SAN DIEGO – The time children spend biking, running or doing other exercise drops sharply between ages 9 and 15, putting them in danger of becoming obese and developing chronic health problems, according to a major national study led by UCSD.

The report appears in Wednesday's edition of the Journal of the American Medical Association.

Researchers measured physical activity with small devices called accelerometers that were attached to the waistbands of about 1,000 children in 10 cities. The purpose was to accurately measure the youngsters' exercise levels, rather than rely on estimates from them or their parents.

Children should engage in moderate to vigorous physical activity for at least an hour each day, the U.S. Department of Agriculture and many health experts recommend.

At age 9, children in the study spent an average of about three hours doing such exercise. Less than one-third of them met that benchmark at age 15.

“I wonder what will it take for (policymakers) to wake up and notice this is a problem?” said Dr. Philip Nader, the study's lead author and a professor of pediatrics at the University of California San Diego.

He said there have been so many reports highlighting poor childhood diets, rising numbers of overweight and obese children, and the growing number of schools cutting physical education classes from their curriculum.

When he was a child in San Francisco, Nader added, “I would get on my bike and ride all day. ... You could explore things and sometimes have social activity with friends and learn self-reliance, how to get around. These are things that kids are missing today, and it could mean a problem for their health in years to come.”

Nader worked with colleagues at the University of Arkansas, the University of North Carolina at Greensboro and RTI International.

They tracked the children from 2000 to 2006. The youngsters wore accelerometers, which measured minute-by-minute physical activity, for at least one week at ages 9, 11, 12 and 15. They received up to $100 for their participation.

The federal government funded the study, which is described as one of the largest and most comprehensive on the topic.

While the new study does not focus on obesity, Nader said it's well established that low levels of physical activity heighten a child's risk of becoming overweight or obese.

In time, excess weight can lead to heart disease, diabetes, high blood pressure and other health problems.

“While there have been many gains in life expectancy for today's adults, there are concerns that because of inactivity and obesity, children may have a shorter life expectancy,” Nader said.

His report comes just a few weeks after a large pediatricians' group recommended that children as young as age 8 be regularly screened for cholesterol and take drugs to lower their cholesterol levels if they're too high.

An important finding in the new study is that levels of physical activity declined more on the weekend than during weekdays. It suggests that school activities may encourage some exercise, while children may spend additional time on the couch or in front of the computer during weekends, Nader said.

“If we just took walks with our family after dinner, or went to a park,” children would not only get more exercise but also be able to spend more time with their families, he said.

The study is significant, said Dr. Robert Ross, head of the California Endowment. His nonprofit organization has given numerous grants in recent years to programs designed to create communities that are safe and more conducive to walking and bicycling.

“It's easy to lecture a kid or a parent that a child needs 45-60 minutes of exercise every day. But that kid's environment has to be supportive of that strategy, and right now, our environments just are not,” Ross said.

He said society must find remedies, such as encouraging computer software makers to build more games that stimulate exercise.

In San Diego County, the Childhood Obesity Initiative is helping to launch a number of projects intended to make children want to go outside and exercise, said Cheryl Moder, director of the initiative.

A program in La Mesa includes a $550,000 grant to build a quarter-mile pathway linking La Mesa High School with a residential neighborhood.

The initiative is supporting another program in which physicians write prescriptions for exercise to children at risk of becoming obese. Those prescriptions can be used to obtain discounts or free admission to designated recreation centers.

Another effort is underway to persuade school districts in the county to use physical education time for physical activity, rather than for driver's education or health clubs. Walking clubs are being formed at many schools, Moder said.

“Most kids don't get daily PE,” she said.

She added that it may seem like reversing the trends of increased inactivity and consumption of junk food, diabetes, may seem insurmountable, with one-third of children being overweight or obese across the U.S. and in San Diego County.

“But fighting this is just like any major public health change,” Moder said. “When we were kids, there were no seatbelts, our parents smoked cigarettes – and look at how our culture has since changed.”

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