Soy is either the answer to our prayers or a menace that’s sure to kill us all, depending on the scientific study du jour. In 1999, the Food and Drug Administration gave manufacturers of foods containing soy permission to label those products “heart healthy,” because some evidence showed that eating a soy-rich diet can help lower cholesterol and fight heart disease. That’s good, right?
Not so fast. Now that soy shows up in almost everything—meal-replacement bars, milk, yogurt, tempeh, tofu, milk, and seemingly every other product under the sun—and the government (through public-health campaigns and subsidies soybean farmers receive) has been encouraging Americans to eat more of it for years, we’re having second thoughts about our infatuation with soy. Is it a superfood, or does it cause our brains to age more rapidly? Is it a great way to ease the symptoms of menopause, or does it increase women’s chances of developing breast cancer? The most current soy battle pertains to its effects on men and boys, the latest allegation being that soy can raise estrogen levels in men, causing them to be “feminized,” become impotent, or even develop breasts. Does consuming soy really interfere with men’s hormones?
The Straight Talk
Soy contains two natural compounds, genistein and daidzein, also known as phytoestrogens. Basically, they’re the plant equivalent of human estrogen, and the manner in which they act on the body is extremely similar to that of the real thing. Many men’s health advocates are incredibly concerned that consuming soy in large quantities—especially in childhood—artificially inflates the levels of estrogen in a man’s body, thereby depressing testosterone. In theory, this can have all sorts of consequences, from low sex drive and lack of muscle tone all the way up to development of female-like breasts (a condition called gynecomastia).
Some studies have shown that when men consume large quantities of soy products, there are hormonal shifts. In one such study, at the University of North Carolina, men who consumed three hundred to six hundred milligrams of genistein per day (60 and 120 times the average amount, respectively) developed nipple discharge, tender breasts, and lower testosterone levels. In another study, conducted at Harvard and published in a 2008 edition of the journal Human Reproduction, high soy intake correlated strongly with low sperm count. The test subjects with the highest average soy consumption averaged 32 percent fewer sperm than men who avoided soy. Other studies on rats have shown that soy consumption can be linked with lower testosterone levels and sexual dysfunction.
However, while these studies established some correlation, they did not definitively establish causation. In fact, other studies have reached exactly opposite findings. One 2001 study at the University of Pennsylvania surveyed adults who were fed soy-based formula as babies, and compared them with adults who were fed cow’s-milk formula. The soy-fed adults had no significant hormonal, sexual, or fertility problems. Another survey of soy-consuming men, published in the March 2002 Journal of Nutrition, found that none of the subjects had the kind of hormonal changes or problems with semen quality that some men worry about.
Experiments showing that phytoestrogens wreak havoc on men’s hormones tend to use extremely large doses of soy—much larger than the average man would eat in a single day. Men in Asian countries where a moderate amount of soy is a common part of the national diet report no hormonal interferences or fertility problems; the Asian diet’s association with low incidence of heart disease and cancer was one of the first indications that soy could be beneficial in the first place. Also, many of the terrifying stories cited as “proof” of soy’s dangers involve men who were consuming much more soy than average—up to three quarts of soy milk per day, in one instance.
Technically, the jury’s still out on this one, but most doctors still recommend eating moderate amounts of soy as part of a healthy diet. Any man who has existing hormonal imbalances or erectile dysfunction, who is at risk for prostate or breast cancer, or who is monitoring his fertility should check with his own doctor, of course, but eating the occasional tofu burger or protein shake shouldn’t make a healthy male start lactating spontaneously or watching Gilmore Girls.
Say What? is a series created to support or debunk common health myths. If you have a question, please send it to the editor at firstname.lastname@example.org.