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.

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"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.

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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.