INDIANAPOLIS (AP) -- The nation's schools, under fire for unhealthy school lunches, well-stocked vending machines and phys ed cuts, may actually do a better job than parents in keeping children fit and trim. A study found that 5- and 6-year-olds gained more weight over the summer than during the school year, casting doubt on the assumption that kids are more active during summer vacation.

The findings don't reveal what's behind the out-of-school weight gain, but the researchers speculate it's because the summer months lack the structure of the school year with all its activities and daily comings and goings.

Doug Downey, an Ohio State University sociologist who co-authored the study, said that for many youngsters, the lazy days of summer may offer plenty of free time to eat snacks and lounge about watching TV or playing video games.

He said the study seems to point to the need for parents to be more involved, as well as raising the idea of a longer school year and more after-school programs to keep children active.

And schools should continue their efforts to promote good health, he said.

"Trying to improve the quality of school lunches, getting the soda machines out of schools - those are still good approaches. But clearly the source of children's obesity problems lie outside of the school," Downey said.

For the study, Indiana University and Ohio State researchers studied the growth rates of the body-mass indexes of 5,380 kindergartners and first-graders. The data came from a National Center for Education Statistics survey that ran from fall 1998 to spring 2000 in 310 schools across the country.

The university sociologists discovered that the youngsters' BMIs increased on average more than twice as much during summer break compared with the school year. That increase was even greater among black and Hispanic students and kids who were overweight at the start of kindergarten.

Once kids were back in school, however, the monthly growth rate of their BMIs fell, and the growth rate gap between the overall population and the minority and overweight groups shrank, the researchers found.

The study will appear in the April issue of the American Journal of Public Health.

Betsy A. Keller, a professor of exercise and sport sciences at Ithaca College in New York, said the pattern seen in the study's snapshot of the kids' kindergarten year, summer break, and first grade is "irregular" and does not mesh with kids' normal growth in height and weight.

Keller said it clearly points to a summer gain in fat mass, although she said data from later school years is needed to see if that trend continues.

Overall, she said the findings point to the need for parents to become actively involved in encouraging their kids to develop healthy habits even as the push continues for schools to focus more on those same goals.

"The big question in my mind is what are the parents doing with these kids during the summer? Unless they're paying attention to their child's level of activity and diet, with each passing summer they're just adding to the risk of them becoming overweight," she said.

"These are 5- and 6-year-olds, after all. So they're not going to the grocery store - it's their parents who are making these choices."

The study's co-author, Brian Powell, a professor of sociology at Indiana University, said earlier studies have indicated that 5- and 6-year-olds with above average BMI and BMI gains are at increased risk for adult obesity.

Some 17 percent of U.S. youngsters already are obese, and millions more are overweight. Obese adults are at heightened risk of developing diabetes, high blood pressure, heart disease and other disorders.

In recent years, criticism has been directed at schools for playing a role in that trend, leading nearly 20 states to enact some form of school nutrition legislation or to emphasize exercise goals.

Nancy Chockley, president of the National Institute for Health Care Management Foundation, said the new research adds to the argument that parents must shoulder more of the responsibility for keeping their children in shape.

Chockley said parents need to make time for regular family bike rides, walks, hikes or other types of exercise during the summer and beyond to help their kids develop good habits.

"I don't think this takes the heat off the schools. I think it spreads the heat around," said Chockley, whose Washington-D.C.-based nonprofit group researches health care issues.

"We ask a lot from the schools, but the fact is that's the easiest environment to reach the most children. Reaching the parents is much harder."

© 2007 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Learn more about our Privacy Policy.