Green Tea Flavan-3-ol Bioavailability

Understanding the real bioavailability of flavan-3-ols is a research topic which is gaining scientific attention in the light of the putative beneficial health effects demonstrated by many dietary sources of this class of polyphenols, green tea included. In general, after ingestion of dietary flavonoids, absorption of some but not all components into the circulatory system occurs in the upper intestinal tract, where phase II metabolism generates sulfate, glucuronide and/or methylated conjugates. Aglycones and metabolites may then reach the liver through portal circulation and undergo additional phase II metabolism, with further conjugations. The relevant fraction of flavan-3-ols not absorbed in the small intestine reach the large intestine where they are subjected to the action of colonic microbiota. Microbial metabolism of flavan-3-ols involves ring-fission enzymatic reactions leading to the production of valerolactones and phenolic acids. In humans, the urinary excretion of the sum of all native molecules and derived compounds may reach substantial amounts, in the order of 70% of the ingested dose.