A regular diet of even modest amounts of food containing soy may halve sperm concentrations, suggest scientists.
The study, published in the journal Human Reproduction, found 41 million fewer sperm per millilitre of semen after just one portion every two days.
The authors said plant oestrogens in foods such as tofu, soy mince or milk may interfere with hormonal signals.
However, a UK expert stressed that most men in Asia eat more soy-based products with no fertility problems.
Animal studies have suggested that large quantities of soy chemicals in food could affect fertility, but other studies looking at consumption in humans have had contradictory findings.
The Harvard School of Public Health study looked at the diets of 99 men who had attended a fertility clinic with their partners and provided a semen sample.
The men were divided into four groups depending on how much soy they ate, and when the sperm concentration of men eating the most soy was compared with those eating the least, there was a significant difference.
The "normal" sperm concentration for a man is between 80 and 120 million per millilitre, and the average of men who ate on average a portion of soy-based food every other day was 41 million fewer.
Dr Jorge Chavarro, who led the study, said that chemicals called isoflavones in the soy might be affecting sperm production.
These chemicals can have similar effects to the human hormone oestrogen.
Dr Chavarro noticed that overweight or obese men seemed even more prone to this effect, which may reflect the fact that higher levels of body fat can also lead to increased oestrogen production in men.
However, the study pointed out that soy consumption in many parts of Asia was significantly higher than even the maximum found in these volunteers.
Dr Allan Pacey, a senior lecturer in andrology from the University of Sheffield, said that if soy genuinely had a detrimental effect on sperm production, fertility might well be affected in those regions, and there was no evidence that this was the case.
"Many men are obviously worried about whether their lifestyle or diet could affect their fertility by lowering their sperm count.
"Oestrogenic compounds in food or the environment have been of concern for a number of years, but we have mostly thought that it was boys exposed in the uterus before birth who were most at risk.
"We will have to look at adult diet more closely, although the fact that such large parts of the world have soy food as a major part of their diet and don't appear to suffer any greater infertility rates than those on western diets suggests that any effect is quite small."