Quercetin is one of the most abundant flavonoids in the human diet. This study aimed to determine the plasma concentrations of quercetin in 10 healthy volunteers after the consumption of a complex meal rich in plant products. Quercetin was determined in plasma (2 h before, and 3, 7 and 20 h after the meal), and in a duplicated portion of the meal by HPLC analysis with an electrochemical detection. The amount of ingested quercetin was estimated to be 87 mg. Before the meal, quercetin concentration in hydrolyzed plasmas ranged from 28 to 142 nM. A marked increase was observed 3 h after the meal in all subjects, with a mean concentration of 373 nM (S.E.M. = 61). After 7 h, quercetin concentration in hydrolyzed plasmas decreased and after 20 h basal levels were found again. The antioxidant capacities of quercetin, 3P-O-methylquercetin, and of some of their conjugated derivatives were compared by the measurement of the conjugated dienes resulting from the Cu P+ -induced oxidation of human LDL. 3P-O-Methylquercetin and conjugated derivatives of quercetin significantly prolonged the lag phase, but the magnitude of their effect was about half that of the aglycone.z 1998 Federation of European Biochemical Societies.

Quercetin is a powerful antioxidant which is widely distributed in edible plants, mainly as glycosides such as rutin. It has been reported to be absorbed in mammals, but its metabolism needs further investigation to evaluate its possible physiological effects. We compared the evolution of the absorption of quercetin and rutin in rats fed with supplemented diets. Rutin was absorbed more slowly than quercetin because it must be hydrolysed by the cecal microflora, whereas quercetin was absorbed from the small intestine. Conjugated derivatives of quercetin, and its methylated forms isorhamnetin and tamarixetin, were recovered in plasma from rats receiving the two kinds of experimental diets after the first meal, but after 10 days, no traces of tamarixetin were detected anymore. The rate of elimination of quercetin metabolites seems very low, and high plasma concentrations are easily maintained with a regular supply of quercetin or rutin in the diet.

Naringenin, the predominant flavanone in grapefruit, mainly occurs as glycosides such as naringenin-7- rhamnoglucoside or naringenin-7-glucoside. This study compared kinetics of absorption of naringenin and its glycosides in rats either after a single flavanone-containing meal or after adaptation to a diet for 14 days. Regardless of the diet, circulating metabolites were glucurono- and sulfoconjugated derivatives of naringenin. The kinetics of absorption of naringenin and naringenin-7-glucoside were similar, whereas naringenin-7-rhamnoglucoside exhibited a delay in its intestinal absorption, resulting in decreased bioavailability. After naringenin-7-glucoside feeding, no glucoside was found in the cecum. However, after feeding naringenin-7-rhamnoglucoside, some naringenin-7-rhamnoglucoside accumulated in cecum before being hydrolyzed by intestinal microflora. Adaptation to flavanone diets did not induce accumulation of plasma naringenin. Moreover, flavanone cecal content markedly decreased after adaptation, and almost no naringenin-7-rhamnoglucoside was recovered after naringenin-7-rhamnoglucoside feeding, suggesting that an adaptation of cecal microflora had occurred. Overall, these data indicate that flavanones are efficiently absorbed after feeding to rats and that their bioavailability is related to their glycosidic moiety.

We studied the bioavailability and the plasma transport of flavonols in rats fed quercetin or rutin diets. Wistar rats were fed one of the following purified diets for 10 d: control; 16.4 or 8.2 mmol rutin/kg diet; or 16.4, 8.2 or 4.1 mmol quercetin/kg diet. Flavonol concentrations were determined in plasma, ileal and cecal contents, and feces. In rats fed diets containing 16.4 mmol quercetin or rutin/kg, the concentration of circulating flavonols was approximately 115 mumol/L. Quercetin or rutin administration resulted in similar concentrations of quercetin in cecal contents. By HPLC analysis and beta-glucuronidase/sulfatase treatment, plasma flavonols have been identified as conjugated quercetin itself, or a conjugated form (4.5-fold as abundant) of an aglycone less polar than quercetin. Rats fed quercetin or rutin diets had a green/yellow-colored plasma that exhibited a peak absorbance at 411 nm, vs. 363 or 375 nm for pure rutin or quercetin solutions, respectively. This shift of band I absorption was obtained when pure quercetin was in the presence of albumin or added to a plasma fraction. The bathochromic properties of flavonoids in the presence of albumin are highly dependent on the presence of the C-2/C-3 double bond on the C-ring and are influenced by the degree of B-ring hydroxylation. The existence of intermolecular bonds between albumin and quercetin is supported by in vitro absorbance and fluorescence studies. With human albumin, the fluorescence intensity and the shift of quercetin absorbance increased in parallel to the albumin/quercetin molar ratio. Conjugated diene formation, resulting from Cu(2+)-catalyzed oxidation of human LDL or rat VLDL+LDL was effectively inhibited in vitro by 0.5 mumol/L quercetin. These results show that dietary flavonols are recovered in rat plasma as conjugated metabolites in non-negligible concentrations, and that these flavonols may be interesting antioxidant micronutrients with a variety of biological effects.

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