A new study found that men who eat just a half of serving of soy each day have much lower sperm counts than men who do not.

Dr. Jorge Chavarro and his colleagues at the Harvard Public School of Health in Boston investigated the effect soy-derived foods and isoflavones have on human semen quality and found that consumption of soy foods and isoflavones is linked to lower sperm concentration.

The findings were presented Monday at the 63rd annual Meeting of the American Society for Reproductive Medicine in Washington, D.C.

The soy industry said the findings do not correlate with its own studies that show soy to have no effect on sperm count.

In previous studies using animals, isoflavones from soy products have been found to decrease fertility when consumed in large amounts, but there have been fewer studies on humans, the Harvard researchers said. The isoflavones in soy mimic the effects of the female hormone estrogen in the body. Because of this, menopausal women suffering from low estrogen often eat soy as means of replacement therapy.

For their study, Harvard researcher surveyed 100 men who were having trouble conceiving with their partners on their consumption of 15 soy-based foods for the preceding three months.

Men with the highest soy food intake (an average 0.5 serving per day) had 41 million sperm per milliliter less than men who did not consume any soy. Measured sperm motility and morphology were not affected by the men’s soy consumption, the researchers said.