Bioavailability and metabolism of the flavonol quercetin in the pig

During the last years, much data pointing to putative health-promoting effects of dietary plant-derived flavonoids (stemming mainly from epidemiological and in vitro studies) have been published. Our knowledge, however, concerning the systemic availability of these substances after ingestion with food is only sketchy. In the present study, we have investigated the bioavailability of the flavonol quercetin after intravenous and oral application in pigs equipped with a permanent jugular catheter. Each animal received a single intravenous dose of quercetin (0.4 mg/kg body weight) and one week later 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 drawn at defined intervals over a total period of three days following the application 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%. Bioavailability was, however, considerably increased to 8.6+/-3.8% after additionally taking into account conjugated quercetin and further increased to 17.0+/-7.1% by including quercetin's metabolites. Our results further indicate, that the conjugation of orally administered quercetin with glucuronic and sulfuric acid appears to preferentially occur in the intestinal wall.