Figure 1.

Schematic representation of the journey of phenolic compounds through the human body. Phenolic compounds (i.e., polyphenols and tannins) ingested with food during our daily dietary consumption mostly consist of glycosides or polymerized higher molecular weight moieties, but also include monomeric forms. In the oral cavity and upper digestive tract, a small fraction of such compounds is metabolized by the resident microbiota, releasing small amounts of saccharides and aglycones. Whether the monomeric aglycones come from this first step of degradation or directly from ingested food, they undergo absorption through the intestinal epithelium. Most of the phenolic moieties—the fraction of which depends on their structure as well as the food matrix they were embedded into—reach the colon unchanged and come into contact with the trillions of commensal microorganisms. Here, deglycosylation reactions release saccharides from the sugar moieties, then dehydroxylation and phenolic ring-targeted metabolism break down the complex phenolic structures into simple and absorbable phenolic acids, among which protocatechuic acid, 3-(3,4-dihydroxyphenyl) acetic acid and 3-(3,4-dihydroxyphenyl) propionic acid. Such metabolic processes provide support for microbial growth, contributing to the production of complex microbial metabolites, still largely unknown. The phenolic end-products are absorbed through the epithelium and—as for the simple monomeric aglycones absorbed in the upper intestine—reach the liver via the portal circulation, where they undergo phase II metabolism reactions such as glucuronidation, sulfation and methylation. The resulting glucuronate, sulfate and methyl compounds are found in the bloodstream and are most likely responsible for the beneficial effects reported, including antitumoral, antioxidant, antibacterial and antiviral effects. Eventually, they can still be metabolized by the gut microbiota after intestinal excretion of the conjugated products. GI, gastrointestinal. Image created in BioRender.com.