A comparison of sperm quality presented at a European fertility meeting Monday showed overweight or obese men, like their female counterparts, have a lower chance of becoming a parent.
In what they described as the largest study of its kind, doctors looked at sperm samples from 1,940 men and matched them to the donor's weight.
The benchmark was the body mass index (BMI), a measure of body fat among adults. Under this, normal weight is defined as a BMI of 18.5-25; overweight as 25-30 and obesity as 30 or more.
The higher the count in excess weight, the more the sperm quality suffered, especially among those in the obese category, according to the findings, presented at the annual conference of the European Society of Human Reproduction and Embryology (ESHRE) in Stockholm.
"Excess weight causes a modification in sperm characteristics, probably as a result of hormonal disturbance, which results in lower sperm numbers, motility and vitality," said Paul Cohen-Bacrie, scientific director at Eylau-Unilabs in Paris, part of a Swiss-based network of labs in 12 European countries.
"This leads to losses in conception potential," he said by phone.
The research found that the sperm count was lower by 10 percent among overweight men, and by 20 percent among obese men, compared with counterparts of normal weight.
The percentage of men who had a zero sperm count rose almost fourfold, from one percent in the normal weight category, to 3.8 percent, in the obese category.