Researchers found that the more minutes children spent exercising at the pace of a fast walk each day, the lower their percentage of body fat. But the time they spent lying around made no difference.
"Our study supports the current physical activity guideline, that's what I want people to know," lead author Soyang Kwon, a paediatric researcher at Northwestern University in Chicago said.
The US Department of Health and Human Services recommends that children and teens exercise at least at the intensity of a fast walk, about 5.6 kilometres per hour (3.5 miles per hour), for 60 minutes every day.
Exercise and body fat
Last year, a study in adults found a different result: regular exercise doesn't protect against the dangers of sitting for many hours at desk jobs. This is likely because children are more active than adults overall.
"In adults, where the activity levels are generally less, the time spent sedentary may have more of an effect," said Russell Pate, who studies physical activity in children at the University of South Carolina in Columbia.
Kwon and her coauthors from the University of Iowa used data from a study in that state that followed children of various ages from 2000 to 2009.
How the study was done
A group of 277 boys and 277 girls were measured at eight, 11, 13 and 15 years old for body composition and fat content using a precise X-ray technique originally developed to assess bone density.
The same children wore an accelerometer, which measures body movement, for several days in a row sometime in the same year.
Even among children who exercised the least, the amount of time sitting didn't make much of a difference.
For the 13-year-olds, those who sat less than, more than or equal to the average six and a half hours per day all had about the same body fat mass.
But boys who spend the least amount of time in moderate to vigorous activity had about 5 kilogrammes (11 pounds) more body fat on average than those who exercised the most. For 13-year-old girls, the low level exercisers had about 3.2 kg (7 lbs) more body fat than the exercisers.
Results were similar in every age group, but the researchers did not record if each child was overweight or not.
In a previous study, the same team of researchers found that even light intensity activity, like walking, every day, was linked to lower levels of fat in teens, but not young children.
"Parents should encourage their children to be physically active, the more the better," said Ulf Ekelund, who studies obesity risk factors in children at the Norwegian School of Sport Science in Oslo. "That might sound simple, but the execution isn't so simple sometimes."
(Reuters Health, January 2013)