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.