Vigorous exercise may be an especially good way to keep kids lean, but sitting around, in and of itself, doesn’t appear to have a major role in making them fat, new research shows.

Nevertheless, there are still plenty of reasons to avoid too much sedentary “screen time,” Dr. Ulf Ekelund of the MRC Epidemiology Unit in Cambridge, UK and colleagues say, given potential negatives including “violence and aggressive behavior, poor academic performance, and poor body image.”

To help tease out the role of time spent in different types of activity in making children fat, independent of screen time and otherwise being a couch potato, Ekelund and his team looked at 1,862 children 9 to 10 years old, 23 percent of whom were overweight or obese.

Using a wristwatch-like device to measure the amount and intensity of activity children got throughout the day, the researchers looked for associations between this activity and children’s waist size, amount of body fat, and body mass index (BMI). Kids also reported how much time they spent watching TV or using a computer.

Sixty-nine percent of the children were getting at least an hour of moderate physical activity a day, while 58 percent reported having less than two hours of screen time daily.

While children who spent more time not moving had bigger waists and a larger percentage of body fat, much of this relationship could be attributed to the fact that they spent less time engaging in moderate physical activity.

But the time children spent engaging in vigorous activity, and their combined moderate activity-vigorous activity time, had the strongest ties to waist circumference and fat mass.

For instance, every 6.5 minutes a child spent doing vigorous activity like playing ball, bicycling, or running around outside was associated with a 1.32-centimeter reduction in waist size, the researchers found. But 13.6 minutes of moderate physical activity only reduced waist size by half a centimeter.

Based on the findings, the researchers say, children should still be encouraged to limit their sedentary time, but this alone won’t be enough to tackle childhood obesity.

“Interventions may therefore need to incorporate higher intensity-based activities to curb the growing obesity epidemic,” they conclude.

Boys in the study got an average of a half-hour of vigorous activity each day, while girls got 22 minutes. “There is no clear cut answer” as to how much vigorous activity is optimal, Ekelund noted in an email to Reuters Health.

“For most health outcomes, the more activity you do the better.” But, he added, people who do lots of strenuous activity may still put on too much weight if they take in too many calories.

SOURCE: American Journal of Clinical Nutrition, November 2009.