Fatherhood makes men fat. That's the conclusion of a study from Northwestern University Feinberg School of Medicine which found men gain weight after becoming fathers, even if they don't live with their children.

The study tracked 10,000 men from adolescence to young adulthood, and found that the average 6-foot-tall man who lives with his child gained an average of 4.4 pounds after the birth of his first child. The same-sized dad who doesn't live with his child gained about 3.3 pounds.

The weight gain translates into a 2.6 percent rise in body mass index (BMI) for resident dads, and a 2 percent rise for dads living elsewhere. But a man in the same group who did not become a dad lost 1.4 pounds over the same time period.

The weight gain was in addition to the increase that usually results from marriage.

"Fatherhood can affect the health of young men, above the already known effect of marriage," said Dr. Craig Garfield, lead author and associate professor of pediatrics and of medical social sciences at Northwestern University Feinberg School of Medicine. "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."

He speculates that new dads' weight gain is due to a change 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."

In addition, the house may be filled with junk food that was in short supply before kids. And then there are the dads who clean their kids' plates. "We all know dads who clean their kids' plates after every meal," Garfield said.

Many new dads don't have personal physicians, he says, but they accompany their children to the pediatrician. "This is an opportunity to talk about things that are important for dad's health and the child's health and to offer dads nutritional counseling and mental health education," Garfield said.

"We now realize the transition to fatherhood is an important developmental life stage for men's health," he said. "It's a magical moment where so many things change in a man's life. Now the medical field needs to think about how can we help these men of child-rearing age who often don't come to the doctor's office for themselves."

The study was published in the American Journal of Men's Health.