Men who are overweight and eat more than a half a portion of soy foods per day may be lower their concentration of sperm, say researchers from the Harvard School of Public Health.
A half portion of soy is the equivalent of one cup (250 ml) of soy milk, one 4-ounce serving of tofu, or one portion of soy burgers.
In this new study, 99 men who attended a fertility clinic were asked about their consumption of 15 different soy foods over the last three months. Their concentration of sperm was analyzed to determine the effect of the soy foods.
The 15 different soy foods included tofu, tempeh (fermented tofu), soy milk and other non-dairy alternatives, energy bars, and vegetarian products using soy as a meat substitute.
The researchers determined the isoflavone content of these foods. Isoflavones, such as daidzein and genistein, are plant compounds that mimic estrogen in the body.
Men who ate the most soy - an average of half a portion of soy foods per day - had the lowest sperm concentrations.
Among men who were overweight, the effect of soy on concentration of sperm was more pronounced.
Researchers believe that men with more body fat already produce more estrogen than slimmer men - and eating soy foods may compound the effect of estrogen on their sperm concentrations.
Previous research has linked excess body fat to lower fertility in men. This is the first study, published in the journal Human Reproduction, to link isoflavones in soy foods to sperm concentration.
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