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

Dietary phytoestrogens are ligands for the estrogen receptor and may mimic estrogenic effects in vivo. To assess the biological activity of isoflavone phytoestrogens, we analyzed the effect of dietary soy isoflavone supplementation on in vivo bioassays of estrogenicity. 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. 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 micro g/mL; thyroid-binding globulin, -16 +/- 8 and -26 +/- 7 nmol/L. Furthermore, gonadotropin and dehydroepiandrosterone sulfate concentrations did not change significantly in either group. In healthy postmenopausal women, dietary soy isoflavones do not affect in vivo biological indicators of estrogenicity, including hepatic protein synthesis and gonadotropin concentrations. This suggests that soy isoflavones have little biologically relevant estrogenic effect in vivo in postmenopausal women.