Dietary soy containing phytoestrogens does not have detectable estrogenic effects on hepatic protein synthesis in postmenopausal women

Background: Dietary phytoestrogens are ligands for the estrogen receptor and may mimic estrogenic effects in vivo. Objective: To assess the biological activity of isoflavone phytoestrogens, we analyzed the effect of dietary soy isoflavone supplementation on in vivo bioassays of estrogenicity. Design: Fifty healthy postmenopausal women aged 50-75 y participated in a double-blind, placebo-controlled trial in which they received either soy protein isolate (40 g soy protein, 118 mg isoflavones) or casein placebo. Measurements were made at baseline and at 3 mo. Urinary isoflavone excretion was measured to reflect compliance. The bioassays of estrogenicity included measurement of hepatic proteins and gonadotropin concentrations. Results: Baseline characteristics were not significantly different between the soy and placebo groups. Urinary isoflavone excretion increased in the soy group and at the end of 3 mo was higher in the soy group than in the placebo group. In plasma samples from both groups, C-reactive protein increased significantly over the 3-mo treatment period, whereas sex hormone-binding globulin and thyroid-binding globulin decreased significantly. However, there were no significant differences between the groups in hepatic protein synthesis (change over 3 mo +/- SEM in the soy and placebo groups, respectively): C-reactive protein, 0.42 +/- 0.2 and 0.48 +/- 0.2 U/mL; sex hormone-binding globulin, -6.9 +/- 1.5 and -10.0 +/- 2.1 mug/mL; thyroid-binding globulin, -16 +/- 8 and -26 +/- 7 nmol/L. Furthermore, gonadotropin and dehydroepiandrosterone sulfate concentrations did not change significantly in either group. Conclusions: In healthy postmenopausal women, dietary soy isoflavones do not affect in vivo biological indicators of estrogenicity, including hepatic protein synthesis and gonadotropin concentrations. 4 This suggests that soy isoflavones have little biologically relevant estrogenic effect in vivo in postmenopausal women.