Heavy men are more likely than normal-weight men to have low sperm counts or no sperm production at all, suggests a new review.
"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.
Sperm motility, plus their shape and the quality of DNA they carry matter too, Dr 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, 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 centre.
Obese men more likely to have low sperm
About one-quarter of the total population of 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.
Based on their findings, Dr Sebastien Czernichow, from the Ambroise Pare Hospital, Boulogne-Billancourt, in France and colleagues report that compared to normal-weight men, overweight men are 11% more likely to have a low sperm count and 39% are more likely to have no sperm – though the latter finding wasn't statistically significant.
Obese men, on the other hand, were 42% more likely to have a low sperm count than their normal-weight peers and 81% more likely to have sperm-free ejaculate, the researchers reported in Archives of Internal Medicine.
They suggest some explanations for the findings, including that male hormones may be converted into oestrogen in fat tissue, affecting sperm-making down the line, or that more fat in the hips and stomach could make the scrotum too hot.
Obesity not to blame
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, Dr 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, Chief of the Division of Endocrinology, Metabolism and Diabetes at the University of Louisville, Kentucky.
Because of that, researchers can't say for sure whether heavy men could boost their sperm production by losing weight.
Maintain healthy weight
Dr Czernichow said 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 align with results from a study last year which 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," Dr 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," Dr Chavarro said.
(Genevra Pittman, Reuters Health, March 2012)