Douglas Kalman, a research scientist who looks like a bodybuilder, says soy protein may not be as bad as we once thought. Writing for Muscular Development, Kalman cites a study presented at the 2005 Experimental Biology Conference (Rubin, S., FASEBJ, Apr., 2005 supplement) that gave male weightlifters two 25-gram servings of soy per day, an equal amount of whey protein, or a combination of soy and whey. The men weight trained three times per week using a program based on the American College of Sports Medicine guidelines.
After 12 weeks, all participants gained a significant amount of muscle with no differences between the groups. The men taking the soy protein supplement showed no negative effects on total testosterone, free testosterone, estradiol, sex hormone binding globulin, or the testosterone-to-estradiol ratio. In fact, in some participants, the T:E ratio improved, showing a favorable impact on testosterone levels. Another study by Canadian researchers also showed the same results (Dillingham, J Nutr, 2005;135:584-591).
For a long time I have cautioned men to stay away from soy protein until we know that it won't turn them into girlie-men with high voices. It appears that there is nothing to be concerned about based on these studies. However, I still would advise pre-menopausal women to limit intake of soy due to its link to breast cancer.
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