A brisk, 15-minute walk daily may be enough to help prevent childhood obesity, British researchers say.

The study in this week's issue of PloS Medicine looked at 5,500 children who wore a motion-sensing device and had their body fat measured with an X-ray scanner, which offers more precise measurement than the body mass index.

'It's been really surprising to us how even small amounts of exercise appear to have dramatic results.'—Prof. Chris Riddoch

Among children who increased their physical activity levels by 15 minutes a day of moderate exercise, the chances of being obese were lowered by almost 50 per cent, the researchers found.

"This study provides some of the first robust evidence on the link between physical activity and obesity in children," said study author Prof. Chris Riddoch of Britain's Bath University.

"We know that diet is important, but what this research tells us is that we mustn't forget about activity. It's been really surprising to us how even small amounts of exercise appear to have dramatic results."

Without accurate measurements of activity levels and body fat, it has been difficult for researchers to determine the relative importance of activity in preventing obesity in children, compared to dietary changes.

Easy to find 15 minutes

Most families would be able to add 15 minutes of moderate exercise a day as a starting point, said Prof. Andy Ness of the University of Bristol, who co-directed the research with Riddoch.

The average age of the children in the study was 12. Those in the 10 per cent levels of fat mass were classified as obese.

"Our findings, if confirmed, suggest that public health policies that increase physical activity levels and in particular [moderate and vigorous physical activity] in children may help to reduce the prevalence of childhood obesity," the researchers concluded.

One limitation of the cross-sectional study is that it is possible that instead of becoming obese because of lack activity, obesity may restrict children from being more active, the study said.

Rates of overweight and obese children nearly tripled among Canadians  over the past two decades, the Canadian Institutes of Health Information reported in 2004. Obesity rates are also on the rise in other developed countries.