Good question. Most likely, you are referring to a recent Harvard study suggesting that men who consumed soy products had lower sperm counts. Like most nutrition research that we hear about in the news, this study only showed a correlation between sperm count and soy intake, not a direct causal relationship. Moreover, the average sperm count of the men who did consume soy products was 82 million per milliliter. That's high. The World Health Organization lists the lower limit of normal as 20 million per milliliter. Additionally, the majority of men with lower counts happened to be overweight or obese, which is a known contributor to lowered sperm counts.
The concern with soy comes from the fact that it contains substances called isoflavones, which mimic the female hormone estrogen. In small doses, this isn't a concern; in fact, human fat converts male hormones to female hormones naturally. One potential explanation for the findings in the Harvard study is that this peripheral estrogen conversionwhich is, of course, increased in men with more body fatmight combine with soy consumption for a one-two punch that would explain the lower counts.
Previous studies have come to differing conclusions, however, with some finding that soy has no effect on fertility, and others suggesting that it might even boost it. Regardless, soy products are still an excellent low-fat source of fiber, protein, vitamins and minerals. So rather than worry whether tofu is killing their sperm, men concerned about fertility might consider reducing alcohol intake, laying off the anabolic steroids, or losing weight. Unlike soy, booze, 'roids, and obesity have all been proven directly to reduce sperm counts.