The study, published in Journal of the American College of Nutrition, reviewed the effects of soy and whey protein supplementation on testosterone, sex hormone binding globulin (SHBG), and cortisol responses to an acute bout of resistance exercise like weight lifting.
The team behind the study noted that there is a common belief among fitness enthusiasts that soy-based products differ from animal-based protein products - with concerns over the potentially pro-estrogenic or anti-anabolic effects of soy protein.
"Although such fears may largely stem from bodybuilding mythologies, there is limited research on soy protein supplementation and its acute effects on hormonal responses to heavy resistance exercise," explained the authors - led by William Kraemer from the University of Connecticut, USA.
The team revealed that while 14 days of supplementation with soy protein does appear to partially blunt serum testosterone, it does not have pro-estrogenic or anti-anabolic effects. In addition, they reported that whey protein influences the response of cortisol (a steroid hormone) following an acute bout of resistance exercise by blunting its increase during recovery.
"Contrary to popular misconceptions, soy protein supplementation does not appear to hinder anabolic signaling postexercise by means of eliciting increases in estradiol concentrations," wrote the researchers.
“This study starts to unravel and provide some insights into one of the real urban myths about soy protein and resistance exercise in men,” said Kraemer.
The study used a sample of ten resistance-trained men in their early 20s. The participants where split between three protein supplementation groups: whey protein isolate, soy protein isolate, or a maltrodextrin placebo control.
For a period of 14 days participants consumed 20g of their assigned supplement at the same time each morning. No other supplements were allowed during the trial. After which the participants would perform six sets of heavy resistance squats at ten reps each using 80% of their maximum lifting weight.
Source: Journal of the American College of Nutrition
Published online ahead of print, doi:abs/10.1080/07315724.2013.770648
“The Effects of Soy and Whey Protein Supplementation on Acute Hormonal Reponses to Resistance Exercise in Men”
Authors: W.J. Kraemer, G. Solomon-Hill, B. M. Volk, B.R. Kupchak, D.P. Looney, C. Dunn-Lewis, B. A. Comstock, T. K. Szivak, D. R. Hooper, S.D. Flanagan, C. M. Mares, and J.S.