Heavy men are more likely than their peers with more normal weights to have low sperm counts or no sperm production at all, a key way to measure fertility, according to an international study.

But the review of past studies, which covered a combined total of 10,000 men and appeared in the Archives of Internal Medicine, can't prove that overweight or obese men will have more trouble fathering a child.

"In general you expect that men with lower sperm counts will have a greater frequency of difficulty conceiving than men with higher sperm counts, but it's not completely straightforward," said Jorge Chavarro at the Harvard School of Public Health, part of the collaborative group that put out the study.

How well sperm move, their shape and the quality of DNA they carry matter too, Chavarro said -- but previous studies have suggested some of those measures of sperm quality may be affected by obesity as well.

For the new analysis, French researchers combined data from 14 studies that compared sperm count in samples from normal weight, overweight and obese men, as well as data from their own infertility center.

About one-quarter of the combined 10,000 men had a low sperm count. In another analysis, just over 250 of almost 7,000 men had no sperm in their ejaculate at all.

Overweight men were 11 percent more likely to have a low sperm count and 39 percent more likely to have no sperm than their normal-weight peers, according to calculations by Sebastien Czernichow and colleagues at the Ambrose Pare Hospital, Boulogne-Billancourt.

Obese men, on the other hand, were 42 percent more likely to have a low sperm count than their normal-weight peers and 81 percent more likely to have sperm-free ejaculate.

The researchers proposed a number of different theories for the findings including that male hormones may be converted into estrogen in fat tissue, affecting sperm-making, or that more fat in the hips and stomach could make the scrotum too hot.

The results don't prove that overweight and obese men will have more fertility troubles, although you wouldn't expect men who have no sperm at all to be fertile, Chavorro said.

It's possible that obesity itself isn't to blame. It could be that in some men an underlying health condition causes them to gain weight and affects their sperm, said Stephen Winters, professor of Medicine and Chief of the Division of Endocrinology, Metabolism and Diabetes at the University of Louisville.

Because of that, researchers can't say for sure whether heavy men could boost their sperm production by losing weight.

Czernichow told Reuters Health in an email that losing weight improves fertility in women, but that there's not much data in men - though small case reports have suggested weight-loss surgery may actually have a negative effect on sperm.

A study late last year found that being overweight was tied to a lower sperm concentration and lower motility.

The current report is not conclusive and the risks are "not huge," said Winters, who wasn't involved in the study. But he added that fertility trouble is one of the health risks of obesity.

"This appears to be yet another health outcome for which maintaining a healthy weight appears to be important," Chavarro told Reuters Health.

"It's not only about your cardiovascular disease risk, it's not only about diabetes and some forms of cancer. Obesity also seems to affect outcomes that may be manifested in younger men."