Pharmacokinetics and bioavailability of quercetin glycosides in humans

Due to its potentially beneficial impact on human health, the polyphenol qu ercetin has come into the focus of medicinal interest. However, data on the bioavailability of quercetin after oral intake are scarce and contradictor y. Previous investigations indicate that the disposition of quercetin may d epend on the sugar moiety of the glycoside or the plant matrix. To determin e the influence of the sugar moiety or matrix on the absorption of querceti n, two isolated quercetin glycosides and two plant extracts were administer ed to 12 healthy volunteers in a four-way crossover study. Each subject rec eived an onion supplement or quercetin-4'-O-glucoside (both equivalent to 1 00 mg quercetin), as well as quercetin-3-O-rutinoside and buckwheat tea (bo th equivalent to 200 mg quercetin). Samples were analyzed by HPLC with a 12 -channel coulometric army detector. In human plasma, only quercetin glucuro nides, bur no free quercetin, could be detected. There was no significant d ifference in the bioavailability and pharmacokinetic parameters between the onion supplement and quercetin-4'-O-glucoside. Peak plasma concentrations were 2.3 +/- 1.5 mug(.)ml(-1) and 2.1 +/- 1.6 mug(.)mL(-1) (mean +/- SD) an d were reached after 0.7 +/- 0.2 hours and 0.7 +/- 0.3 hours. respectively. After administration of buckwheat tea and rutin, however, peak plasma leve ls were-despite the higher dose-only 0.6 +/- 0.7 mug(.)ml(-1) and 0.3 +/- 0 .3 mug ml(-1), respectively Peak concentrations were reached 4.3 +/- 1.8 ho urs after administration of buckwheat tea and 7.0 +/- 2.9 hours after inges tion of rutin. The terminal elimination half-life was about II hours for al l treatments. Thus, the disposition of quercetin in humans primarily depend s on the sugar moiety. To a minor extent, the plant matrix influences both the rate and extent of absorption in the case of buckwheat tea administrati on compared with the isolated compound. The site of absorption seems to be different for quercetin-4'-O-glucoside and quercetin-3-O-rutinoside. The si gnificance of specific carriers on the absorption of quercetin glycosides, as well as specific intestinal B-glucosidases, needs to be further evaluate d. (C) 2001 the American College of Clinical Pharmacology.