Papa's got a brand new bag around his waist. (James Day Photography Ltd/Getty) Papa's got a brand new bag around his waist. (James Day Photography Ltd/Getty)

Dad bod is real. Science says so.

Men who become fathers experience weight gain and an increase in body mass index, a measurement of body fat based on height and weight, according to a new, large-scale study that tracked more than 10,000 men over a 20-year period. Men who didn't become dads actually lost weight over the same time period.

The findings by Northwestern University's Feinberg School of Medicine were published Tuesday in the American Journal of Men's Health. Researchers describe the study on fatherhood and young men's BMIs as the first of its kind.

"Fatherhood can affect the health of young men, above the already known effect of marriage," lead author Craig Garfield, a Northwestern associate professor, said in a release. "The more weight the fathers gain and the higher their BMI, the greater risk they have for developing heart disease as well as diabetes and cancer."

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Weight gain differed for dads who lived with their children ("resident dads") and those who didn't. First-time resident dads experienced an average 2.6 percent increase in their BMIs over the study period. Non-resident dads experienced 2 percent increase. That translates to a 4.4 pound weight gain for a 6-foot-tall dad who lives with his child and a 3.3 pound weight gain for a non-resident dad. Meanwhile, a similar 6-foot-tall man who had no kids? He lost 1.4 pounds.

The BMI increase may be the result of lifestyle changes, researchers said.

"You have new responsibilities when you have your kids and may not have time to take care of yourself the way you once did in terms of exercise," Garfield said. "Your family becomes the priority."

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Starting in 1994, BMI measurements were taken of 10,253 men at four different stages in life, ranging between early adolescence and the early 30s. While BMIs generally change over time as men age, the way these men's changed depended on whether they were dads.

Researchers controlled for other factors that may account for differences in weight gain, such as age, race, education, income, daily activity and marriage status, which is already known to be tied to weight gain.

While the BMI difference appears to be small, researchers write that the estimates might be "conservative."

The findings underscore the need to focus on preventive strategies for new dads, especially since a father's weight can also influence children's health outcomes, researchers write.

"We now realize the transition to fatherhood is an important developmental life stage for men's health," Garfield said.