Both obesity and poor cardiovascular fitness are growing problems among children worldwide, Dr. Michael Sjostrom of the Karolinska Institute in Huddinge, Sweden and colleagues note. Physical activity is proven to fight both of these epidemics.

Evidence is mounting that more intense activity may be more effective in preventing excess weight gain, they add.

To investigate how the amount and intensity of activity might be related to obesity and fitness in kids, the researchers looked at 780, 9- and 10-year-old children, measuring their activity levels over four consecutive days using a device called an accelerometer.

The study is published in the American Journal of Clinical Nutrition.

The children who engaged in vigorous physical activity for more than 40 minutes daily had less body fat than those who were this active for just 10 to 18 minutes a day, and also had higher cardiovascular fitness, the researchers found.

There was no association between the total amount of a child's daily physical activity and his or her levels of body fat, but kids who were active for a longer amount of time each day did have greater cardiovascular fitness.

"Our...results suggest that vigorous-intensity physical activity may have a greater impact in preventing obesity in children than lower physical activity intensity levels, whereas both total and at least moderate to vigorous physical activity may improve children's cardiovascular fitness," the researchers conclude.

They acknowledge that vigorous activity may be more difficult for overweight and obese children and teens. The doctors suggest that these individuals begin with moderate activity and gradually increase both the intensity and amount of exercise.

SOURCE: American Journal of Clinical Nutrition, August 2006.