A plant chemical found in soya, tofu and legumes could potentially damage sperm and affect male fertility, a British researcher suggest.

Professor Lynn Fraser, of King’s College London has shown that genistein, which can mimic the effect of the female hormone estrogen, affects sperm in mice.

But it seems to have an even stronger impact on human sperm.

In laboratory tests, Fraser found that small amounts of the chemical can cause human sperm to “burn out” and lose fertility.

“Human sperm proved to be even more responsive than mouse sperm to genistein,” Fraser told a fertility meeting.

She added that if women eat soya and other foods high in genistein it might have a bigger impact on male fertility because the chemical is likely to affect sperm when it is in the female preparing to fertilize an egg.

“Maternal exposure to the compounds is probably more important than paternal exposure,” Fraser explained.

Although it is very preliminary research, Fraser speculated that the findings could have an impact on women trying to conceive.

“On the basis of what we have seen, it might be a practical thing to do if you are in the habit of eating lots of soya-based products to restrict your diet for a short time over the window of ovulation,” she told the European Society of Human Reproduction and Embryology meeting.

Professor Richard Sharpe, of Edinburgh University in Scotland, described the research as interesting and said the results were surprising.

“It could of course have human health/fertility implications as indicated by the authors,” he said in a statement. But Sharpe added that oriental societies that traditionally eat a soya-rich diet show no signs of reduced fertility that he is aware of.

The effects on sperm in the laboratory may also not be directly related to what may happen in real life.

Dr Allan Pacey, of the University of Sheffield in England, said it is worrying if these compounds affect mature sperm.

“It’s early days, but clearly if what happens in the laboratory also occurs in the woman’s fallopian tube as the sperm make their way to the egg, then there would be the potential for fertilization to fail,” he explained.