Physical exercise has long been touted as the best way to combat childhood obesity, spurring governments and school boards to implement programs to get youngsters moving at school.

Perhaps they deserve a failing grade.

A study published yesterday in the Canadian Medical Association Journal found that school-based interventions had no effect on body mass index, a measure of body fat, suggesting that increased physical activity at school may not be the best way to stall the growing waistbands of Canadian children.

"It's definitely a misnomer to say that these programs alone will solve childhood obesity," said Kevin Harris, a pediatric resident at B.C. Children's Hospital and lead author of the report, which analyzed 18 studies involving over 18,000 children in school-based physical education programs that lasted between six months and three years.

Finding effective strategies to combat childhood obesity has baffled and frustrated researchers and educators for years. While the alarming increase in obesity among North American children has inspired governments and school boards to implement policies that promote physical activity and better nutrition - proven methods of prevention or changing bad habits have largely been elusive.

One of the main reasons is that scientists still aren't sure why children become obese, says Tom Baranowski, a psychologist and professor of pediatrics at Baylor University's medical school in Houston, who has been researching obesity-prevention programs for children since the 1980s.

Many school-based intervention programs assume that the roots of childhood obesity are too much food, too little exercise. But recent research has shown that the underlying causes are much more complex, Dr. Baranowski said.

"It's not clear just how big a role physical activity plays in combatting weight gain in children," he said. Other factors such as genetics, behavioural problems, mental health, parental influence and the amount of time spent watching television may also play a significant role.

In a paper published in the January, 2009, issue of the International Journal of Behavioral Nutrition and Physical Activity, he argued that researchers need to focus more on understanding those root causes. "We need to stop jollily going forward into interventions before we understand what the problem is," he said.

That understanding could come from more targeted studies on specific factors that could be driving the obesity epidemic such as the increasing consumption of sugary drinks. Or, whether family-based interventions might have a better chance of affecting a child's weight.

Dr. Harris agrees: "I think these policies are well intentioned. But perhaps we need to do more to understand the complex causes of childhood obesity so we can develop effective interventions."

His study, conducted by several B.C. researchers, is not the first to highlight the ineffectiveness of school-based programs on BMI. Other major meta-analysis, including those done in Britain and the U.S., have found similar results, said Dr. Baranowski.

Still, the Canadian researchers emphasized that even though these physical education programs may not be a cure-all for obesity, they can still have a positive impact on other measures, including reducing blood pressure and increasing lean muscle mass, bone mineral density, aerobic capacity and flexibility.

"We don't want people to get the wrong message," Dr. Harris said. "Physical activity is very important."

Kieran Kennedy, principal at East Front Public School in Cornwall, Ont., knows this all too well. Alarmed by the high rate of childhood obesity at his school, he has introduced several programs. Children at his school now get 38 minutes of physical education a day, have healthy snacks at their fingertips all day long, and are given a list of exercises to do at home after school.

Yet, despite these efforts, which have been recognized with awards from the Canadian Association of Physical Education, he says obesity is still an issue for his students.

"All the other factors in society are dwarfing that really good 38 minutes," he said, listing things like too much sweets, fast food and too much television.

But even if BMI isn't affected, these programs are having an impact on children's lives, said David Barnum, co-ordinator for Healthy Buddies, a school-based program developed by researchers at B.C. Children's Hospital, which focuses on older children teaching younger peers about good nutrition and exercise. The program has been implemented in dozens of schools in B.C. and Alberta, and will spread to Manitoba next year.

A study on Healthy Buddies' pilot program - published last year in the journal, Pediatrics - found it had a measurable impact on children's self-esteem and knowledge about healthy living, he said. Hopefully, it will lead to behaviour changes that last a lifetime.