Tea polyphenols: absorption, bioavailability and potential toxicity.

Abstract Tea polyphenols (catechins) have various reported properties that may benefit health and lower risk of non-communicable disease. However, their bioavailability is low. There is rapid metabolism and elimination of most absorbed catechins, and they do not accumulate beyond nanomolar amounts in plasma, even with regular intake. There is extensive biotransformation of unabsorbed catechins by colonic microflora, which produces ring scission products that, if included, increase bioavailability from an average of <4% to around 40%. Strategies exist to increase bioavailability, such as taking tea catechins in the fasting state, or taking more concentrated, purified, stable or bioaccessible formulations of catechins. However, catechins demonstrate pro-oxidant activity under certain conditions, and administration of high levels induces toxic effects on liver and kidney. In humans catechins are generally well tolerated at up to 400 mg/day, but even this dose can cause gastrointestinal upset and in some people induces hepatic toxicity. Furthermore, catechins bind iron and high doses can interfere with iron absorption, and catechins ingestion affects pharmacokinetics and action of some drugs used to treat hypertension and cardiovascular disease. Therefore, strategies to overcome low bioavailability of catechins must be viewed with caution, and very high doses should be avoided. In this article, absorption, metabolism and bioavailability of green tea catechins, in particular epigallocatechin gallate, are reviewed, focusing mainly on human data. Issues of potential toxicity of catechins are examined, and a recommendation in regard to 'safe' intake is given.