and neurological disorders. These effects may be related to the release of phenolic compounds by the bacteria from plant-based foods and drinks. Interactions with dietary fibre may also be important. Polyphenols may also help shape the microbiome via antibacterial and prebiotic effects. There is considerable inter-individual variability in the colonic metabolism of polyphenols, which may be due to differences in their microbiome.