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.
source:
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