Researchers in Scotland have found yet another reason for men to avoid becoming overweight.

In addition to an increased risk of heart disease and high blood pressure, they're also more likely to have lower quality sperm.

In research presented Wednesday at a meeting of the European Society of Human Reproduction and Embryology, scientists found that obese men have lower levels of normal sperm.

"There is a very long list of health hazards from being overweight," said Ghiyath Shayeb, the study's lead researcher at the University of Aberdeen. "Now we can add poor semen quality to the list."

Experts aren't sure if that necessarily means obese men face major difficulties having children.

"If you have a man who isn't fantastically fertile with a normal partner who is fertile, her fertility will compensate," said Dr. William Ledger, a professor of obstetrics and gynecology at Britain's University of Sheffield, who was unconnected to the study.

But if both partners are heavy, Ledger said that could be a problem, since obesity is also known to decrease women's fertility.

5,000 samples analyzed

Shayeb and colleagues analyzed the sperm samples of more than 5,000 men in Scotland, and divided the men into groups according to their Body Mass Index. Men who had an optimal BMI (20 to 25) had higher levels of normal sperm than those who were overweight or obese.

Obese men had a 60 per cent higher chance of having a low volume of semen, according to Shayeb's research. They also had a 40 per cent higher chance of having some sperm abnormalities.

Shayeb and colleagues found that underweight men were just as likely to have the same problems as obese men. "But there were not many underweight men in Scotland," he noted.

The researchers adjusted their analysis to account for other factors that could have affected men's sperm count, like smoking, alcohol intake, history of drug abuse and age.

"Male fitness and health are clearly linked to a man's fertility," said Neil McClure, professor of obstetrics and gynecology at Queen's University in Belfast.

There are several theories about why obese men might have bad sperm. Because fat tissue influences the metabolism of sex hormones, scientists think it might also disrupt sperm production.

It could also be a temperature problem. Sperm is best produced at a temperature slightly cooler than normal body temperature. But because obese men have more fat, Shayeb said their bodies might be overheated.