NEW YORK >> Many men gain a new sense of responsibility and purpose when they become fathers. A new study suggests they also gain 3 to 5 pounds.

The research wasn’t designed to prove fatherhood causes weight gain and raises more questions than it answers. But one outside expert, while noting its limitations, said the research is provocative and should spark further study.

Doctors pay attention to the weight gain of mothers — both before and after pregnancy. But the waistline of dads? That’s not on most doctors’ radar, said Tom Wadden, director of the University of Pennsylvania’s Center for Weight and Eating Disorders.

The study’s lead author — Dr. Craig Garfield of Northwestern University — said he could only speculate about what’s behind the extra pounds.

“For men who become fathers, their whole life changes,” Garfield said. They may sleep less, exercise less, and experience more stress — all of which can lead to weight gain, he said.

It doesn’t help that the food selection at home may gradually change to include more things like “making chocolate chip cookies with the kids,” said Garfield. A dad himself, Garfield said his weakness is finishing his kids’ leftover cheese pizza.

For their work, the researchers looked at results from another study, which tracked the health of adolescents over two decades. The researchers focused on teen boys and young men, comparing weight changes in the 3,400 who became dads and the 6,800 who didn’t.

The researchers made statistical adjustments to iron out the potential influences on weight gain by other factors, like age and marriage.

Simply by becoming a first-time dad, a typical 6-foot-tall man who lives with his child can expect to gain an average of about 4½ extra pounds, the study suggested. A same-sized man who does not live with his child can expect to gain nearly 3½ pounds.

But a 6-foot man who does not have children typically had a weight gain that was 1½ pounds less than would otherwise have been expected, the researchers found.

The study checked weights of the men at four times over the two decades. The researchers were not able to determine at what point in time dads put on the weight. Most of it could have gone on during the pregnancy, Wadden noted.

The study found men who lived with their children were a little heavier to begin with, on average, and ended up heavier than the absent fathers and the men who didn’t have kids.

Nearly three-quarters of U.S. men are overweight or obese, according to government statistics.

The study was published online Tuesday by the American Journal of Men’s Health.

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Online:

Study: http://jmh.sagepub.com/content/early/recent