Quercetin is a dietary polyphenolic compound with potentially beneficial effects on health. Claims that
quercetin has
biological effects are based mainly on in vitro studies with
quercetin aglycone. However,
quercetin is rapidly metabolized, and we have little knowledge of its availability to tissues. To assess the long-term tissue distribution of
quercetin, 2 groups of rats were given a 0.1 or 1%
quercetin diet [approximately 50 or 500 mg/kg
body weight (wt)] for 11 wk. In addition, a 3-d study was done with pigs fed a diet containing 500 mg
quercetin/kg body wt. Tissue concentrations of
quercetin and
quercetin metabolites were analyzed with an optimized extraction method.
Quercetin and
quercetin metabolites were widely distributed in rat tissues, with the highest concentrations in lungs (3.98 and 15.3 nmol/g tissue for the 0.1 and 1%
quercetin diet, respectively) and the lowest in brain, white fat, and spleen. In the short-term pig study, liver (5.87 nmol/g tissue) and kidney (2.51 nmol/g tissue) contained high concentrations of
quercetin and
quercetin metabolites, whereas brain, heart, and spleen had low concentrations. These studies have for the first time identified target tissues of
quercetin, which may help to understand its mechanisms of action in vivo.