The human distal gut harbours a variety of microbes (the microbiota) whose genomes complement our own set of genes. The human colonic microbiota is capable of relevant metabolic functions, including the extraction of energy from indigestible dietary polysaccharides, the production of putatively beneficial short chain fatty acids, which, in turn, are responsible of the modulation of lipid metabolism. The microbiota also modifies the endocrine function and modulates the inflammatory tone, and could therefore be considered an active factor able to influence the onset of obesity and other metabolic diseases.

A peculiar interaction with the human gut microbiota is that of dietary polyphenols, which are plant secondary metabolites present in relevant concentrations in many foods such as tea, cocoa, fruits and vegetables and have been associated with several beneficial health effects in epidemiological and intervention studies. The intact forms of complex dietary polyphenols all share a very scarce bioavailability and a large proportion of ingested polyphenols pass to the large intestine, where substantial structural modifications are mediated by the colonic microbiota. The resultant low-molecular-weight compounds may be absorbed and circulate in the body prior to excretion in urine in amounts that greatly exceed those of metabolites absorbed in the small intestine, whose concentration in plasma rarely exceeds the nmol/l levels. This lecture will highlight the complex colonic microbial metabolism undergone by dietary polyphenols and describe the most recent advancements in the study of polyphenol biological activity in the light of these newly investigated chemical modifications. Based on the most recent observations, polyphenols could be considered as a key factor in the health effects derived by the diet/gut microbes interaction.