The rap: “Soy protein powder strips your masculinity!”
The real story: “Some people assume that the plant estrogens in soy must be interfering with reproductive hormones in men,” says Christopher D’Adamo, an assistant professor at the University of Maryland School of Medicine in Baltimore. But the evidence to support that claim is scarce.
Testosterone. Fourteen trials have looked at men with a wide range of testosterone levels who consumed different amounts of soy protein or a placebo (typically a dairy food) every day for 25 days to four years.
“In all 14 studies, testosterone levels did not differ between the two groups,” says Jill Hamilton-Reeves, of the University of Kansas, who led a review of them.
Estrogen. Thenewcenturyman.com, a website that helps men “reclaim their masculinity,” gets right to the point: “Soy is creating man boobs.”
The scare may have started with a 2008 report in a medical journal about a 60-year-old Texas man who complained of sore, enlarged breasts and a decreased libido.
Tests revealed that his level of blood estrogens was up to eight times higher than the top of the normal range. The man said he was lactose intolerant and drank three quarts of soymilk a day. That would have given him a daily dose of 360 mg of isoflavones, about 10 times what the average man in Asia consumes.
After the man stopped drinking soymilk, his estrogens returned to normal and his breast tenderness disappeared. No other similar cases have ever been reported.
That’s not surprising. In nine studies, estrogen levels remained within the normal range whether men consumed soy foods, isoflavone supplements, or a placebo.
In the largest trial, Australian researchers told 42 healthy men aged 35 to 62 to eat a diet with either 5 oz. of lean meat or 10 oz. of tofu every day. After a month, there was no difference in blood levels of estradiol (the major estrogen) or testosterone.
Fertility. “Tofu a day, sperm goes away,” read the headline in Canada’s The Globe and Mail. The newspaper was describing the results of a 2008 study by researchers at the Harvard School of Public Health.
But the men in the study didn’t eat tofu every day…and their sperm didn’t go away.
Among 99 male patients at a Boston fertility clinic, total sperm count and sperm quality (that is, how well the sperm moved and were shaped) were the same in those who reported consuming the most soy foods (three to four servings a week) as in those who said they never ate soy.
However, the men who ate the most soy had lower sperm concentration, though it was still in the normal range.
Only two good studies have measured what happens when men are fed soy. In the more recent one, researchers gave 32 Canadian young men soy protein with high levels (62 mg) or low levels (2 mg) of isoflavones or milk protein every day for two months.
“Soy protein and isoflavones had no effect on their sperm concentration, sperm count, sperm quality, or semen volume, compared with milk protein,” says co-author Hamilton-Reeves. She concedes, however, that if soy altered the formation of new sperm, two months may have been too short a time to see it.
The earlier study found essentially the same results.
Sources: Fertil. Steril. 94: 997, 2010; Endocr. Pract. 14: 415, 2008; Fertil. Steril. 93: 2095, 2010; Br. J. Nutr. 84: T557, 2000; Hum. Reprod. 23: 2584, 2008; Fertil. Steril. 94: 1717, 2010.
Other relevant links:
• Can eating soy help with certain health problems? See: How to Diet: What are the Benefits of Soy?
• Should you avoid soy to protect your thyroid? See: How to Diet: Does Soy Affect Thyroid Function?
• What affect does soy have on our brains? See: How to Diet: Does Soy Put You at Risk for Cognitive Decline?