Men who eat soy foods are more likely to have lower sperm counts than men who don't, new research suggests.

Scientists at the Harvard School of Public Health have found that men who eat an average of half a serving of soy foods per day have 41 million sperm per millilitre less than men who do not consume any soy products.

The normal sperm concentration range is between 80 and 120 million per millilitre.

The findings are published in the online edition of Human Reproduction.

Previous research has linked a high consumption of plant compounds known as isoflavones, which can be found in soy products, with infertility in animals. Isoflavones are known to mimic the hormone estrogen in the body.

Three types of isoflavones -- daidzein, genistein and glycitein -- are found mainly in soy beans and other soy-based products.

For this study, researchers led by Dr. Jorge Chavarro studied the soy intake of 99 men who visited a fertility clinic with their spouses between 2000 and 2006.

They included foods such as tofu, tempeh and soy-based meat and dairy alternatives and energy bars.

Chavarro speculated that the estrogen-like activity caused by the soy products could interfere with hormonal signals that influence sperm production.

The researchers also found the link between soy food consumption and sperm counts was strongest in men who were overweight or obese.

This strong link could be explained by the fact that overweight and obese men already produce more estrogen than thinner men.