The quality of a protein is judged based on the combination of amino acids that the protein has in its structure. Humans require a mix of essential amino acids that cannot be made in the body. Soy protein, egg white, and whey protein all contain the full complement of amino acids needed to maintain your body protein. Many plant foods such as corn, beans, and rice do not have all the amino acids needed by the body. However, vegetarians can combine rice and beans or corn and beans to improve the mix of amino acids to a level as good as beef or chicken.
Soy protein contains a phytonutrient called soy isoflavones which act as an antioxidant, a prebiotic for your gut bacteria and may benefit your heart, brain, and bones. You may have heard some people calle these isoflavones phytoestrogens. These do not act like estrogen in the breast where they have only 1/1000th the potency of estrogen and act as an anti-estrogen. There are two different types of estrogen receptor proteins. In the breast, there are alpha-estrogen receptors where as indicated the soy isoflavones are anti-estrogenic. There is a beta-estrogen receptor in the brain and bones where soy isoflavones act as a beneficial estrogen.
The experiments that showed estrogen-like actions of soy protein or isoflavones on breast cells or in animals were all carried out in the absence of estrogens. In mice with their ovaries removed there is no source of estrogen since they lack the enzyme in their fat tissues that makes estrogen. Therefore, soy looked like an estrogen in this animal experiment. In mice with ovaries, soy acts as an anti-estrogen.
In humans, there are always estrogens produced by the fat tissue. Obese women have more estrogen than lean women. In women over 50 years of age where 75% of all breast cancer occurs, body fat is the major source of estrogens. In men, the fat tissue accounts for 80% of the low levels of estrogen circulating in the blood with about 20% coming from the testes.