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.

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The findings by Northwestern University's Feinberg School of Medicine were published recently in the American Journal of Men's Health. "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," said lead author Craig Garfield.

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. Nonresident 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 nonresident 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."