Ileal and faecal digestibility of daidzein and genistein and plasma bioavailability of these isoflavones and their bioactive metabolites in the ovariectomised rat.

Consumption of the soya isoflavones genistein and daidzein may provide protection against postmenopausal bone loss. The purpose of this study was to determine ileal and faecal digestibility of daidzein and genistein and the extent of formation of metabolites in the gastrointestinal (GI) tract in the ovariectomised rat, a model for postmenopausal bone loss. Twenty female rats were ovariectomised and fed either genistein or daidzein (0.026% of diet) for 4 wks. Genistein, daidzein and their GI-derived metabolites were quantitatively determined in plasma, urine, faeces and ileal digesta using GC/MS. Ileal and faecal digestibility of genistein (93 and 99.9%, respectively) were significantly greater than that of daidzein (32 and 77.5%, respectively). In genistein-supplemented animals, 4-ethylphenol was present in plasma in relatively high concentrations. The bioactivity of 4-ethylphenol may contribute to the physiological effects attributed to genistein consumption. The daidzein metabolite equol, was present in relatively high amounts in ileal digesta indicating substantial biotransformation of daidzein occurred in the small intestine presumably as a result of the activity of the resident microbiota. Further studies are required to determine whether 4-ethylphenol is a major metabolite of genistein in humans and the extent of biotransformation of daidzein to equol in the small intestine in humans.