Men who eat soy have lower sperm counts than those who do not, according to a new study.

Researchers at the Harvard School of Public Health said in a news release that the effect was even more pronounced in men who are overweight.

Men who ate the most soy had about 41 million sperm per milliliter. Between 80 million and 120 million is considered normal.

The study was based on analysis of 99 men who went to a fertility clinic when their partners were having trouble conceiving. The men were divided into four groups according to their intake of soy foods.

The group that ate the most still only had about half a serving a day, on average, but they saw the dramatic effects on sperm concentrations. Some in the group, however, had up to four servings a day.

The study did not say why soy foods have this effect on sperm, though author Dr. Jorge Chavarro said it could be because of soy's tendency to act like estrogen interfering with other hormones.

That could also be why overweight men had more trouble, since body fat can produce more estrogen, as well.

The study was published in the European journal Human Reproduction.