Men not only gain "a new sense of responsibility and purpose" when they become fathers, they also pile on about 5 pounds, The Associated Press reported.

Researchers from Northwestern University found that regardless of whether they live with their children or not, men gain between 3.5 to 4.5 pounds after they become fathers for the first time.

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

For the 20-year study, researchers tracked the weight of more than 10,000 men from adolescence to young adulthood. Over the years, the study participants had their BMI measured at four different time points: early adolescence, later adolescence, mid-20s and early 30s. Each participant was categorized either as a non-father, resident father or non-resident father. Then researchers looked at each person's BMI at each time point and took the average of all those measurements to determine whether their fatherhood status was associated with their BMI.

They found a difference in the weight gained by fathers who lived with their children and those who didn't, The Washington Post reported. They found that the typical 6-foot-tall man who lives with his child gained an average of about 4.4 pounds after becoming a first-time dad; the 6-foot-tall dad who does not live with his child gained about 3.3 pounds, the study reports. That's a 2.6 percent rise in BMI for resident dads and a 2 percent rise in BMI for non-resident dads after controlling for other variables.

Researchers believe the new fathers' weight gain may be due to changes in lifestyle and eating habits.

"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."

The findings are detailed in the American Journal of Men's Health.