Lead investigator Dr. Catherine Davis of the Medical College of Georgia said exercise isn't a magic wand that turns overweight children into lean children but although the children are still overweight they have less fat, a healthier metabolism and an improved ability to handle life.

The study participants ages 7 to 11 learned about healthy nutrition and the benefits of physical activity. One-third exercised 20 minutes after school, another one-third exercised for 40 minutes and another one-third did nothing different. The children played running games, with hula hoops and jump ropes, raising their heart rates to 79 percent of maximum -- considered vigorous psychical activity.

Functional magnetic resonance imaging scans, which show brain activity, were performed on a percentage of children in each group and the scans found those who exercised had different patterns of brain activity during an executive function task.

"We hope these findings will help persuade policymakers, schools and communities that time spent being physically active enhances, rather than detracts, from learning," Davis said.

The findings are being presented at The Obesity Society’s annual scientific meeting in New Orleans.