For kids, time spent inactive seems less of a factor in higher body fat than does a lack of exercise, according to a new study.
Researchers found that the more minutes kids spent exercising at the pace of a fast walk each day, the lower their body fat percentage was. But the time they spent as couch potatoes made no difference, according to results published in the Journal of Pediatrics.
"Our study supports the current physical activity guideline, that's what I want people to know," lead author Soyang Kwon, a pediatric researcher at Northwestern University in Chicago, told Reuters Health.
The U.S. Department of Health and Human Services recommends that children and teens exercise at least at the intensity of a fast walk, about 3.5 miles per hour, for 60 minutes every day.
Last spring 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 kids 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 kids 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 kids of various ages from 2000 to 2009.
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. At each age, the exposure to radiation from the test was about equal to one transatlantic flight.
The same children wore an accelerometer, which measures body movement, for several days in a row sometime in the same year.
Even among kids who exercised the least, 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 spent the least amount of time in moderate to vigorous activity had about 11 pounds more body fat, on average, than those who exercised the most.
For 13 year old girls, the low level exercisers had about seven pounds 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 an earlier 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.
But experts caution this doesn't mean kids should camp out in front of a screen for hours at a time. Previous research has shown a link between sedentary screen time and weight gain, probably because watching TV often goes hand in hand with snacking, Ulf Ekelund said.
"Parents should encourage their children to be physically active, the more the better," said 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."
"I support parents establishing rules that limit TV and videogames," but more is needed, said Pate, who was not involved in the new study. "If we get kids away from the screen that doesn't mean that they start to jump up and do vigorous activity."