Bioavailability of [2-(14)C]quercetin-4'-glucoside in rats.

[2-(14)C]quercetin-4'-glucoside (4 mg/kg body weight) was fed by gavage to rats housed in metabolic cages, and over an ensuing 72 h period, radiolabeled products in body tissues, plasma, feces, and urine were monitored by high-performance liquid chromatography with online radioactivity and MS2 detection. One and 6 h after ingestion, while in the small intestine, the flavonol glucoside was converted to glucuronide and methylated and sulfated derivatives of quercetin, but only trace amounts of these metabolites were excreted in urine. On entering the cecum and the colon, the flavonol metabolites declined as they were converted to phenolic acids, principally 3-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid, by the colonic microflora. Feces contained mainly 3-hydroxyphenylacetic acid. Urine collected 0-12 and 0-24 h after ingestion contained radiolabeled hippuric acid and 3-hydroxyphenylacetic acid. 14C-Hippuric acid declined markedly in the 24-48 and 48-72 h urine samples, and there was a concomitant increase in labeled benzoic acid. There was minimal accumulation of radioactivity in plasma, despite a 69% recovery of label in urine over the 72 h period, and likewise, very little radioactivity was detected in body tissues out with the gastrointestinal tract. This is reflected in the fact that 72 h after ingestion 96% of the ingested radioactivity was recovered in feces, urine, and the cage washes, which comprise a mixture of urine and feces. The study reveals that as it passes through the gastrointestinal tract, almost all of the of [2-(14)C]quercetin-4'-glucoside is converted to phenolic acids, compounds not monitored in previous flavonol bioavailability studies with model animal systems, some of which have used exceedingly high doses of the aglycone quercetin (500 mg/kg body weight), which is not a normal dietary component.