By Caroline Parkinson
Health reporter, BBC News, Barcelona
The world's obese population is rising
Obese men have poorer quality sperm, perhaps because too much fat around their testicles causes them to heat up, scientists have suggested.
University of Aberdeen researchers looked at the sperm of over 2,000 men in couples having problems conceiving.
The heaviest men had a higher proportion of abnormal sperm, as well as other problems.
The scientists told a European fertility conference losing weight probably boosted fertility.
Being obese is already known to affect women's chances of getting pregnant.
The men were divided into four groups, according to their BMI (body mass index).
Other factors which could affect fertility, such as smoking, high alcohol use and age, were taken into account by the researchers.
Men who had a healthy BMI of 20 to 25 were had higher levels of normal sperm than those who were heavier.
They also had higher semen volume, the European Society for Human Reproduction and Embryology meeting in Barcelona was told.
But those with a higher BMI had lower volumes of seminal fluid, and a higher proportion of abnormal sperm.
There was no significant difference between the four groups in sperm concentration or activity.
Other studies have also linked obesity to DNA damage in the sperm.
Dr Ghiyath Shayeb, who led the research, said: "Our findings were quite independent of any other factors and seem to suggest that men who are trying for a baby with their partners, should first try to achieve an ideal body weight.
"This is in addition to the benefit of a healthy BMI for their general well being.
"Adopting a healthy lifestyle, a balanced diet, and regular exercise will, in the vast majority of cases, lead to a normal BMI.
"We are pleased to be able to add improved semen quality to the long list of benefits that we know are the result of an optimal body weight."
The researchers will now look at male BMI in fertile and infertile couples to see if the poorer semen quality is directly linked to poor fertility, and examine further how obesity can damage sperm.
Dr Shayeb said there were a number of possible explanations, including different hormone levels in obese men, overheating of the testicles caused by excessive fat in the area, or simply the lifestyle and diet that leads to obesity also causing poorer semen quality.
Dr Ian Campbell, chair of the charity Weight Concern, said it was known that overweight people had a tendency to have fewer children.
He said there had been a suspicion that was mainly due to lack of opportunity.
"But if weight actually has a detrimental effect on sperm quality, that's really interesting," he said
"It's one more reason for men to lose weight."