Men who eat soy-based foods may be harming their fertility, doctors say after a study found a link between soy-rich diets and lower sperm counts.

The study showed men who ate more than two portions of soy-based foods a week had, on average, 41 million fewer sperm per ml of semen than men who had never eaten soy products.

The apparent fall in sperm count is unlikely to make healthy men infertile, but some experts say it could have a significant impact on those who already have a lower than average sperm count.

A sperm count of between 80 million and 120 million per ml is regarded as normal, while men who produce fewer than 20 million sperm per ml are regarded as clinically sub-fertile.

The study, by Jorge Chavarro at Harvard School of Public Health in Boston builds on previous research in animals and on human tissues that have suggested certain ingredients in soy can harm sperm production.

Male fertility has been in decline in the West for several decades, with about 20 percent of young Europeans having a low sperm count, while levels of soy have risen steadily in the Western diet since the 1940s because it is a cheap source of protein.

Soy-based products are now found in two-thirds of manufactured food, including sweets, pasta, bread, and cookies and crackers, according to the Institute of Food Research in Norwich, England.

In the biggest human study into the effects of soy on fertility, Chavarro and colleagues at Massachusetts General Hospital recruited 99 men who had visited a fertility clinic between 2000 and 2006.

The men were asked to fill out a questionnaire regarding the amounts of 15 different soy foods they had eaten over the previous three months. The researchers then put 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, estrogen. Each man then provided a sperm sample for testing.

Chavarro found that men who consumed at least half a portion of soy food a day had the lowest sperm counts.

"Our findings suggest that the greater the soy food intake is, the lower the sperm concentration, compared with men who never consume soy food," said Chavarro, whose study appears in the journal of Human Reproduction.

Questions:

1. Where is the Harvard School of Public Health?

2. According to the article normal sperm count is how much?

3. Isoflavones are chemicals in soy that do what?

Answers:

1. Boston.

2. Between 80 million and 120 million per ml.

3. Mimic the female sex hormone, estrogen.