Protein supplementation has become very popular among athletes and gym-goers. Whey protein, a derivative of milk, is the most popular choice. Soy protein is particularly popular among vegans and individuals intolerant to dairy. However, concerns have been raised regarding soy’s effects on testosterone and estrogen. A study from the University of Connecticut has provided clarity on the effects of whey versus soy protein supplementation on testosterone, estrogen and cortisol levels following resistance training.
Ten resistance-trained individuals supplemented 20 grams of whey protein, soy protein, or carbohydrate (control) for two weeks. Then, participants’ serum hormone levels were tested after performing a resistance workout. After two-week flush periods, the participants repeated the experiment using the alternative protein or carbohydrate source.
Researchers found that post-workout testosterone levels were significantly decreased when soy protein was supplemented compared to whey protein. Consuming whey protein produced the same level of testosterone as consuming the carbohydrate, indicating that soy protein has a direct negative impact on testosterone levels. This action is independent of estrogen, as estradiol (the most prevalent form of estrogen) was not altered by soy or whey protein supplementation.
Researchers also found that whey protein has beneficial effects on cortisol levels compared to soy protein. Cortisol is a hormone released during times of stress, including exercise. Cortisol increases during a workout, and then decreases as muscles are repaired. Whey protein supplementation yielded a significant post-workout decrease in cortisol levels relative to soy protein or carbohydrate supplementation.
On the whole, the study indicates that soy protein supplementation reduces testosterone, possibly blunting the anabolic response following a workout. Alternatively, whey protein supplementation does not affect the normal testosterone response and reduces cortisol levels, promoting a pro-anabolic state. These results suggest individuals interested in building muscle should consume whey rather than soy protein. Vegans might be advised to use alternatives to soy, such as hemp protein, although the effects of hemp protein on hormone levels have not yet been explored in resistance-trained individuals.