Premenopausal Equol Excretors Show Plasma Hormone Profiles Associated with Lowered Risk of Breast Cancer

Increased urinary excretion of equol, a metabolite of the isoflavone daidzein, has been associated with lowered risk of breast cancer (Ingram et al. 1997). This risk reduction has generally been presumed to be a consequence of increased isoflavone consumption. However, only 30-40% of the population excretes more than trace amounts of equol regardless of isoflavone intake. Accordingly, we hypothesized that the observed protective effect of equol may be due to hormonal differences between equol excretors and nonexcretors. To evaluate the effects of equol status per se, we compared plasma hormone and binding globulin concentrations between premenopausal equol excretors (n = 5) and nonexcretors (n = 9) consuming identical isoflavone doses (10, 64 and 128 mg/d) as components of soy protein isolates for 3.5 menstrual cycles each. P [is less than] 0.05 was considered significant. Urinary equol for excretors far exceeded that of nonexcretors, even at the lowest dose. At all doses, equol excretors generally had lower concentrations of estrone, estrone sulfate, testosterone, androstenedione, dehydroepiandrosterone, dehydroepiandrosterone sulfate and cortisol and higher concentrations of sex hormone--binding globulin and midluteal progesterone, a hormonal pattern generally consistent with lowered breast cancer risk. Thus, the association of lowered breast cancer risk with equol excretion may largely reflect the tendency of equol excretors to have more favorable hormonal profiles rather than merely reflecting increased isoflavone intake. Equol may be a marker for the presence of colonic bacterial enzyme activity that increases fecal steroid excretion. Alternatively, equol itself, even with very modest isoflavone intake, may exert beneficial effects on the regulation of endogenous hormones. [Supported by NIH grants CA-66016 and MO1-RR00400, and a gift from Protein Tech. International.]