Men who eat just half a serving of soya a day have drastically fewer sperm than those who do not consume such foods, according to a small, preliminary study.
The study's researchers say larger trials are needed to determine whether men hoping to conceive a child should try to avoid soya foods, such as tofu, tempeh and soya milk. However, soya industry representatives caution that the new findings contradict earlier studies that have shown no impact on sperm count from soya-based products.
Soya foods contain high amounts of isoflavones, compounds that mimic the effects of oestrogen in the body. For this reason, women sometimes increase their intake of soya foods to treat hot flushes caused by declining oestrogen levels in menopause.
Oestrogen-like compounds can also have a dramatic impact on the male body. And previous rodent studies have suggested that high intake of soya products can reduce male fertility. This has led scientists to wonder how isoflavones might influence men's reproductive function, which is highly sensitive to hormones.
Jorge Chavarro at the Harvard School of Public Health in Boston, Massachusetts, US, and his colleagues identified 100 couples seeking treatment for infertility. Researchers asked the men to provide semen samples and complete a questionnaire about their intake of 15 soya-based foods such as miso soup, "power bars", and tofu over the preceding three months.
An analysis of the data, which controlled for factors such as age and weight, revealed that those men who consumed half a serving of soya-based food each day - about the equivalent of half a soya burger - had 65 million sperm per millilitre on average.
That is about 40 percent less than the typical sperm count of men who do not eat such foods -- normally between 80 million to 120 million sperm per millilitre. Men with counts lower than 20 million sperm per millilitre are generally considered infertile.
Chavarro presented the new findings at the annual meeting of the American Society for Reproductive Medicine in Washington, DC on Monday.
Researchers speculate that isoflavones from soya products might lower sperm count by interfering with other hormonal signals that drive sperm production in the testes. They say that further studies are needed to assess whether the drop in sperm count seen in men consuming soya could translate into reduced fertility.
But some experts are sceptical about the new findings. Earlier studies involving men and primates with carefully controlled isoflavone intake have not observed an effect on sperm, according to Stephen Barnes, a toxicologist at the University of Alabama at Birmingham, US, who has studied the influence of soya foods.
The result from Chavarro's study stands in stark contrast to previous findings. "It doesn't fit in with the rest of what has been done experimentally," says Barnes. "But that's not to say it's wrong," he adds.
Industry representatives also note that because isoflavone content in a given food can vary widely based on harvesting conditions, it is difficult to draw conclusions about the impact of soya from retrospective surveys of food consumption.
"Unless researchers are measuring the actual isoflavone content it would be difficult to know the impact of soya foods on sperm count," says Nancy Chapman, executive director of the Soyfoods Association of North America based in Washington, DC. "When you do an observational trial like this there could have been other behaviours causing this decline in sperm count."
Chavrro admits that many east Asian men consume much more soya than the participants in his trial and do not develop fertility problems.
He speculates that his study found a link between soya and low sperm count because many of the participants were overweight or obese. Men with high levels of body fat produce more oestrogen than their slim counterparts. "They already have a lot of background estrogen," says Chavarro.
He believes that the oestrogen-mimicking isoflavones in soya might push the hormonal levels in overweight and obese men even higher, to the point at which sperm begin to suffer.