Soy protein neither lowers testosterone levels nor hinders lean body mass gain

The effectiveness of soy protein for bodybuilders has been repeatedly questioned because soy protein contains chemicals called isoflavones, which dock onto the body's estrogen receptors and can affect estrogen metabolism (female hormones) as well as androgen metabolism (male hormones). Many of the best-selling protein supplements contain soy protein - either alone or in combination with whey protein, casein or egg protein.
Numerous studies have investigated the effect of different protein sources on protein and hormone metabolism, but few have ever looked at the effect of training with weights. A study conducted by Miami Research Associates under the direction of Douglas Kalman concluded that soy protein supplements did not reduce blood testosterone levels or affect lean muscle mass gain in men (average age 30) participating in a 12-week weight training program.
Twenty men were given daily protein supplements (50 grams) in the form of a soy concentrate, soy isolate or a mixture of soy isolate and whey protein. Since this study included only five subjects, further research is needed before the relative value of soy as a protein source for bodybuilders can be assessed.
(Journal International Society Sports Nutrition, 4: 4 doi:10.1186/1550-2783-4-4, 2007, published online)