Programs encouraging children to be physically active have little impact on their weight, a review suggests.

British researchers examined 20 studies on after-school and community programs to encourage kids to get active.

"This review provides strong evidence that physical activity interventions have had only a small effect [approximately four minutes more walking or running per day] on children's overall activity levels," Terence Wilkin, a professor of endocrinology and metabolism at Plymouth University Campus in England and his co-authors concluded in this week's issue of the British Medical Journal.

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Physical activity clubs for kids could be replacing periods of time they would usually spend outdoors, researchers say. (John Dunham/The Messenger-Inquirer/Associated Press)

"This finding may explain, in part, why such interventions have had limited success in reducing the body mass index or body fat of children."

Unlike previous reviews, Wilkin's team used objective measurements of kids' physical activity levels using accelerometer devices worn over their hip instead of relying on self reports or estimates from parents.

The physical activity programs resulted in small improvements in time spent in modest-to-vigorous intensity activities like jogging or playing competitive sports.

They said such increases would have a minimal impact on the waist circumference of about two millimetres.

Other physical activity benefits

The study's authors proposed that the activity clubs could be replacing periods of equally intense activity that would usually be spent outdoors. Previous studies suggested that physical activity programs for children don't work because they lead kids to eat more calories.

"However counterintuitive or discomforting it may be, strong evidence from this review shows that physical activity interventions have little effect on the overall activity of children," they wrote. "Organized physical activity may nevertheless still offer benefits such as improved co-ordination skills, greater self confidence, team participation, and social inclusivity."

One of the limitations of using accelerometers to measure physical activity is that water-based activity like swimming is excluded, as is cycling.

Many people believe that lack of physical activity is a major contributor of childhood obesity but the evidence of a link is weak, according to a journal editorial published with the study.

"Perhaps the focus should shift away from looking at overweight and obesity as primary outcomes in physical activity interventions towards outcomes that relate to improved cardiometabolic health in children, regardless of their weight status," editorial writers Mark Hamer and Abigail Fisher of University College London said.

The editorial called the review the best evidence so far on the effectiveness of physical activity programs for children.

But the effect of such programs on reducing sedentary time was not considered, the pair said, adding that emerging evidence points to sitting time as a long-term risk factor for poor health, including diabetes and death from heart disease and stroke.

They concluded that changing indoor and outdoor built environments to foster kids' physical activity is a key area for research.