(CNN) When John Kinnear's wife became pregnant, her boss warned the soon-to-be father that he gained 10 pounds for every child he'd had.

"I shook it off at the time, and now here I am two kids and 25 pounds later," Kinnear said. It's a good thing we're done having kids!"

Kinnear, who writes the " Ask Your Dad " blog, may have been personally surprised by his path to extra pounds, but a study shows he's not the only dad with such an experience.

In fact, a regular 6-foot-tall man living with his child gained an average of 4.4 pounds after becoming a dad for the first time, according to the study. There was only a slight difference for the nonresident dads. Those fathers gained, on average, 3.3 pounds.

Meanwhile, nonfathers lost 1.4 pounds over the same period.

The results were surprising to the study's lead author, Dr. Craig F. Garfield, especially given his earlier research that fatherhood motivated men to improve their health habits.

"Regardless of what kind of father you were," Garfield said, "Your weight actually increased. So all fathers ended up increasing their weight as they go through fatherhood." Garfield serves as associate professor of pediatrics and of medical social sciences at Northwestern University Feinberg School of Medicine and attending pediatrician at Ann & Robert H. Lurie Children's Hospital of Chicago.

The study accounted for factors such as race, education, income and screen time as well as marriage status , which has been shown to affect weight. Though all the young men's BMI showed an increase over time due to age, additional weight gain was shown to be contingent on fatherhood status, according to the study.

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The study only speculates why fatherhood leads to extra pounds. But Garfield's noted his weakness for finishing his kids' leftover frozen pizza.

Kinnear was not surprised by the study. "I have so many more priorities as a father than I did before I had kids," said Kinnear. "I love to cook and eat healthy, but when it comes to compromises on time I will trade a healthy meal for more time with my kids every time."

While Kinnear admitted it's a common struggle among his friends, it's about accepting a "new normal," for Adrian Kulp, author of the " Dad or Alive " blog. Kulp is a stay-at-home dad to Ava, age 5, Charlie, 4, and Mason, 18 months.

"When you have your first child," Kulp said, "they are your priority. You put everything else on hold, which sometimes means working out and staying active."

He's gained a little "sympathy weight" he added, but Kulp said it's more about a lifestyle change than any extra snacking. Sometimes it's hard to wake up for a 5:30 a.m. run, he said, if the kids get to bed late.

But there's very little literature on how fatherhood influences male health, according to Kermyt G. Anderson, who co-wrote the 2012 book "Fatherhood: Evolution and Human Paternal Behavior," with Peter B. Gray.

Anderson said the new study might be a bit skewed given its narrow age range, which doesn't account for changes past early fatherhood, but overall, the research is interesting because it really pinpoints fatherhood as a contributor to weight gain, isolated from other factors.

"It's common sense in a way," said Anderson, who is an associate professor at the University of Oklahoma's department of anthropology and not affiliated with the study. The study doesn't track lifestyle changes such as decreased physical activity and eating, he said, but those behaviors would affect fathers living with children slightly more, which may account for the difference in weight gain between resident and nonresident dads.

"It's not the biology of fathering a child," Anderson said. "It's after you have a baby. Are you living with that kid? Are you getting up in the middle of the night? Are you spending time with the kid instead of going to the gym?"

"It certainly echoes my own experience," Anderson added, and said there was much less time for eating healthy once he became a dad. He himself has had to work off the 30 pounds he gained since fatherhood.

That connection — and the fact that obesity is on the rise -- calls for a greater conversation around young men's health, according to Garfield, who said this group tends not to regularly see a primary care physician.

As a result, pediatricians are in a good position to counsel dads about taking care of their health, according to Garfield.

"The transition of fatherhood is a magical moment," said Garfield. "We need to find a way to engage men for their improved health," he added, including having doctors and the health care system be there in such a transitional period of their lives.

"It's a two for one," according to Garfield, who explained the importance of fathers setting the precedents for healthy behaviors. "By helping a father take care of himself, he ends up role-modeling for his children."