Higher levels of pesticide residue in fruit and vegetables are associated with lower quality of semen, according to a study published on Tuesday.
Its authors said the research was only an early step in what should be a much wider investigation.
In a first recommendation, they urged men not to stop eating fruit and veg, and pointed to organically-grown food, or food that is low in pesticides, as options for lowering any apparent risk.
The U.S. team analysed 338 semen samples from 155 men attending a fertility centre between 2007 and 2012.
The volunteers were aged between 18 and 55, had not had a vasectomy, and were part of a couple planning to use their own eggs and sperm for fertility treatment.
The men were asked to fill out a questionnaire about their diet, asking them how often, on average, they consumed portions of fruit and vegetables.
These portions were then placed into categories of being low, moderate or high in pesticide residues, on the basis of U.S. Department of Agriculture data.
Peas, beans, grapefruit and onions, for instance, fell into the low category, whereas peppers, spinach, strawberries, apples and pears were in the high category. The data factored in whether the items had been peeled and washed before being eaten.
Men who had the greatest consumption of high-category fruit and vegetables had a total sperm count of 86 million sperm per ejaculate. This was 49 per cent less than men who ate the least. They had a sperm count of 171 million per ejaculate.
In addition, men with the lowest pesticide residue intake had an average of 7.5 per cent of normally-formed sperm -- but this tally was nearly a third lower, at 5.1 per cent, among those who had the highest intake.
There were no significant differences between the low-and moderate-residue groups.
"To our knowledge, this is the first report on the consumption of fruit and vegetables with high levels of pesticide residue in relation to semen quality," said the study, published in the journal Human Reproduction.
"These findings suggest that exposure to pesticides used in agricultural production through diet may be sufficient to affect spermatogenesis in humans."
The study acknowledged limitations: men attending fertility clinics are prone to having semen quality problems, and the diet in this case was assessed only once and could have changed over time.
In addition, the pesticide residues were estimated rather than actually measured in the lab, and it was not known whether the fruit and vegetables that were consumed were conventionally-grown or organic.
"These findings should not discourage the consumption of fruit and vegetables in general," said Jorge Chavarro, assistant professor of Nutrition at the Harvard T.H. Chan School of Public Health in Boston, who co-led the study.
"In fact, we found that total intake of fruit and vegetables was completely unrelated to semen quality.
"This suggests that implementing strategies specifically targeted at avoiding pesticide residues, such as consuming organically-grown produce or avoiding produce known to have large amounts of residues, may be the way to go."
Outside commentators said the research was interesting but limited. Further work was needed to confirm the findings, and see if they applied beyond this small group of men.
"This paper may cause unnecessary worry," said Jackson Kirkman-Brown of the Birmingham Women's Fertility Centre in central England. "Men wishing to optimise their sperm quality should still eat a healthy balanced diet until more data is available," he told Britain's Science Media Centre.
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