By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) - We know we have an obesity problem in this country and for parents of young children who think the problem will disappear during the teen years, don’t be so sure of it. Children’s activity levels decline as they reach adolescence.

Researchers followed more than 1,000 children for six years. They tracked their typical activity levels with a monitor worn around the waist for about a week during each year measured. They were measured at ages 9, 11, 12 and 15. At 9 years-old, children engaged in 3 hours of moderate to vigorous activity per day on both weekdays and weekends, but each year that activity decreased. By the time they were 15, physical activity levels dropped to about 49 minutes on weekdays and, believe it or not, 35 minutes on weekend days.

I have long been a supporter of gym class in school and expanded after school sports opportunities – not just for the best athletes. This is another reason it makes sense.