Bioavailabilities of quercetin-3-glucoside and quercetin-4 '-glucoside do not differ in humans

The flavonoid quercetin is an antioxidant which occurs in foods mainly as g lycosides. The sugar moiety in quercetin glycosides affects their bioavaila bility in humans. Quercetin-3-rutinoside is an important form of quercetin in foods, but its bioavailability in humans is only 20% of that of querceti n-4'-glucoside. Quercetin-3-rutinoside can be transformed into quercetin-3- glucoside by splitting off a rhamnose molecule. We studied whether this 3-g lucoside has the same high bioavailability as the quercetin-4'-glucoside. T o that end we fed five healthy men and four healthy women (19-57 y) a singl e dose of 325 mu mol of pure quercetin-3-glucoside and a single dose of 331 mu mol of pure quercetin-4'-glucoside and followed the plasma quercetin co ncentrations. The bioavailability was the same for both quercetin glucoside s, The mean peak plasma concentration of quercetin was 5.0 +/- 1.0 mu mol/L (+/-SE) after subjects had ingested quercetin-3-glucoside and 4.5 +/- 0.7 mu mol/L after quercetin-4'-glucoside consumption. Peak concentration was r eached 37 +/- 12 min after ingestion of quercetin-3-glucoside and 27 +/- 5 min after quercetin-4'-glucoside, Half-life of elimination of quercetin fro m blood was 18.5 +/- 0.8 h after ingestion of quercetin-3-glucoside and 17. 7 +/- 0.9 h after quercetin-4'-glucoside. We conclude that quercetin glucos ides are rapidly absorbed in humans, irrespective of the position of the gl ucose moiety. Conversion of quercetin glycosides into glucosides is a promi sing strategy to enhance bioavailability of quercetin from foods.