Want your kids to be slim? Make them run

Washington: Just 15 minutes a day of kicking around a ball or swimming might be enough to keep children from becoming obese, British and US researchers said on Monday.

A study of 5,500 children who agreed to wear a motion sensor device showed that those who exercised more were less likely to be obese, and that short bursts of intense activity seemed to be the most helpful.

Children who did 15 minutes a day of moderate exercise equivalent to a brisk walk -- were 50 per cent less likely than inactive children to be obese, the researchers reported in the Public Library of Science journal PLoS Medicine.

"Our data suggest that higher intensity physical activity may be more important than total activity," Andy Ness of the University of Bristol and colleagues wrote.

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

"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."

Obesity is on the rise in many countries, including the United States, where 60 per cent of the population is overweight or obese, Britain and elsewhere in Europe.

It is clearly a matter of people eating more calories than they burn off, but experts cannot agree whether diet or exercise is more important -- and which kind of exercise might be best.

Ness' team studied 5,500 children, with an average age of 12, who with their mothers have been taking part in a larger, long-term study of health.

The children agreed to wear a device called an accelerometer, which measures total activity, and they had X-ray scans for body fat.

The researchers rated the children with the top 10 per cent levels of fat mass as obese.

The less the children exercised, the more likely they were to be obese, the study found.

"These associations suggest even a modest increase of 15 minutes moderate and vigorous physical activity might result in an important reduction in the prevalence of overweight and obesity," the researchers wrote.