Men who drink about a quart or more of cola every day could be causing harm to their sperm, results of a Danish study hint.

On average, these men’s sperm counts were almost 30 percent lower than in men who didn’t drink cola. While most of the sperm counts would still be considered normal by the World Health Organization, men with fewer sperm generally have a higher risk of being infertile.

The link is unlikely to be due to caffeine, the researchers say, because coffee did not have the same effect, even though its caffeine content is higher. Instead, other ingredients in the beverage or an unhealthy lifestyle could be involved.

“It’s important to note that the men who drank a lot of cola were also different in many other ways,” Dr. Tina Kold Jensen of Rigshospitalet in Copenhagen, Denmark, told Reuters Health.

Kold Jensen, who led the new research, said only a few studies have looked at caffeine’s impact on reproductive health in men. The participants have generally been a very select group, such as infertile men, and the results have been conflicting.

Because Danish youth has been upping their consumption of caffeine-containing soft drinks over the last decades, the researchers decided to study how this might affect their reproductive health.

More than 2,500 young men were included in their study. Those who didn’t drink cola had better sperm quality - averaging 50 million sperm per milliliter semen - and tended to have a healthier lifestyle.

In contrast, the 93 men who drank more than one liter (about 34 ounces) a day had only 35 million sperm per milliliter. They also ate more fast foods, and less fruit and vegetables.

When looking at caffeine from other sources, such as coffee and tea, the decrease in sperm quality was much less pronounced, the researchers note in the American Journal of Epidemiology.

It is still not clear if the cola or the unhealthy lifestyle, or both, is to blame. However, Dr. Fabio Pasqualotto, of the University of Caxias do Sul in Brazil, who was not involved in the study, said the drink itself probably wasn’t the most important factor.

“I imagine it’s the lifestyle,” he said.

SOURCE: American Journal of Epidemiology, online March 25, 2010.