Hi Abhimanyu,

Soy protein is a source of isoflavones, compounds known to have estrogen-like effects in the body (subsequently referred to as “phytoestrogens”). They are also implicated as the main culprits in soy protein’s testosterone-lowering claims.

Clinical and epidemiological evidence have backed soy protein’s cholesterol-reducing, total lipid-lowering, and cancer-preventative effects, but concerns still remain over its role as a muscle-building supplement, especially in men.

One study, for example, found that 56g of soy protein for 28 days significantly reduced testosterone levels in healthy male.