Men who consume a diet rich in saturated fat - the type found in junk food - have lower sperm counts than men whose diets contain low levels of such fats, report scientists.
Indeed, the men in the study who ate the most saturated fat had a 43 percent lower sperm count than the men who ate the least. Furthermore, the team found that having a healthier diet high in omega-3 polyunsaturated fats - found in fish and plant oils - increased the men's chances of having sperm of normal sizes and shapes.
Lead author Dr Jill Attaman, from the Dartmouth-Hitchcock Medical Centre in the USA, said: 'If men make changes to their diets so as to reduce the amount of saturated fat they eat and increase their omega-3 intake, then this may not only improve their general health, but could improve their reproductive health too'.
Ninety-nine men attending fertility clinics at Massachusetts General Hospital, Brigham and Women's Hospital and Harvard Medical School took part in the study. Dr Attaman and her co-workers caution that as there were a small number of participants, further research is required to conclusively establish a link between junk food intake and low sperm quality.
In addition, 71 percent of the study participants were overweight or obese. However, the researchers say this factor was taken into account, as they showed the frequency was no different to that found in the general US population.
Dr Attaman told ABC News that there are not many clearly identifiable lifestyle modifications that can be made to optimise natural fertility and that 'this is the first report of a relationship between specific dietary fats and semen quality'.
The study results were published in the journal Human Reproduction.
In a separate systematic review in the Archives of Internal Medicine, a research team working from the Harvard School of Public Health report that overweight and obese men are more likely to have low sperm counts or no sperm at all than their normal-weight peers.
The findings are a combination of those from 14 studies that compared sperm count in groups of men of varying weights. Specifically, overweight men were 11 percent more likely to have a low sperm count than their normal-weight peers, and 39 percent more likely to have no sperm.
One suggested explanation is that male hormones may be converted into the female hormone oestrogen in fat tissue, which could affect sperm production, say the researchers.