In vitro catabolism of quercetin by human fecal bacteria and the antioxidant capacity of its catabolites

Background: Part of quercetin flows into the colon after escaping the absorption of the small intestine and will be degraded by colonic microbiota. The catabolites in the colon partially determine the physiological activity of quercetin. Methods: Seven gut bacteria isolated from human feces were utilized to in vitro ferment quercetin. Their catabolites were analyzed with high-performance liquid chromatography and mass spectrometry, and the antioxidant activities of their fermented broths were compared with that of quercetin. Results: One metabolite, 3,4-dihydroxyphenylacetic acid, was produced by both C. perfringens and B. fragilis transforming quercetin. No other metabolites were detected in the other fermented broths. The antioxidant activities of all strains fermenting quercetin reached the highest values at the concentration of 1 mg/mL quercetin in broth; the fermented products of C. perfringens and B. fragilis presented stronger activities than those of other strains at most concentrations of quercetin in broth. Additionally, all of the fermented broths presented a decline of the antioxidant activities compared to quercetin. Therefore, the antioxidant activity of quercetin will be lost when it reaches the human colon because of the gut bacterial fermentation. Conclusions: This is the first study to report that quercetin can be degraded by C. perfringens and B. fragilis and transformed to the same metabolite, 3,4-dihydroxyphenylacetic acid, and that antioxidant activities decline when quercetin is fermented by seven gut bacteria.