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.
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Testicular atrophy is one such issue, although many of the studies suggesting it does cause shrinkage are relatively outdated — from the 1980s. Nevertheless, the majority of it suggests that smoking weed lowers testosterone levels, either directly or through precursor hormones; or causes testicular atrophy in monkeys and dogs. A study from 2012 revives this idea, and finds some evidence in men living in the Niger delta area of Nigeria. In that study, 75 men who smoked marijuana were also likely to have shrunken testicles, infertility, and even male breasts.