Alcohol might make young men less fertile.  (REUTERS/Michaela Rehle)

It goes without saying that drinking in excess is bad for your health, but it might also hurt your chances of reproducing. In a survey of men between the ages of 18 and 28, researchers reported Thursay in the British Medical Journal, as few as five drinks a week were found to affect sperm quality.

In the study, men who drank an average of five to 10 units (one beer, glass of wine, or 40mL of liquor) of alcohol a week had a slight decrease in sperm concentration, total sperm count, and percentage of healthy and normal sperm in the samples they provided.

The effect became much more pronounced at 25 units a week (3.5 drinks a day) and men who consumed 40 units a week on average had a 33 percent reduction in sperm concentration compared to the lightest drinkers.

The study is just a preliminary one -- surveying 1,221 Danish men during medical exams required before their compulsory military service -- but lead author Tina Kold Jensen believes that it may paint a more accurate picture of male drinking and its effects than previous studies.

Jensen and her colleagues started by asking the young men to give a detailed account, unit by unit, of what they'd had to drink the week before. But then the researchers asked the men to decide whether or not that week had been typical. It was easier for them to remember the specifics of the previous week's consumption than it would have been to estimate their overall "averages" -- a prompt that usually leads to people underestimating how much they drink. But it also weeded out those who'd happened to binge drink the week before.

Sure enough, heavy drinking only affected sperm quality in the men who reported that it was just their normal routine.

Young Danes drink more than any other youth in Europe, Jensen says, so the findings are particularly worrisome in her own country. "But the effect was there, albeit smaller, with just five units a week," she said, "so it's troubling."

Jensen can't be sure whether this effect would be permanent. "Hopefully it would reverse within three months, because that's how long it takes new sperm to mature," she said. But she and her colleagues will have to perform a long-term study and follow up with drinkers down the road if they want to know that for sure.

"We cannot answer these questions yet. We can't say that drinking less will improve sperm quality," Jensen said, "But having said that, it's easy to recommend that young men drink less. We know it's healthier for your liver, and most other things."