(HealthDay News) -- If you think young children spend their summers running around, keeping trim, think again. A new study finds that kindergartners and first-graders get fatter during summer vacations than during the school year, suggesting they're eating more and exercising less when they're out of the classroom.

And the extra pounds aren't normal growth spurts: During the summer, children gain weight at twice the rate as during the school year, pushing some kids into obesity and making fat kids even fatter.

The study authors don't know for sure why kids gain extra weight during the summer, but they suspect that schools -- with their vending machines and fattening foods -- might not be the culprit after all.

"Our research indicates that people may have overstated the negative influence of schools on children's weight," said study co-author Brian Powell, a professor of sociology at Indiana University.

Powell and his colleagues examined a 1998-2000 survey of 5,380 children in kindergarten and first grade. The kids were weighed twice during each school year, and the researchers calculated their body mass index -- a ratio of weight to height. The study authors then looked for patterns to see if the children were most likely to gain weight during the school year or during summer.

The researchers found that the children gained weight at a faster rate -- double that of the rest of the year -- during summer. Black and Latino children, along with those who were overweight at the beginning of kindergarten, were most affected by the discrepancy.

"We're not saying all children are getting fat, but they're moving up," Powell said. "The extent of their BMI gain is troubling during the summer."

The researchers noted that obesity among kids ages 6 to 11 has tripled in the last 20 years, to 15 percent. They also pointed to earlier studies that found that 5- and 6-year-olds with above average BMI and BMI gains are at greater risk of obesity as adults.

The findings are published in the March issue of the American Journal of Public Health .

Other researchers have found signs of weight problems among young children. In a study released last year that covered the years 1988-1994, researchers reported that children aged 2 to 7 consumed between 110 and 165 calories more than they needed each day, resulting in a weight gain not related to growth of almost a pound a year.

Some might assume that the weight gains found in the new study are just a product of ordinary growth spurts. But such spurts shouldn't be seasonal, Powell said. "Compared to the summer, schools reduce the problem (of weight gain), or at least buffer the problem."

The study authors weren't able to pinpoint what it is about school that is better for children when it comes to weight. But they suspect it has something to do with the controlled environment inside a school, which makes students eat and exercise at specific times.

"People criticize schools for having vending machines, but when you're sitting in class, you don't have the opportunity to get something out of the vending machine whenever you like," said study lead author Paul von Hippel, a research statistician at Ohio State University.

But kids at home can eat whenever they like.

What should parents do? "You need to think about reforms that are going to affect how kids behave when school is out after the bell rings, like nutrition classes that emphasize making smart decisions when you're home," von Hippel said.

More information

Learn more about childhood obesity from the American Obesity Association.

SOURCES: Brian Powell, Ph.D., professor of sociology, Indiana University, Bloomington; Paul von Hippel, Ph.D., research statistician, Ohio State University, Columbus; March 2007, American Journal of Public Health
Last Updated: January 2008
Copyright © 2008 ScoutNews LLC. All rights reserved.