Obese men are more likely to be infertile than their slimmer peers, according to the first study to look at whether a man’s weight influences a couple’s fertility.

Every excess 10 kilograms, or 20 pounds, may cut a man’s fertility by 10 percent, Dr. Markku Sallmen of the Finnish Institute of Occupational Health in Helsinki and colleagues at National Institute of Environmental Health Sciences (NIEHS) in Research Triangle Park, North Carolina, write in the September issue of Epidemiology. Sallmen was a post-doc at NIEHS when he conducted the study.

The researchers looked at couples participating in the Agricultural Health Study who had attempted pregnancy over the past four years. The analysis was limited to couples for whom the wife was younger than 40.

The researchers compared the men’s body mass index (BMI), a measure of weight in relation to height, to pregnancy success. A BMI of over 25 is considered overweight. Infertility was defined as failure to become pregnant after 12 months of unprotected intercourse.

Fertility was lower among men with BMIs of 26 or greater, and decreased as BMI rose, Sallmen and colleagues found.

For every three-point increase in BMI, the risk of infertility rose by 12 percent.

There are a number of mechanisms by which being overweight could affect fertility in males, Sallmen noted in an e-mail to Reuters Health. For example, excess weight may reduce sperm concentration/count, alter hormonal balance and increase scrotal temperature, or overweight men may simply have lower libidos and less sex than normal-weight men.

The study can’t answer such questions of mechanism, Sallmen added, nor can it determine whether losing weight could restore fertility.

Sallmen and his team say their findings should be considered a first step in evaluating the relationship between overweight and obesity and male fertility.

###

SOURCE: Epidemiology, September 2006.