Soy does not contain any estrogens, but has antioxidant soy isoflavones (genistein and daidzen) which act like anti-estrogens on the alpha-estrogen receptor in both men and women for estrogen actions in stimulating the breasts. In men and women even after age 50, there are always estrogens coming from body fat in the blood stream. They are formed by an enzyme called aromatase in fat cells.
Overweight and obese women have higher estrogen levels from their excess body fat. This leads to “man boobs” (gynecomastia) in 40% of men over 40 in the U.S. However, isoflavones bind only 1/1000th as much as estradiol (the strongest estrogen) to the alpha estrogen receptor. So when it is present it actually blocks the binding of estrogen to this protein. If you put isoflavones in an experiment with zero estrogens around in a test tube or in a mouse with no ovaries that cannot make estrogen, it can act like an estrogen but this artificial condition never occurs in real life!! In a normal mouse soy inhibits experimental breast cancer.
There is also a good beta-estrogen receptor in bone, heart, and brain that binds isoflavones, estrogens, and other Phytonutrients called Phytoestrogens. In the brain, isoflavones reduce hot flashes in some studies and build bone in other studies. You will see conflicting studies in the scientific and popular press calling soy a “Phytoestrogen” confusing its good actions and implying bad ones – BUT now you know why Soy is a great protein for your Protein-Rich Shakes and that Soy is NOT an estrogen!!
Resources: Dr David Heber