Dietary quercetin is recovered in rat plasma as conjugated derivatives of isorhamnetin and quercetin

Quercetin is present in noticeable amount in human diet and this polyphenolic molecule is supposed to exert beneficial effects on human health. However, its intestinal absorption and its metabolic fate in the organism have not received much attention. In the present study, rats were fed a control or a 0.25% quercetin diet, and the plasma, urine, and bile metabolites of the dietary quercetin were analyzed by HPLC. Conjugated derivatives of quercetin and isorhamnetin, a 3'-O-methylated form of quercetin, were identified in the plasma from the rats fed quercetin. After deconjugation, the concentration of aglycones in the plasma reached 120 +/- 16 mu mol/L, with an isorhamnetin/quercetin ratio of about 5. In bile and in urine, where the 4-oxo-flavonoid concentration were 378 +/- 42 and 128 +/- 19 mu mol/L respectively, conjugated derivatives of quercetin and isorhamnetin, but also of tamarixetin, a 4'-O-methylated form of quercetin were recovered. In plasma, the 4-oxo-flavonoid metabolites are bound to albumin, which induces a bathochromic effect. The bathochromic and chromogenic responses depend on the presence of the unsaturated C2-C3 bound of the C-ring and on the presence of hydroxyl groups on the B-ring. Studies on 4-oxo-flavonoid bioavailability could allow a better understanding of the nutritional effects of various type of plant products.