Men who consume half a serving of soy-based food on average each day have been found to have lower concentrations of sperm, making them less fertile than those who don't include soy in their diet, research suggests.

A study published online today in the European journal of reproductive medicine, Human Reproduction, discovered that men who ate the largest amounts of soy foods had 41 million sperm per millilitre less than men who didn't eat soy.

Previous studies carried out on animals have associated high intake of the plant-derived isoflavones - found mainly in soy beans - with infertility.

Lead researcher Dr Jorge Chavarro, a research fellow in the department of nutrition at Harvard School of Public Health, Boston, explained that men in the highest intake group had a mean soy food intake of half a serving per day: in terms of their isoflavone content that is comparable to having one cup of soy milk or one serving of tofu or soy burgers every other day.

"It is important to highlight that the figure of half a serving a day is the average intake for men in the highest intake group. Some men in this group had intakes of soy foods as high as nearly four servings per day," he added.

"The implication is that men who have normal or high sperm counts may be more susceptible to soy foods than men with low sperm counts, but this remains to be evaluated."

According to fertility experts, 'normal' sperm concentration for men ranges between 80-120 million/ml.

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