Dietary flavonoids have attracted much attention because of their potentially beneficial effects in preventing various diseases, especially coronary heart disease. Quercetin is a major flavonol-type flavonoid commonly found in vegetables and fruits, and in especially high amounts in onions. The bioavailability of quercetin in humans has been studied by many research groups (1). Quercetin metabolites have been detected in human plasma as the glucuronide and/ or sulfate with or without additional

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... methylation (2). One of the major quercetin metabolites, quercetin 3-O-β-D-glucuronide, has been shown to protect lowdensity lipoprotein (LDL) from copper ion-induced oxidative damage in vitro (3). However, it is not known whether quercetin metabolites present in human plasma following consumption of a flavonoid- PROCEEDING Abstract : Quercetin is a major dietary flavonoid found in onions and other vegetables. It is known that dietary quercetin is metabolized in the intestinal mucosa and the liver and is present as its glucuronide/sulfate conjugates with or without methylation. Although quercetin is known to possess strong antioxidant activity, there are only limited reports on the antioxidant activity of its metabolites. In this study, the antioxidant capacity of quercetin metabolites under physiological conditions was investigated. After consumption of cooked onion, more than 80% of quercetin metabolites were localized in the human plasma fraction containing concentrated serum albumin. Other lipoprotein fractions contained only small amounts of quercetin metabolites. Addition of quercetin 3-O-β-glucuronide to the lipoprotein-eliminated plasma fraction generated antioxidant activity against LDL oxidation in a dose-dependent manner. However, onion consumption failed to enhance the antioxidant activity of the lipoprotein-eliminated plasma fraction against LDL oxidation, probably because the amount of quercetin metabolites bound to albumin was less than the effective level in an ex vivo study. The physiological role of plasma albumin in retaining quercetin metabolites needs to be further clarified.