Objectives: Quercetin is one of the main dietary flavonoids and undergoes a substantial intestinal phase-II metabolism. Quercetin conjugates have been detected in plasma and in urine, but their presence in the small intestine has not been assessed. This study aimed to investigate the intestinal metabolism and metabolite excretion of quercetin by the human small intestinal wall after oral dosing.
Methods: Six healthy volunteers were given a capsule of 500 mg of quercetin with 240 ml of water. Duodenal fluids were collected using the intraluminal sampling technique for 4 h and analysed by LC-MS/MS.
Key findings: Phase-II metabolites of quercetin were detected and quantified in aspirated intestinal fluids. Metabolites appeared almost immediately after administration, indicating an intestinal metabolism and apical excretion into the lumen. Quercetin-3′-O-glucuronide was found to be the main intestinal metabolite. Our results could not conclude on the enterohepatic recycling of quercetin or its metabolites, although several individual profiles showed distinctive peaks.
Conclusions: This study highlights the intestinal metabolism and excretion of quercetin and its conjugates in humans and gives insights into the relevant concentrations which should be used to investigate potential food-drug interactions in vitro.
Keywords: clinical study; dietary supplements; intestinal efflux; phase-II metabolism; quercetin.
© 2018 Royal Pharmaceutical Society.