July 15 (Bloomberg) -- Children in the U.S. become more sedentary as they become teenagers, with 15-year-olds engaging in less than an hour a day of physical activity, increasing their risk of obesity, a study found.

By the time the children in the study reached 15, they spent an average 49 minutes a day during the week and 35 minutes on the weekends doing moderate-to-vigorous activity, including walking, riding bicycles, swimming or jogging, compared with about three hours a day when they were 9, according to research in tomorrow's Journal of the American Medical Association. The U.S. government recommends children exercise for at least an hour a day.

About 25 million U.S. children and teens are estimated to be overweight or obese, leaving them at a higher risk for diabetes and heart disease, including high blood pressure. Changes in the home, in which more children stay indoors after school as part of parent-working families, and at school, as physical education classes are reduced or eliminated, may account for some of the drop in exercise as kids become teenagers, lead author Philip Nader said.

``Parents have to really take a hard look at what their own activity patters are and really plan things as a family. Even talking a walk 15 minutes a day would really start reversing some of these trends,'' said Nader, a doctor and professor emeritus at the University of California, San Diego, School of Medicine, in a July 11 telephone interview. ``Schools are still cutting out physical education and recess. Parents should get active about that. They shouldn't allow that to happen.''

Ten Cities

Researchers in the study followed more than 1,000 children from 10 U.S. cities. The children, who were 9 years old when the study began, were asked to wear an accelerometer, which records minute-by-minute movement, for a week each at ages 9, 11, 12 and 15.

When the children were 9 years old, almost all of them were ``well above'' the recommended 60 minutes of physical activity during the week and on the weekends, researchers said. By the age of 15, only 31 percent met the guidelines during the week and 17 percent on the weekends.

The study did not include time spent playing organized sports.

Boys were more likely than girls to be physically active. Girls on average dropped below the recommended activity level at 13.1 years for weekdays and 12.6 years on the weekends. Boys dropped below the level at 14.7 years for weekdays and 13.4 years on weekends, the study found.

Public Health Goal

``Lack of physical activity in childhood raises the risk for obesity and its attendant health problems later in life,'' said Duane Alexander, a doctor and director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, in a statement by the agency. ``Helping American children maintain appropriate activity levels is a major public health goal requiring immediate action.''

About 186,000 children and adolescents younger than 20 have diabetes, and 2 million teenagers have blood glucose levels higher than normal, a condition called pre-diabetes, according to the American Diabetes Association.

A U.S. government study in May showed that the soaring rates of childhood obesity steadied after 1999, but the researchers said more data is needed before they can say a reversal in the increases has occurred.

That study found about 32 percent of children ages 2 to 19 were at risk for being overweight or obese from 2003 to 2006, little changed from 1999. Of those, 16 percent were overweight or obese and 11 percent were considered the heaviest kids. Future research should focus on how much physical activity is needed to improve a child's health and why kids don't exercise more, the authors of today's study wrote.

``A culture change will be required,'' Nader said, to encourage teenagers to exercise more and give them the opportunities to do so.

To contact the reporter on this story: Nicole Ostrow in New York at nostrow1@bloomberg.net.

Last Updated: July 15, 2008 16:00 EDT