"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."
But 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 known to decrease women's fertility.
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.
Fat men had a 60 percent higher chance of having a low volume of semen, according to Shayeb's research. They also had a 40 percent 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.
The study supported results of an earlier sperm study done by doctors at hospitals and universities in Denmark.
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 two degrees cooler than normal body temperature. But because obese men have more fat, Shayeb said their bodies might be overheated.
Another study presented at the conference concluded that diabetes in men damages their sperm and is linked to male infertility.
Con Mallidis and colleagues at Queen's University in Belfast examined semen samples from nearly 40 men who were being treated for diabetes, but were not overweight. They found significant DNA damage linked to the excess sugar in the body from diabetes.
They found that diabetic men had twice the rate of DNA damage in their sperm as men without diabetes.
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