Men who consumed the greatest amount of fruit and vegetables with high levels of pesticide residue had 49 percent lower sperm count and 32 percent lower normal sperm than men who ate the least amount of produce with high pesticide residue.
However, the scientists from Harvard University's TH Chan School of Public Health say that further research is needed, and that men should not reduce the amount of fruit and vegetables in their diet on the basis of their findings.
Dr Jorge Chavarro, assistant professor of nutrition and epidemiology and lead author of the study, said: 'These findings should not discourage the consumption of fruit and vegetables in general. In fact, we found that total intake of fruit and vegetables was completely unrelated to semen quality.'
He added: '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.'
The study, published in Human Reproduction, analysed 338 semen samples from 155 men aged 18 to 55, presenting at fertility centres between 2007 and 2012. The men also completed a food frequency questionnaire to assess how often and how many portions of fruit and vegetables they consumed.
Fruits and vegetables were classified as having high, medium or low pesticide residues, according to the US Department of Agriculture pesticide data programme. Produce considered high in pesticide residue included, among others, apples, pears, strawberries, spinach and peppers. Produce considered low in pesticide residue included peas, beans, grapefruit and onions.
The men were grouped into four categories according to pesticide exposure from fruit and vegetables, ranging from high (1.5 servings per day) to low (less than half a serving per day). Men in the high-exposure group had 86 million sperm per ejaculation and 5 percent normally formed sperm, compared to 171 million sperm per ejaculation and 7.5 percent normally formed sperm in the low-exposure group.
The authors acknowledge a number of limitations to the small observational study. Men who present at fertility clinics are more likely to have problems with sperm quality than men in the general population. Diet was only assessed once during the study, and pesticide exposure from diet was not directly measured in individual participants.
In a Science Media Centre briefing, Professor Sheena Lewis of Queen's University Belfast, who was not involved in the research, noted that the sperm counts of men in both the high- and low-exposure groups were still within the normal range according to the World Health Organization.
She added that therefore 'the authors' conclusion is correct that more studies are needed, NOT that we should stop eating our 5-a-day'.