The debate over the value of soy as a bodybuilding protein supplement has raged for many years. The controversy derives from the natural "estrogenic" isoflavones in soy. Do they decrease testosterone levels and affect strength? Obviously, this would not be a good thing for bodybuilders or weight trainers, especially men, because testosterone helps build muscle. On the other hand, it could be of value to women approaching and beyond menopause as soy estrogens are thought to assist with symptoms of menopause -- although the evidence for this is scant.
A new study provides reassuring evidence that soy protein does not reduce testosterone levels when used as a bodybuilding protein supplement.
In the study in the Journal of the International Society of Sports Nutrition, researchers gave four groups of men 50 grams (1.7 ounces) of one of four different protein supplement powders each day for 12 weeks in conjunction with a resistance training program:
- Soy concentrate
- Soy isolate
- Soy isolate plus whey
- Whey only
Soy "concentrate" and "isolate" are different forms of processed soy protein and whey is an extracted milk protein.
The researchers found that both soy and whey supplementation resulted in muscle mass increases without negatively affecting serum testosterone levels. This study seems to provide further assurance that soy protein, at the quantity tested, does not affect normal hormone levels or the ability to build muscle in men.
Source: Kalman D, Feldman S, Martinez M, Krieger DR, Tallon MJ. Effect of protein source and resistance training on body composition and sex hormones. J Int Soc Sports Nutr. 2007 Jul 23;4(1):4.
I was reading on nurturenatural.com that the relative amounts of amino acids in different protein supplements is important. I remember hearing that soy protein has a lot more glutamine than other animal sources of protein. Testoterone aside, wouldn’t that make it a superior form of protein?
Michael, in a mixed diet, a little extra of one amino acid over another is not going to make much difference in the scheme of things. Getting all of the essential amino acids is important. Why not mix it up with some soy, some whey or casein supplements?
Protein should not be from only one sources. Get a variety of proteins to get maximum effect.