I've recently become a vegetarian, so I'm eating a pretty soy-heavy diet. But I've heard too much soy can cause man boobs in a lot of guys. Any truth to that?
— Tom N., Colorado Springs, CO
Well, all men have breasts, but what you're referring to are BIG breasts as in "man titties," "Andro-Jugs" or "tasty man treats." The medical term for large breasts in a male is "gynecomastia" ("gyneco" = ladylike, "mastia" = boobies). There are a ton of causes for gynecomastia including obesity, genetics and hormones. In addition, certain medications like cimetidine (Tagamet, a medication for acid reflux) and marijuana (well, marijuana is considered "medicine" in some states . . . not where I'm from, dammit, but that's another issue) can also cause breast tissue growth. So where does soy fit in?
Soybeans are a rich source of a set of molecules called "isoflavones." These substances have weak estrogen (female hormone) activity and have been used for years as "natural" remedies for hot flashes in menopausal women. Since male transgender patients take estrogens to stimulate breast growth, it has been assumed that soy might have the same effect. This appears to be a myth, or at least is not supported in the medical literature.
Probably the greatest amount of research on the long-term effects of soy has to do with soy infant formulas. Some kids can't tolerate regular formula and get switched to formula derived from soybeans and they'll drink this stuff from infancy to one-year of age and beyond. Some kids are raised on soy milk instead of cow's milk their whole lives. A 2004 Italian study of kids raised on soy protein formulas showed no gynecomastia, no early puberty, no changes in their bones and no other signs of screwed up hormones.
I should point out that searching "soy and gynecomastia" in the National Institute of Health database only spews out three articles, but you'll be happy to know that the reproductive tracts of male macaques were not adversely affected by ingesting soy. There is one interesting case report of a 60-year-old man who developed big ol' hooters and presented to a gland clinic for an answer. His gynecomastia eventually went away when they discovered he was drinking 3/4 of a GALLON of soy milk A DAY and told him to cut it out. With all the millions of people who ingest soy daily (there are countries where soy is a staple of the "normal" diet) the fact that there is one case report of one guy whose breast tissue was sensitive to soy products is reassuring to me.
Dr. Steve is The Opie & Anthony Show's resident medical expert and the host of his own Sirius XM Radio program, Weird Medicine.