A diet high in saturated fat has been linked with a reduced sperm count.
A study of 99 men attending a US fertility clinic found those eating junk food diets had poorer sperm quality.
High intakes of omega-3 fatty acids, found in fish and plant oils, were associated with higher sperm concentration.
More work is needed to confirm the findings, the researchers report in the journalHuman Reproduction.
The team, led by Prof Jill Attaman from Harvard Medical School in Boston, questioned men about their diet and analysed sperm samples over the course of four years.
Compared with those eating the least fat, men with the highest fat intake had a 43% lower sperm count and 38% lower sperm concentration (number of sperm per unit volume of semen).
Men consuming the most omega-3 fatty acids had sperm with a more normal structure than men with the lowest intake.
Prof Attaman said: "The magnitude of the association is quite dramatic and provides further support for the health efforts to limit consumption of saturated fat given their relation with other health outcomes such as cardiovascular disease."
However, 71% of participants were overweight or obese, which could have had an impact on sperm quality. Furthermore, none of the men had sperm counts or concentrations below the "normal" levels defined by the World Health Organization of at least 39 million and 15 million per millilitre.
Commenting on the research, British fertility expert Dr Allan Pacey, of the University of Sheffield, said: "This is a relatively small study showing an association between dietary intake of saturated fats and semen quality.
"Perhaps unsurprisingly there appeared to be a reasonable association between the two, with men who ate the highest levels of saturated fats having the lowest sperm counts and those eating the most omega-3 polyunsaturated fats having the highest.
"Importantly, the study does not show that one causes the other and further work needs to be carried out to clarify this. But it does add weight to the argument that having a good healthy diet may benefit male fertility as well as being good general health advice."