Men who are infertile may have an increased risk of dying sooner than men with normal semen, according to a recent study.

Researchers from Stanford University in California found that men with two or more abnormalities in their semen were more than twice as likely to die over a roughly eight-year period as men who were fertile.

Smoking and diabetes - either of which doubles mortality risk - both get a lot of attention, "but we're seeing the same doubled risk with male infertility, which is relatively understudied," Michael Eisenberg, lead author of the study and Stanford's director of male reproductive medicine and surgery, said in a statement.

For the study, researchers examined records of nearly 12,000 men, between the ages of 20 and 50, who had visited one or two centers to be evaluated for possible infertility.  By keying identifiers for the patients to data in the National Death Index and the Social Security Death index, the investigators were able to monitor these men's mortality for a median of about eight years.

Infertility is a widespread medical complaint in developed countries, where about one in seven couples is affected at some point. But this is only the third study worldwide, and the first in the United States, to address the question of a connection between male infertility and mortality, Eisenberg said.

"We were able to determine with better than 90 percent accuracy who died during that follow-up time," Eisenberg said. "There was an inverse relationship. In the years following their evaluation, men with poor semen quality had more than double the mortality rate of those who didn't."

The difference in death rates between those who had semen abnormalities and those who didn't was statistically significant. Those with two or more semen abnormalities were more than twice as likely to die during the follow-up period as those without any.

"It's plausible that, even though we didn't detect it, infertility may be caused by pre-existing general health problems," Eisenberg said. 

The findings were recently published in Human Reproduction.