Background: There are a number of theories which place certain types of protein powder, notably whey protein, as being superior to soy protein powders with arguments made that soy can have an estrogenic effect leading to lowered testosterone levels. We went through the arguments on the impact of soy protein on testosterone already so we turn now to examining a new study which compared results with soy and whey proteins. The researchers in this study set this up to explore if the previous inverse correlation shown between soy and testosterone (2) would impact lean body mass gains.
Methods: For twelve weeks, twenty subjects supplemented their diets with one of four different protein sources – A whey blend, whey plus soy isolate, soy isolate, and soy concentrate. The subjects also performed a comprehensive weight training programme and measurements were taken for body composition, testosterone, estradiol, and sex hormone binding globulin (SHBG) at both the start and week 12.
Results: The use of protein supplements led to a significant increase in lean body mass independent of protein source used. No significant differences were observed for total and free testosterone, SHBG, body fat percentage, BMI, or body weight. The testosterone to estrogen ratio increased across all groups. In group analysis revealed the T/E ratio showed significant increases in the soy isolate/whey blend and estradiol was significantly lower in the whey blend group.
Conclusion: The researchers concluded that a 12 week program of supplementation with soy did not decrease serum testosterone or inhibit lean body mass changes.