A study from the University of Medicine and Dentistry of New Jersey set out to determine if a commonly used soy protein supplement exhibits biological activity. Researchers studied healthy male volunteers, aged 18 years or older that were in good health. They consumed two scoops of pure soy protein powder daily for 28 days. It turns out soy protein powder actually decreased testosterone levels, thanks to the estrogen. This same estrogen may also cause testicular shrinkage. The good news is, when study participants discontinued the soy protein powder, testosterone levels increased within two weeks.