New research shows a possible link between male infertility and obesity.

The link -- based on male BMI (body mass index, which relates height to weight) -- hasn’t been proven and needs further study, the researchers stress.

“To our knowledge this was the first study to examine male BMI and couple fertility. Thus, it is important that the findings will be confirmed or refuted in future studies,” researcher Markku Sallmen, PhD, tells WebMD in an email.

Sallmen worked on the study while at the U.S. National Institute of Environmental Health Sciences and is now based at the Finnish Institute of Occupational Health in Helsinki, Finland.

The study appears in Epidemiology’s September edition.

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Complex Issue

It’s often hard to pin down the source of infertility. Men and women may each have risk factors for infertility.

For instance, women may have a harder time conceiving as they near the end of their childbearing years.

Also, “it is well documented that women who are overweight or obese are at higher risk of reproductive problems, including reduced fertility,” the researchers write.

But little has been known about what effect, if any, men’s BMI has on infertility, Sallmen’s team notes.

Sallmen and colleagues studied 2,111 couples in Iowa and North Carolina. The men in those couples were mainly farmers; their wives were less than 40 years old.

The researchers asked the men’s wives about infertility, defined as not conceiving a pregnancy after at least 12 months of unprotected sex in the previous four years, even if the wives later got pregnant.

The husbands and wives reported their height and weight. The researchers used that data to calculate BMI.

The couples didn’t get checkups or infertility tests for the study.

But the researchers noted factors including the wives’ BMI and the age, smoking status, alcohol use, and exposure to solvents and pesticides for husbands and wives.

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Study’s Findings

Compared with men with normal BMI of 20-22, those who had a three-point increase in BMI were 10 percent more likely to be a partner in an infertile couple during the four-year study period.

“The results were the same when we limited the analysis to couples with female BMI of less than 26,” Sallmen tells WebMD.

“I think that this finding offers further support for the idea that men's BMI is an independent risk factor for infertility,” Sallmen adds.

However, the study has limits.

For example, Sallmen’s team doesn’t know how often the couples had sex. It’s possible, but not certain, that heavier men had sex less often.

Also, the data don't show if BMI changed over time for the husbands or wives. And about 30% of the husbands didn’t provide full information for BMI calculations.

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'Plausible' but Not Proven

“It is biologically plausible for high male BMI to increase the risk of infertility,” the researchers write.

However, they write that their findings “must be viewed as supportive but not confirmatory of an association given the limitations of the study data."

In other words, don’t count on the findings being correct unless other studies back up the results.

If confirmed, the findings suggest that some cases of male infertility may be “an additional price associated with obesity epidemic,” write Sallmen and colleagues.

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By Miranda Hitti, reviewed by Louise Chang, MD

SOURCES: Sallmen, M. Epidemiology, September 2006; vol 17: pp 520-523. Markku Sallmen, PhD, Finnish Institute of Occupational Health, Helsinki, Finland. News release, National Institute of Environmental Health Sciences, National Institutes of Health.