Men who consume soy daily and want to start a family may want to reconsider their dietary choices in light of new research.

A study published in the July 24 issue of Human Reproduction finds that men who eat half a serving of soy per day — such as 115 g of tofu or 240 ml of soy milk — had lower concentrations of sperm than men who didn't eat soy.

Researchers at the Harvard School of Public Health found that men who consumed the most soy (half a serving per day) had 41 million sperm per millilitre less than men who did not eat soy at all.

An average sperm concentration is between 80 and 120 million per millilitre.

They found that there was an "inverse association between soy food intake and sperm concentration that remained significant after accounting for age, abstinence time, body mass index, caffeine and alcohol intake and smoking," reads the study.

Men who were obese or overweight, who accounted for 72 per cent of the study's participants, were more likely to have lowered sperm concentrations, the study also found.

The 99 participants were recruited from the Massachusetts General Hospital Fertility Center, where they were evaluated for sub-fertility between 2000 and 2006.

The researchers believe that the estrogenic properties of soy may negatively affect the production of sperm, an effect that may be heightened in obese or overweight men who have higher estrogen levels than men of normal weight.