Does Soy “Feminize” Men? Fact Vs. Myth

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The benefits and dangers of soy are hotly debated, but one of the more disturbing accusations is that soy promotes the development of breasts in men. Should guys avoid eating this little green bean, or are the rumors overblown?

Soy beans are rich in phytoestrogens, a subclass of a group of antioxidants known as isoflavones. The chemical structure of phytoestrogens is very similar to that of the human sex hormone estrogen (phytoestrogen literally means plant estrogen). The similarity of the structures allows phytoestrogens to sometimes mimic or inhibit the hormone in animals with estrogen receptors (i.e. mammals).

Because they can interact with estrogen receptors, phytoestrogens have the potential to play a role in any biological process that involves estrogen (here is my extensive review on the pros and cons of soy). This introduction of estrogen-like copycats has led some to speculate that soy might “feminize” men if consumed too frequently.

Indeed, studies have shown that diets with very high in phytoestrogens can reduce fertility in rodents, which is more than a little scary. Fortunately, soy consumption has not been shown to act as a potent regulator of sex hormones in the vast majority of human men or in certain primates.

However, there have been some exceptions. One study reported that a 19-year-old male developed decreased libido, erectile dysfunction, and low testosterone after consuming very large quantities of soy as part of a vegan diet. The researchers did not, however, report the presence of gynecomastia (a.k.a male breasts). It should also be noted that the patient had type 1 diabetes, indicating some level of metabolic dysfunction, and relied on soy as his primary protein source. The symptoms disappeared one year after discontinuing his vegan diet.

Another study reported that one 60-year-old man developed breasts, as well as sexual dysfunction, after consuming three quarts of soy milk daily for six months. Again, the symptoms faded after discontinuation of the soy intake.

Extreme case studies aside, soy intake appears to be safe at normal to moderately high levels, even those above the relatively high consumption level of Asian men. Problems only seem to arise when soy is consumed at levels 9 – 10 times the norm for several months at a time.

If you’re a man and trying to avoid even small amounts of something that may lead to breast development, that’s probably not the right approach. There are benefits of soy that may make regular consumption worthwhile, such as its role in protecting against prostate cancer. Remember, it’s the dose that makes the poison.

To summarize, in very high doses, soy may cause some reversible sexual disruption in men (and possibly breasts), but in normal-to-high ranges, soy consumption appear to be safe and may offer health benefits.

When it comes to the science of wellness, distinguishing the facts from the urban legends can be tough. That’s why we’ve enlisted Darya Pino – a scientist, foodie, and self-proclaimed geek girl. Check out the ZocDoc Blog every other week to see her bust the biggest myths in health.

Image: edamame, a Creative Commons Attribution (2.0) image from Wesley Chan’s photostream.

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