"Although the physical activity interventions in the studies we analyzed were not successful in improving BMI (body mass index), the underlying reasons for failure were unclear," says Dr. Kevin Harris of B.C. Children's Hospital and his co-authors. They suggest the amount of activity may have been insufficient to improve BMI -- a formula that measures body fat based on height and weight -- or that other factors may have more influence on BMI.

"It is therefore important to promote school-based physical activity for its demonstrated health benefits, even though there is currently no evidence that it is an effective method to reverse the trend of increasing BMI in children," they conclude.