Beter even geen soja als je zwanger wilt worden"*

Uit een studie blijkt dat stoffen zoals genisteïne uit soja mogelijk zaadcellen op weg naar bevruchting in de vrouw kunnen doden. Ook blijkt dat mannen wellicht ook dan beter soja even weg kunnen laten omdat de kwaliteit van het sperma daardoor sterk verminderd, althans dit blijkt uit een studie onder ratten. Het hoe en waarom is nog onduidelijk maar even geen soja kan geen kwaad wellicht alleen maar goed doen.

Would-be mums told to avoid soya

Women trying to conceive should consider not consuming soya for the few days around ovulation, according to a UK researcher. Her study shows a compound found in soya causes human sperm in a dish to “pop their caps” prematurely, rendering them useless. But it remains unclear whether eating soya has any actual effect on fertility.

Lynn Fraser of King’s College London studied the effect of very low levels of genistein - a compound found in leguminous plants such as soya - on human sperm in a liquid medium similar to that found in the female reproductive tract. “It was very striking,” she says. “Within an hour a third of the sperm had gone all the way.”

This means that the genistein had prematurely triggered the sperm to undergo what is known as the acrosome reaction. The acrosome is the cap on the tip of sperm that contains the enzymes needed to penetrate the thick outer layer of the female’s egg once the sperm has reached it. If it is lost early, sperm have no chance of fertilising an egg.

Fraser says other studies have shown that genistein gets into the blood of people who eat soya products. She believes that in women, it could end up in the reproductive tract and damage their chances of conceiving. “From what we have seen, women should restrict their diet for a short time over the period of ovulation.”

Effects on males

But other experts are not convinced such advice is necessary. James Kumi-Diaka of Florida Atlantic University, US, says his team has also found that genistein has a dramatic effect on sperm - so much so that he has toyed with the idea of incorporating genistein into condoms as a contraceptive.

His team has also found that when genistein is injected into male rats three times a week, it reduces the size of the litters they father, from about 11 pups at most to five. Even low doses had an effect, he says. That would seem to hint that men, too, should worry about eating soya when trying to father children.

But Kumi-Diaka stops short of such advice. “It depends on so many things,” he says. “How the food is prepared, how often you eat it, whether it is eaten alone.” If genistein really does affect fertility, Kumi-Diaka points out, you would expect to see fertility problems in Asian countries, where many people consume soya products daily - but there is no such evidence.

Combining chemicals

Fraser first reported that genistein triggers the acrosome reaction in mouse sperm in 2003. In other studies on mouse sperm, she has found two other chemicals can also trigger the acrosome reaction. One, called 8-prenylnaringenin, is found in hops and is thus is present in some beers, but Fraser does not know what levels are typical. The second chemical, nonylphenol, is found in products such paints, pesticides and cleaning products. “There could be a whole range of chemicals with this effect,” she says.

What is more, Fraser found that combinations of these chemicals, which she calls xenobiotics, had a much greater effect than any one alone. “Given the likelihood that we are exposed to several xenobiotics at any one time, we need to investigate their possible effects on fertility as quickly as possible.”

Her latest studies were presented at a meeting of the European Society for Human Reproduction and Embryology in Copenhagen, Denmark, on Wednesday. (Juni 2005)

    Printen