Effect of Bile Acids on Daidzin Metabolism to Equol during In Vitro Incubation of Human Feces

Equol (Eq), produced by intestinal bacteria, is a more active metabolite than its precursor, daidzein; however, its production varies greatly among individuals. Bile acids (BAs) influence the balance of intestinal microbiota and may affect Eq production. Therefore, this study aimed to investigate the effects of BAs on Eq production in humans. Twenty-four-hour urine and feces were collected simultaneously from 23 participants after a 3-d soy challenge. Fecal samples from three participants with confirmed Eq-production ability, as determined by urinary analysis and in vitro fecal incubation, were used for incubation with BAs. Furthermore, the effects of co-addition of substances with BA-binding ability were examined. Fecal incubation from the three Eq producers showed distinctive daidzin (D) metabolic patterns, and the inhibitory concentrations of BAs on Eq production differed among individual samples. These differences were thought to be due to variations in individual microbiota. However, in all samples, cholic, chenodeoxycholic, and deoxycholic acids (CA, CDCA, and DCA, respectively), the major human BAs, inhibited D metabolism to Eq in a concentration-dependent manner at physiological concentrations. CDCA and DCA had approximately 10 times stronger inhibitory effects than CA. Co-addition of cholestyramine attenuated the inhibitory effects of BAs; therefore, ingestion of substances with BA-binding ability was expected to improve Eq production. However, the addition of sodium alginate, which has BA-binding ability and may affect the microbiota, showed variable response among individual samples. These results strongly suggest that BAs secreted into the intestinal tract are significant factors affecting Eq production in humans.