Decreased serum estradiol concentration associated with high dietary intake of soy products in premenopausal Japanese women

We examined the cross-sectional relationship of soy product intake to serum concentrations of estradiol (E-2) and sex hormone-binding globulin (SHBG) in 50 healthy premenopausal Japanese women. Intakes of sou products and various nutrients were estimated through a semiquantitative food-frequency questionnaire. Blood samples were drawn from each subject on Days 11 and 22 of her menstrual cycle. The intake of soy products was inversely correlated with E-2 on Days 11 and 22 of her cycle after controlling for age, body mass index, cycle length, and intakes of total energy, fat, and crude fiber (r = -0.23, p = 0.08 and r = -0.32, p = 0.04, respectively). No significant correlation was observed between soy product intake and SHBG. There was a tendency for the intake of individual soy foods such as tofu and mise to be inversely correlated with E-2 On Days 11 and 22 of the menstrual cycle. Mise intake was inversely significantly correlated with SHBG on Day 22 of the cycle (r = -0.36, p = 0.02). If the observed association is causal, our results suggest that the consumption of soy products lowers the risk of developing breast cancer risk modifying estrogen metabolism.