Soy Protein Powder
Soy protein is a mixed bag.
While research has shown it’s an all-around effective source of protein for building muscle, it’s also a source of ongoing controversy for men. 21 According to some research, regular intake of soy foods has feminizing effects in men due to estrogen-like molecules found in soybeans called isoflavones.
For instance, a study conducted by Harvard University researchers analyzed the semen of 99 men and compared it against their soy and isoflavone intake during the three previous months. 22 What they found is that both isoflavone and soy intake were associated with a reduction in sperm count. Men in the highest intake category of soy foods had, on average, 41 million sperm per milliliter fewer than men who did not eat soy foods. On the other hand, a study conducted by scientists from the University of Guelph had 32 men eat low or high levels of isoflavones from soy protein for 57 days and found that it didn’t affect semen quality. 23 Furthermore, literature reviews like those conducted by researchers from Loma Linda University and St. Catherine University suggest that neither soy food nor isoflavones alter male hormone levels. 24 What gives, then?
Well, there isn’t a simple answer just yet, but we do know that soy’s effects in the body can vary depending on the presence or absence of certain intestinal bacteria. These bacteria, which are present in 30 to 50 percent of people, metabolize an isoflavone in soy called daidzein into an estrogen-like hormone called equol. 25
In a study published in 2011, researchers at Peking University found that when men with the equol-producing bacteria ate high amounts of soy food for three days, their testosterone levels dropped while their estrogen levels rose. 26 These effects were not seen in women, regardless of equol production or lack thereof.
Related to this is a study conducted with women by scientists at Sungkyunkwan University, which found that in a high-estrogen environment, isoflavones suppressed estrogen production, and in a low-estrogen environment, they increased estrogen production. 27 Research has also shown that soy protein contains substances that inhibit the digestion of protein molecules and the absorption of other nutrients as well as several known allergens. 28
So, all things considered, I think you understand why I generally recommend for men to avoid soy if at all possible. There are just too many unknowns for my liking.
Matthews, Michael (2014-01-05). Bigger Leaner Stronger: The Simple Science of Building the Ultimate Male Body (The Build Muscle, Get Lean, and Stay Healthy Series Book 1) (p. 83). Oculus Publishers, Inc.. Kindle Edition
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