Decreased ovarian hormones during a soya diet: Implications for breast cancer prevention

Ovarian hormones are biomarkers for breast cancer risk. Soybean consumption may be responsible in part for lower levels of ovarian hormones and decrea sed rates of breast cancer in women in Asia compared with Western populatio ns. Soybeans contain a significant amount of the isoflavones daidzein and g enistein, which are weak estrogens. The purpose of this study was to determ ine whether soya feeding decreases circulating levels of ovarian hormones a nd gonadotropins. Ten healthy, regularly cycling women consumed a constant soya-containing diet on a metabolic unit, starting on day 2 of a menstrual cycle until day 2 of the next cycle. Blood and urine samples were obtained daily for one menstrual cycle before and during soy feeding. The diet was c alculated to maintain constant body weight, included 400 kilocalories from a 36-ounce portion of soymilk, and provided 113-207 mg/day (154.0 +/- 8.4 m g/day, mean +/- SE) of total isoflavones. For the group, the soya diet prov ided more carbohydrate and less protein than the home diets. Daily consumpt ion of the soya diet reduced circulating levels of 17 beta-estradiol by 25% (P < 0.01, Wilcoxon signed rank test, two-tailed) and of progesterone by 4 5% (P < 0.0001) compared with levels during the home diet period but had no effect on luteinizing hormone or follicle-stimulating hormone. Mean menstr ual cycle length did not change during the soya diet; a slight decrease in mean luteal cycle length was marginally statistically significant (P = 0.06 ). Urinary excretion of isoflavones was 33.8 +/- 5.3 mg/day (mean +/- SE) a nd when expressed as percentage of intake, varied substantially (21.9 +/- 3 .3% of intake; range, 9.1-36.7%) among the subjects. Mean daily serum level s of daidzein and genistein (free and conjugated forms) 15 h after soymilk were 2.89 +/- 0.53 mu g/ml and 0.85 +/- 0.22 mu g/ml, respectively, indicat ing systemic bioavailability of these substances. Secondary analyses by mul tiple regression showed that decreases in follicular and luteal phase 17 be ta-estradiol levels were positively associated with urinary isoflavone excr etion, an association affected by age, and were inversely associated with d ecreases in protein intake. Decreases in progesterome levels during the soy a diet were inversely associated with increases in intakes of genistein and were affected by the interaction of the intakes of daidzein with energy or with fiber. Consumption of an isoflavone-containing soya diet reduced leve ls of ovarian steroids in normal women over the entire menstrual cycle with out affecting gonadotropins. This suggests that at least under the conditio ns of this study, soya-induced reductions of circulating ovarian steroids a re not mediated by gonadotropins. Decreases in ovarian hormones are related to isoflavones contained in soy and also to energy intake and other compon ents such as protein and fiber but not fat. Our results may explain decreas ed ovarian hormone levels and decreased risk of breast cancer in population s consuming soya diets and have implications for reducing breast cancer ris k by dietary intervention.