Jorge Chavarro, leader of the Harvard School of Public Health's Department of Nutrition study, attributed the finding to the isoflavones content of soy, which can mimic the effect of estrogen. Previous studies in animals have shown that high consumption of isoflavones results in infertility.

The study was based on the semen analysis of 99 men who went to U.S. fertility clinics between 2000 and 2006. The respondents were asked the volume of soy products they used including tofu, tempeh, soy milk and other soy products,

Their average intake was half a serving per day, although some had as much as four servings. A standard tofu serving is about 115 grams.

Chavarro pointed out tofu's negative effect is felt among obese western men, who already have elevated levels of estrogen. Asian men in general are not affected despite tofu being a staple, because obesity is not widespread in the continent compared to western men, he added.

Because Chavarro's respondents were mostly white men, he plans to repeat the study on other ethnic groups using larger sample sizes to establish if ethnicity is a factor.

Isoflavones is found not only in soy, but also in some fruits and vegetables, such as black beans, peas and other legumes, but its highest concentration is in soy.

The study was published in Sunday's on-line version of Human Reproduction.