According to Sol Orwell, co-founder of Examine.com, soy protein powders have a bad rap in some circles because of concerns over the estrogenic properties of soy isoflavones. The theory is that isoflavones can inhibit the activity of enzymes involved in testosterone production, resulting in lower testosterone and higher estrogen levels.
Unless your intake is incredibly high (think: vegan bodybuilder), you’re worrying for no reason. A randomized controlled trial in resistance-trained men reported that there were no differences in total or free testosterone, sex hormone-binding globulin (SHBG), estradiol or the estradiol-to-testosterone ratio between groups supplementing with 50 grams of protein from soy concentrate or whey protein concentrate. Notably, the soy concentrate was providing 138 milligrams of isoflavones per day, which is higher than the 99th percentile in Asian countries.
Further evidence comes from a meta-analysis of 15 randomized controlled trials, which found no significant effect on total testosterone, free testosterone or SHBG from soy. These studies used a variety of soy foods and protein powders that provided 10 to 70 grams of soy protein per day.
Soy doesn’t appear to impact fertility, either. A study in healthy young men reported no differences between groups on semen parameters after two months of supplementing with 30 grams of protein from either milk protein or soy protein. A separate study reported similar findings in young men supplementing with 40 milligrams of pure soy isoflavones per day for two months.
If you’re still worried about soy’s impact on your testosterone, find a protein powder made using an ethanol wash, which removes virtually all the isoflavones.