A new study has found that men who consume more soy products have lower sperm counts, especially if they are overweight.
The study, published in online version of the journal Human Reproduction, found that men who ate the most soy food had a sperm count 41 million less than those who did not consume soy products.
“What we found was men that had the highest intake of soy foods had a lower sperm concentration,” said Jorge Chavarro, of the department of nutrition at the Harvard School of Public Health, who led the study.
He believes this happens because soy contains isoflavones, a naturally occurring compound that can mimic the effects of estrogen.
Previous studies have linked high consumption of isoflavones with infertility in animals, but Chavarro’s research is the first to show a similar effect in humans.
The study analyzed the semen of 99 men who attended a U.S. fertility clinic between 2000 and 2006. The men were asked how often they ate soy products, including tofu, tempeh, soy milk and other soy products such as ice creams, cheeses and energy bars.
The average intake of soy for those with the highest consumption levels was about half a serving a day, although some men in that group ate as many as four servings a day.
The researchers also found that soy food intake had the greatest impact on sperm concentrations among those who were also overweight or obese.
This may be because overweight men already have higher levels of estrogen, Chavarro said.
“It’s possible that when you already have very high levels of estrogen, an additional source of an estrogen-like compound may become an issue, but not if you’re a lean man,” he said.
This may explain why soy does not seem to affect fertility among Asian populations, for whom it is a food staple, he said. Obesity is not as common in Asian populations as it is among Western men.
In Chavarro’s study, it was not clear whether the reduction in sperm count was a factor in the subjects’ fertility issues, and he believes it is too early to warn people away from soy.
“I guess if somebody already knows they have a low sperm count and they’re very avid consumers of soy foods they could try stopping it, but there’s no guarantee that’s going to help them,” he said.
Isoflavones are present in a wide variety of fruits and vegetables, including black beans, peas and other legumes, but are most concentrated in soy.
Even with reduced sperm count, men are capable of conceiving a child, Chavarro said, and soy seemed to have no other adverse effect on the sperm.
The study corrected for other factors that may affect the count, such as age, caffeine intake and smoking.
There have been two other studies on the connection between soy and sperm count. One, a study of 14 young men, found no change in semen quality, while the other, which followed 48 men, found that isoflavone intake had a positive effect on sperm count.
The men in Chavarro’s study were predominantly white, and he plans to test the connection with different ethnic groups, as well as with larger sample sizes.
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