Bioavailability and metabolism of the flavonol quercetin in the pig

During the last years, much data pointing to putative health-promoting effe cts of dietary plant-derived flavonoids (stemming mainly from epidemiologic al and in vitro studies) have been published. Our knowledge, however, conce rning the systemic availability of these substances after ingestion with fo od is only sketchy. In the present study, we have investigated the bioavail ability of the flavonol quercetin after intravenous and oral application in pigs equipped with a permanent jugular catheter. Each animal received a si ngle intravenous dose of quercetin (0.4 mg/kg body weight) and one week lat er an oral dose of 50 mg/kg. A single animal additionally received an oral dose of 500 mg/kg one week after the lower oral dose. Blood samples were dr awn at defined intervals over a total period of three days following the ap plication of quercetin. Analysis of quercetin and some of its metabolites ( isorhamnetin, tamarixetin, kaempferol) in plasma samples were performed by HPLC. The calculated apparent bioavailability of free, unchanged quercetin after intake of 50 mg quercetin/kg body weight was 0.54 +/- 0.19%. Bioavail ability was, however, considerably increased to 8.6 +/- 3.8% after addition ally taking into account conjugated quercetin and further increased to 17.0 +/- 7.1% by including quercetin's metabolites. Our results further indicat e, that the conjugation of orally administered quercetin with glucuronic an d sulfuric acid appears to preferentially occur in the intestinal wall. (C) 2000 Elsevier Science Inc.