The study: Data was collected on 99 men who visited a fertility clinic for an evaluation. They were asked specifically how much of 15 soy-based foods they had consumed in the previous three months, including tofu, tempeh, tofu or soy sausages, bacon, burgers, soy milk, cheese, yogurt, ice cream and other soy products like roasted nuts and energy bars.

The results: The men who ate the most soy had 41 million fewer sperm per milliliter of semen, compared with men who did not eat soy foods. Normal sperm counts range between 80 million and 120 million per milliliter. The link was especially true for overweight and obese men, who produce more estrogen than thinner men. Soy could increase these estrogen levels even further.

Why? Lead researcher Dr. Jorge Chavarro, a research fellow in the department of nutrition at the Harvard School of Public Health, says the reason isn't conclusive, but he suspects soy increases estrogen activity, which may have a negative effect on sperm production and also interfere with other hormonal signals, reports HealthDay News. He added that previous research in animals has shown that isoflavones and estrogen can have a potentially negative effect on reproduction, including decreased fertility. However, there has been very little evidence of how these animal findings applied to humans. Until now. Although Chavarro considers the results preliminary and inconclusive, he says they lend support to the results of the animal studies.

Should you stop eating soy? Chavarro doesn't advise any dietary changes just yet. "It's way too early to say stop eating soy foods," he told HealthDay News. "It's not time to worry about whether you're eating too much soy. There's not enough information to conclusively say that."

The study findings were published in the online edition of the journal Human Reproduction.