Young children who don't get much exercise have a greater chance of heart problems, with symptoms showing up as early as the teen years, researchers from the University of North Carolina at Chapel Hill said.

Nearly 400 children were studied when they were in grade school and seven years later when they were teens. Their height, body mass, body fat, blood pressure and cholesterol levels were checked, along with surveys about activity.

In the later test, 4.6 percent had three signs of metabolic syndrome, When the same children were examined again seven years later, 4.6 percent had three or more characteristics of metabolic syndrome, which is a collection of abnormalities that are linked to insulin resistance. It's meant to identify people who are at increased risk for heart attacks, strokes and peripheral vascular disease.

Previous studies have found that somewhere from 4 percent to 9 percent of adolescents have the condition, researchers said in a news release.

Robert McMurray said that adolescents with the syndrome were six times more likely to have had low aerobic fitness as children and five times more likely to have low levels of physical activity at the time they joined the study.

Low levels means no vigorous exercise and less than 20 minutes a day of moderate activities such as walking or riding a bike.

"This shows efforts need to begin early in childhood to increase exercise," McMurray said. "Children today live a very sedentary life and are prone to obesity. This is the first study to examine the importance of childhood fitness levels on your metabolism as a teenager. Previously we didn't know if low fitness levels were an influence," said McMurray.

The findings were published in the open access journal Dynamic Medicine.