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No, Eating Soy Isn't Going to Give You Manboobs

Vegan dude bros can rest easy.

Photo by Jamie Lee Curtis Taete

As a relatively new vegan, soy is a constant, low-level source of anxiety for me. It's a plant, and it's ubiquitous, and I'm hungry, but I get this I-think-I-read-it-somewhere feeling like a soy-free existence is better, or healthier for some reason (other than soy being a fairly common allergy). I recall a report from Greenpeace ten years ago which said soy farming depleted the Amazon, but earlier this year, Greenpeace wrote that the deforestation problem looks like it's been mostly resolved.


The more urgent and personal reason to avoid eating soy—if you're a man—is that it will supposedly give you gynecomastia, a.k.a. manboobs.

Wherever soy is discussed on bodybuilding forums, or on Reddit, users wanting to avoid or reduce unwanted breast tissue claim to avoid soy "like the plague." Articles in publications like Men's Health tell horror stories about man's men with tender, throbbing breasts and enlarged nipples, lives basically ruined by the hidden dark side of soy. And for my part, I don't want d-cups either.

So do I need to go soy-free?

No I don't, according to Dr. Richard J. Santen, a medical professor at the University of Virginia with a focus on hormones. He explained to me that there is no reason for any man who wants to eat a big pile of soybeans, or even eat a steady diet of soy-based foods, to avoid them over a fear of growing boobs. In his lifetime of hormone research, Santen explained that his male test subjects have never exhibited anything like a clear connection between soy and feminization. "I have only seen one man where I suspected soy induced gynecomastia, but we measured the levels of soy products in urine in that patient and it did not support a causative correlation," he told me.

A 2010 review of medical literature by Mark Messina, a researcher at the Loma Linda University School of Public Health, agrees with Santen. The compounds in soy that supposedly give men boobs are called isoflavones, which can also be found in chick peas, and peanuts. The cause of all this suspicion around soy isoflavones in the first place is that they're phytoestrogens, naturally-occuring plant compounds that can be processed a little bit like estrogen. Estrogen is, of course, the central hormone involved in transitioning from male-to-female, but there's no sign so far that trans women can save money on hormone therapy by just switching over to phytoestrogens found in soy.


And according to Messina's paper, in all the lab work that's ever been performed on isoflavones with regard to feminization, "isoflavones do not exert feminizing effects on men at intake levels equal to and even considerably higher than are typical for Asian males."

The remark about Asian diets is there because according to conventional wisdom, and a detailed report from 2003 by the journal Environmental Health Perspectives, consumers in certain Asian countries—Japan for instance—get way, way more of their protein from soy than Americans. Japanese people, for instance, ate 8.7 times as much per day. In that case, one might expect more Japanese men to have gynecomastia, but in my research, I spotted no claims one way or the other about rates of gynecomastia in Japan compared to the US. But for what it's worth, I did spot some reports from 2013 that some Japanese men might have a growing interest in wearing bras.

The medical literature does include one notable case study in which soy consumption is linked—more or less anecdotally—to manboobs. A 2008 paper tells the story of a 60-year-old male patient suffering from gynecomastia, along with reduced libido and erectile dysfunction. It turned out he'd been drinking three standard cartons of soy milk per day—which is about 1,200 calories. "After he discontinued drinking soy milk, his breast tenderness resolved and his estradiol concentration slowly returned to normal," the paper says.


Virginia Miller, an estrogen researcher at The Mayo Clinic told me all that soy milk was "excessive," given that the guy was rolling the dice with phytoestrogens, but she noted that "the amount of phytoestrogens in various soy products varies by process method," and that "eating tofu is probably OK."

"There are estrogen receptor disrupters in environmental compounds and plastics which are probably more of a concern," Miller pointed out.

Santen agreed that it is reasonable to suspect that binge-level doses of soy might give men boobs given what we know about it. But that might be the least of your worries if you're planning to become the Takeru Kobayashi of tofu dogs. "In rats, soy can cause estrogen-like effects of breast cancer growth, so these is a concern," Santen told me.

But overall he was sanguine about the soy in my diet, telling me I can probably eat tofu and edamame to any reasonable extent that I want without sweating it.

In summary, according to Santen, "I do not think men should have any concern about this as a practical matter."

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