The soybean is rich in isoflavone phytoestrogens, which are ligands for estrogen receptors, but it is unknown if soy/phytoestrogens have similar procoagulant effects to estrogen. In this randomized double blind trial, 40 healthy postmenopausal women of age 50-75 yr received soy protein isolate (40 g soy protein, 118 mg isoflavones) (n = 19) or casein placebo (n = 21). Plasma markers of coagulation, fibrinolysis and endothelial dysfunction were measured at baseline and 3 months. The baseline characteristics of the two groups were similar. Compared with casein placebo, soy decreased triglycerides (P < 0.005) and LDL/HDL ratio (P < 0.001) and increased lipoprotein (a) (P < 0.05). Activity of coagulation factor VII (VIIc) decreased similarly in both groups (P < 0.005). Prothrombin fragments 1 + 2 (a marker of thrombin generation) decreased in the soy group (P < 0.005), but the change was not different to the casein group. There was no effect of soy on soluble fibrin (a marker of fibrin production), plasminogen activator inhibitor-1 (a marker of fibrinolytic inhibitory potential), D-dimer (a marker of fibrin turnover), or von Willebrand factor (a marker of endothelial damage).

In conclusion, the results of the current study do not support biologically significant estrogenic effects of soy/phytoestrogens on coagulation, fibrinolysis or endothelial function.