Dietary flavonoids as antioxidants in vivo: conjugated metabolites of (-)-epicatechin and quercetin participate in antioxidative defense in blood plasma.

Flavonoids are present in mainly plant foods and have attracted much attention in relation to disease prevention. Their antioxidant activity at least partly accounts for their potential health effect, because oxidative stress leads to a variety of pathophysiological events. It is essential to know the bioavailability of flavonoids involving intestinal absorption, metabolic conversion and urinary excretion, in order to evaluate their in vivo antioxidant activity after intake. Here (-)-epicatechin and quercetin were selected as typical flavanol- and flavonol-flavonoids present in vegetables, fruits and tea. Our rat study suggests that their metabolic conversion begins in the intestinal mucosa where the activity of uridine-5'-diphosphoglucuronosyltransferase (UGT) is at its highest. Both flavonoids accumulated mostly as glucuronide and sulfate conjugates in blood plasma after oral administration. No intact quercetin was found in the circulation. However, on the oral administration of these flavonoids, the antioxidative ability of rat plasma was enhanced indicating that conjugated metabolites participate in the antioxidant defense in blood plasma. Therefore, the intake of vegetables, fruits and tea rich in flavonoids may help to prevent oxidative damages in the blood.