Just one rasher of bacon a day can damage a man's fertility, while eating a portion of white fish such as cod or halibut every other day can improve it, researchers have suggested.
The study by Harvard University on 156 men in couples suffering problems conceiving examined their diet and the size and shape of their sperm.
Researchers found that men who regularly ate processed meat had significantly lower amounts of normal sperm, compared with those who limited the amount of foods like bacon, sausages, hamburgers, ham and mince.
On average, those who ate the equivalent of less than a rasher of bacon a day had 30 per cent more normal sperm than those who ate higher quantities of processed meats.
Meanwhile, those who ate a portion of white fish every other day had a similar edge over those who ate foods such as cod more rarely.
Dr Myriam Afeiche, from the Department of Nutrition at Harvard School of Public Health, said, "We found that processed meat intake was associated with lower semen quality and fish [with] higher semen quality."
Few studies have examined the relationship between processed meat and fertility, and Dr Afeiche said it wasn't clear why such foods might negatively affect sperm quality.
Dr Allan Pacey, a fertility expert at the University of Sheffield, said it was already known that a healthy diet could improve male fertility, but it was less clear whether specific foods could be blamed for a deterioration in sperm quality.
"The relationship between diet and men's fertility is an interesting one, and there is convincing evidence that men who eat more fresh fruit and vegetables have better sperm than men who don't," he said. "However, less is known about the fertility of men with poor diets."
Dr Pacey said it was extremely difficult to accurately measure the size and shape of sperm, but said advice to eat less processed meat and more fish was good health advice, regardless.
"It is already known that high intake of processed meat is linked to other health issues, and so advising men to limit their intake of processed food may improve their health generally, as well as possibly be good for their fertility," he said.