Saturated fats, like those found in rich cheeses and meats, may do more than weigh men down after a meal – a Danish study also links them to dwindling sperm counts.
Researchers, whose report appeared in The American Journal of Clinical Nutrition, found that young Danish men who ate the most saturated fats had a 38 percent lower concentration of sperm, and 41 percent lower sperm counts in their semen, than those who ate the least fat.
"We cannot say that it has a causal effect, but I think other studies have shown that saturated fat intake has shown a connection to other problems and now also for sperm count," said Tina Jensen, the study's lead author from Rigshospitalet in Copenhagen, the Danish capital.
The research is not the first to connect diet and other lifestyle factors to sperm production and quality. In 2011, Brazilian researchers found that eating more grains - such as wheat, oats or barley - was associated with improved sperm concentration and mobility, and fruit was also linked to a speed and agility boost in sperm.
But that study and most others looked at these associations using data on men seeking fertility treatments, which may not be representative of all men.
For their study, Jensen and her colleagues surveyed and examined 701 young Danish men who were about 20 years old and getting checkups for the military between 2008 and 2010.
They were asked about the food they ate over the prior three months, and then asked for a semen sample. The researchers then broke the results into four groups, depending on how much of the men's energy intake came from saturated fats, and compared how much sperm the men in each group produced.
The men who got less than 11.2 percent of their energy from saturated fats had an average sperm concentration of 50 million per milliliter of semen and a total sperm count of about 163 million.
That compared to 45 million sperm per milliliter of semen and a 128 million count in men who got more than 15 percent of their energy from saturated fats.
The World Health Organization defines anything over 15 million sperm per milliliter of semen as normal. In the study, 13 percent of men in the lowest-fat group and 18 percent of men in the highest-fat group fell below that level.
Although the study cannot determine whether other lifestyle factors might account for the link, Jensen said her team's findings may partially explain studies that have found sperm counts decreasing around the world.
Last year, French researchers reported the number of sperm in one milliliter of the average 35-year-old Frenchman's semen fell from about 74 million in 1989 to about 50 million in 2005.
"I think obesity is another cause, but (saturated fats) could also be a possible explanation," Jensen said.
She said that the next step is to find the mechanism by which saturated fat could influence sperm count, and then to see whether sperm counts improve when men cut down on saturated fat in their diets.