Catechin is metabolized by both the small intestine and liver of rats.

Flavan-3-ols are the most abundant flavonoids in the human diet, but little is known about their absorption and metabolism. In this study, the absorption and metabolism of the monomeric flavan-3-ol, catechin, was investigated after the in situ perfusion of the jejunum + ileum in rats. Five concentrations of catechin were studied, ranging from 1 to 100 micromol/L. The absorption of catechin was directly proportional to the concentration, and 35 +/- 2% of the perfused catechin was absorbed during the 30-min period. Effluent samples contained only native catechin, indicating that intestinal excretion of metabolites is not a mechanism of catechin elimination. Catechin was absorbed into intestinal cells and metabolized extensively because no native catechin could be detected in plasma from the mesenteric vein. Mesenteric plasma contained glucuronide conjugates of catechin and 3'-O-methyl catechin (3'OMC), indicating the intestinal origin of these conjugates. Additional methylation and sulfation occurred in the liver, and glucuronide + sulfate conjugates of 3'OMC were excreted extensively in bile. Circulating forms were mainly glucuronide conjugates of catechin and 3'OMC. The data further demonstrate the role of the rat small intestine in the glucuronidation and methylation of flavonoids as well as the role of the liver in sulfation, methylation and biliary excretion.