Genistein and daidzein are the two principle isoflavones in soy protein and are structurally similar to 17b-estradiol. These molecules bind to the two types of estrogen receptors, ERa and ERb with differing affinities. While genistein and daidzein have very weak affinity for ERa, they bind more avidly to ERb. The ability to activate estrogen-mediated pathways may not be their only mechanism of action of the phytoestrogens. Genistein is also a tyrosine kinase inhibitor and has been shown to have this activity in in vitro studies.
Several studies support the isoflavones as having a role in preventing cardiovascular disease.
The effects on coronary artery atherosclerosis mirrored the effects on LDL cholesterol. In one group the protein was casein, and in four groups soy protein isolates with varying concentrations of isoflavones were used. Baseline plasma lipids were then measured and they were randomly assigned to one of the supplement groups for nine weeks. Importantly, there was also a significant trend toward lower LDL cholesterol concentrations with increasing isoflavone dose. These studies and others suggest that the isoflavones modulate plasma lipid concentrations and cardiovascular disease.