Soy isoflavones exert modest hormonal effects in premenopausal women

Soy isoflavones are hypothesized to be responsible for changes in hormone a ction associated with reduced breast cancer risk. To test this hypothesis, we studied the effects of isoflavone consumption in 14 premenopausal women. Isoflavones were consumed in soy protein powders and provided relative to body weight (control diet, 10 +/- 1.1; low isoflavone diet, 64 +/- 9.2; hig h isoflavone diet, 128 +/- 16 mg/day) for three menstrual cycles plus 9 day s in a randomized cross-over design. During the last 6 weeks of each diet p eriod, plasma was collected every other day for analysis of estrogens, prog esterone, LH, and FSH. Diet effects were assessed during each of four disti nctly defined menstrual cycle phases. Plasma from the early follicular phas e was analyzed for androgens, cortisol, thyroid hormones, insulin, PRL, and sex hormone-binding globulin. The low isoflavone diet decreased LH (P = 0. 009) and FSH (P = 0.04) levels during the periovulatory phase. The high iso flavone diet decreased free T-3 (P = 0.02) and dehydroepiandrosterone sulfa te (P = 0.02) levels during the early follicular phase and estrone levels d uring the midfollicular phase (P = 0.02). No other significant changes were observed in hormone concentrations or in the length of the menstrual cycle , follicular phase, or luteal phase. Endometrial biopsies performed in the luteal phase of cycle 3 of each diet period revealed no effect of isoflavon e consumption on histological dating. These data suggest that effects on pl asma hormones and the menstrual cycle are not likely to be the primary mech anisms by which isoflavones may prevent cancer in premenopausal women.