Men who drink around a litre of cola every day could be harming their sperm, according to a new Danish study.
On average, these men's sperm counts were almost 30 per cent 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.
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 millilitre semen - and tended to have a healthier lifestyle.
In contrast, the 93 men who drank more than one litre a day had only 35 million sperm per millilitre. However, 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.
It is still not clear if the cola or the unhealthy lifestyle, or both, is to blame and the scientists said further research was needed.
The study was published in the American Journal of Epidemiology.