A new study linked soy to male fertility. It suggests that eating half a serving soy food a day lowers sperm concentrations, particularly in obese men.

Soy contains isoflavones (a substance which can mimic the effect of estrogen), which have long been tied to infertility in animal studies, Jorge Chavarro, leader of the study undertaken by the Harvard School of Public Health’s Department of Nutrition, explains the results of the research study.

The study was based on the semen analysis of 99 men who visited the Massachusetts General Hospital Fertility Center between 2000 and 2006. The respondents were asked the amount of soy products they consumed, including tofu, tempeh, soy milk, soy sausages, bacon, burgers, soy cheese, yogurt, ice cream and other soy products like roasted nuts and energy bars in the past three months. Their average intake was half a serving per day. The Soya Protein Association dismissed the findings, saying that the researchers took into consideration participants’ recall of their previous soy foods intake.

Researchers found that men who ate the most soy had 41 million fewer sperm per milliliter of semen compared with men who did not eat soy foods.

But Chavarro’s team considers the findings preliminary and inconclusive. “It’s way too early to say stop eating soy foods,” the researcher said. “It’s not time to worry about whether you’re eating too much soy. There’s not enough information to conclusively say that.”

The report was published in the July 24 online edition of the journal “Human Reproduction.”

Chavarro plans to make further studies on other ethnic groups using larger sample sizes to establish if ethnicity is a factor.