Overweight men are not more likely to be infertile, as past research has shown to be true in obese women, according to a new study.

Findings of the study, performed in New York in nearly 300 very overweight men, were unexpected, said coauthor Nanette Santoro, MD, an Albert Einstein College of Medicine obstetrician-gynecologist who is trained in reproductive endocrinology.

“We see pretty significant deficits in fertility in women due to obesity, so we thought we’d see an effect in men,” Santoro said. “But that wasn’t the case.”

Santoro and her colleagues studied 292 men who gave semen samples at fertility clinics. The men were ages 18 to 50 and, on average, had a body mass index (BMI) of 28, which is considered nearly obese. The authors found that greater body weight was not associated with worse sperm production or sperm motility. Impaired sperm production is the cause of infertility in 90 percent of infertile men, according to Santoro. About 6 percent of reproductive-age men are infertile, she said.

Increasing body weight was linked to lower testosterone in the 31 study participants for whom testosterone was tested. However, Santoro said low testosterone causes infertility only if it is very low or occurs in morbidly obese men.

“Our results show that the process of making sperm is pretty robust and is hard to interrupt,” Santoro said. “It’s good news for men that body size may be less related to fertility than it is in women.”

She cautioned that the findings should not serve as an excuse for overweight men to avoid losing weight.

Santoro and her coworkers plan to further study a trend they saw toward reduced sperm motility in the six men who were taking an antidepressant in the selective serotonin reuptake inhibitor (SSRI) class. This class of drugs includes Prozac, Celexa, Lexapro, Paxil and Zoloft.

“We know that SSRIs affect sex drive, but there aren’t much data on their effect on fertility,” she said.

The results will be presented at The Endocrine Society’s 90th Annual Meeting in San Francisco.