Soy protein is derived from soybeans and represents an excellent source of high quality amino acids including twice the arginine and glutamine than milk-based proteins like whey and casein. Soy protein is a good alternative for vegetarians. In fact, it’s the highest quality plant-derived protein there is.
One reason some men avoid soy protein is because certain nutrients in soy called isoflavones (or phytoestrogens) have the potential to alter hormonal responses, including decreased testosterone and impaired muscle gains when training. Men do not want to reduce their testosterone levels. They already have to contend with several other factors that drive testosterone levels down such as age (starting in the late thirties testosterone drops about 1% per year), obesity (testosterone levels are on the order of 25% lower in obese compared to normal-weight men) and low fat diets.
A comprehensive analysis of 32 different studies examining the supposed feminizing or testosterone-lowering effects of soy protein was recently published. The studies involved the consumption of different soy products (soy protein isolate, soy milk, grits, soy flour, isoflavones) and measured the effect on testosterone levels in men.
The results showed soy protein or isoflavones did not exert a significant estrogenic effect on men and did not alter their testosterone or free testosterone levels. These findings should diminish fears about soy protein having an adverse effect on testosterone.