Men who eat a lot of saturated fat could be damaging their sperm, new research suggests.
Experts at Harvard Medical School in the US found men who consume lots of saturated fat and monounsaturated fat may have fewer sperm.
In contrast, those who eat healthier fats - specifically omega 3 and omega 6 - may have healthier sperm that is more active, the study suggests.
Saturated fat is found in processed meats such as bacon, sausages and ham, dairy and butter. Monounsaturated fat is found in items such as olive oil.
Researcher Dr Jill Attaman, said: "We were able to demonstrate that in men who took in higher amounts of fats, such as saturated fat and monounsaturated fat, there was an association with decreased sperm concentration.
"But polyunsaturated fat - those seen in fish primarily - was associated with improved sperm morphology and motility."
Morphology relates to the size and shape of the sperm and is an indicator of fertility.
The research was carried out on 91 men seeking fertility treatment but Dr Attaman said she would expect to see a "similar pattern" in the general population, although this would need to be studied.
The men were asked how often they ate certain foods, what types of oil they used in cooking and baking and the types of margarine they consumed.
Of the group, 21 men also had levels of fatty acids in their sperm and semen measured.
Overall, those men in the highest saturated fat intake group took in 13% of their daily calories as saturated fat, while those in the lowest third took in 8%.
The study concluded: "Men in the highest third of saturated fat intake had 41% fewer sperm than those in the lowest third.
"Likewise men in the highest third of monounsaturated fat intake had 46% fewer sperm than those in the lowest third."
Asked if the findings suggested a potentially large impact on a man's ability to father a child, Dr Attaman said it depended on the men.
Those with a borderline level of sperm concentration might see more of an effect than those who already had a healthy number of sperm, she said.
She added that the exact reasons for the apparent connection are unclear and require more work.
But "dietary modifications could be beneficial for global health as well as reproductive health," she said.
The findings were presented at the American Society for Reproductive Medicine conference in Denver.
Dr Tony Rutherford, chair of the British Fertility Society, said the study was small but the conclusion was that people should eat a healthy, balanced diet.