Modest hormonal effects of soy isoflavones in postmenopausal women

Soy isoflavones have been hypothesized to exert hormonal effects in postmen opausal women. To test this hypothesis, we studied the effects of three soy powders containing different levels of isoflavones in 18 postmenopausal wo men. Isoflavones were consumed relative to body weight [control: 0.11 +/- 0 .01; low isoflavone (low-iso): 1.00 +/- 0.01; high isoflavone (high-iso): 2 .00 +/- 0.02 mg/kg/day] for 93 days each in a randomized crossover design. Blood was collected on day 1 of the study (baseline) and days 36-38, 64-66, and 92-94 of each diet period, for analysis of estrogens, androgens, gonad otropins, sex hormone binding globulin (SHBG), prolactin, insulin, cortisol , and thyroid hormones. Vaginal cytology specimens were obtained at baselin e and at the end of each diet period, and endometrial biopsies were perform ed at baseline and at the end of the high-iso diet period, to provide addit ional measures of estrogen action. Overall, compared with the control diet, the effects of the low-iso and high-iso diets were modest in degree. The h igh-iso diet resulted in a small but significant decrease in estrone-sulfat e (E-1-S), a trend toward lower estradiol(E-2) and estrone (E-1), and a sma ll but significant increase in SHBG. For the other hormones, the few signif icant changes noted were also small and probably not of physiological impor tance. There were no significant effects of the low-iso or high-iso diets o n vaginal cytology or endometrial biopsy results. These data suggest that e ffects of isoflavones on plasma hormones per se are not significant mechani sms by which soy consumption may exert estrogen-like effects in postmenopau sal women. These data also show that neither isoflavones nor soy exert clin ically important estrogenic effects on vaginal epithelium or endometrium.