The review of past studies can't prove that overweight or obese men will have more trouble fathering a child. But researchers said that how many sperm men make is one of the key ways doctors measure their fertility.

"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 Dr. Jorge Chavarro, from 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 of ejaculate 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 out of almost 7,000 men had no sperm in their ejaculate at all.

Dr. Sebastien Czernichow, from the Ambroise Pare Hospital, Boulogne-Billancourt, and colleagues calculated that compared to normal-weight men, overweight men were 11 percent more likely to have a low sperm count and 39 percent more likely to have no sperm -- though it's possible the second finding was due to chance.

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 reported in the Archives of Internal Medicine.

They proposed a number of different theories for the findings, including that male hormones may be converted into estrogen in fat tissue, affecting sperm-making down the line, 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 trouble -- although you wouldn't expect men who have no sperm at all to be fertile, Chavarro said. And it's possible that the obesity itself isn't to blame; rather, in some men, an underlying health condition causes them to gain weight and affects their sperm, said Dr. 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.

The findings jibe with a study from late last year which found that being overweight was tied to a lower sperm concentration and lower motility -- how well sperm swam (see Reuters Health story of November 18, 2011).

The current report "is not conclusive, and the risks are not huge," Winters, who wasn't involved in the research, told Reuters Health. But fertility trouble, he added, "is there among 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." SOURCE: Archives of Internal Medicine, online March 12, 2012.

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