Bioavailabilities of tea polyphenols in humans and rodents

Consumption of tea (Camellia sinensis) has been suggested to prevent cancer and heart disease. Animal studies have shown that tea and tea constituents inhibit carcinogenesis of the skin, lung, oral cavity, oesophagus, stomach, liver, prostate and other organs. Studies with human cancer cell lines have demonstrated a number of potential cancer prevention mechanisms for tea polyphenols, including protection from or induction of oxidative stress, inhibition of enzymes (mitogen-activated protein (MAP) kinases, cyclin-dependent kinases and topoisomerase I) and inhibition of growth factor-related cell signalling (epidermal growth factor and others). Whereas some studies report effects of epigallocatechin-3-gallate (EGCG) at submicromolar levels, most experiments require concentrations of greater than 10 or 20 μM to demonstrate the effect. In humans, mice and rats, tea polyphenols undergo glucuronidation, sulphation, methylation and ring fission. Recent reports also suggest that EGCG and other catechins may be substrates for active efflux. The peak plasma concentrations of EGCG, epigallocatechin (EGC) and epicatechin (EC) following oral administration of green tea are 0.04-1 μM, 0.3-5 μM and 0.1-2.5 μM, respectively, in humans and rodents. The plasma levels of theaflavins are much lower (~2 nM). The present chapter reviews the literature concerning the biotransformation and bioavailability of tea polyphenols. Such a review should serve as the foundation for future experiments on the bioavailability of tea polyphenols, and as a guide in extrapolating mechanistic data from cell-line studies to animal or human studies.