FIRST it was tight jeans and hot spas, but now research suggests soy-based products may reduce a man's sperm count.

A study that monitored the soy consumption of 99 men has found those who consumed more than two portions of soy-based foods a week had, on average, 41 million fewer sperm per millilitre of semen than men who had never eaten soy food.

The apparent fall in sperm count is unlikely to make healthy men infertile, but some experts said it could have a significant impact on those already with lower than average sperm counts. A sperm count of 80 million to 120 million per millilitre is regarded as normal, while men who produce fewer than 20 million sperm per millilitre are regarded as clinically subfertile.

As a cheap source of protein, soy-based products have risen steadily in western diet popularity since the 1940s and are now included in many products, including biscuits, sweets, pasta and bread.

The study, by Dr Jorge Chavarro at Harvard school of public health in Boston, builds on research in animals and on human tissues that has suggested certain ingredients in soy could harm sperm production.

In the biggest human study into the effects of soy on fertility, Dr Chavarro and colleagues at Massachusetts General Hospital recruited 99 men who had visited a fertility clinic between 2000 and 2006. They were asked to fill out a survey which asked them about the amounts of 15 different soy foods they had eaten over the previous three months.

The researchers then divided the men into four groups according to the levels of chemicals called isoflavones in their diets. (Isoflavones are ingredients in soy products that mimic the female sex hormone, oestrogen.) Each man then provided a sperm sample for testing.

Dr Chavarro found that men who consumed at least half a portion of soy food a day had the lowest sperm count. "Our findings suggest that the greater the soy food intake, the lower the sperm concentration, compared with men who never consume soy food," said Dr Chavarro, whose study appears in the journal Human Reproduction.