"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.