Feb. 14, 2012 -- Kids who exercise vigorously for more than 30 minutes a day may be at lower risk of heart disease than their peers who don’t break a sweat quite as often.

This is true regardless of how much time the kids spent sitting on the couch.

Kids who exercised at a moderate or vigorous pace for more than 35 minutes a day had lower levels of cholesterol, blood fats called triglycerides, blood sugar or glucose, blood pressure, and a smaller waist size than their counterparts who clocked about 18 minutes of vigorous physical activity each day.

High blood pressure and high blood cholesterol levels are known risk factors for heart disease. Once seen only in adults, many of these conditions are increasingly being diagnosed in children because of skyrocketing rates of childhood obesity.

The new study appears in the Journal of the American Medical Association.

“Kids should be active,” researcher Ulf Ekelund, PhD, says in an email. Ekelund is a group leader in the epidemiology unit of the Institute of Metabolic Science of Addenbrooke’s Hospital in Cambridge, U.K.

This includes moderate-intensity exercise such as brisk walking, outdoor play, dancing, cycling, aerobics, ball games, and other sports. "Parents should encourage these types of activities rather than [only] reducing sedentary time.” But the study did not distinguish whether sedentary time was spent in front of a TV as opposed to other non-active activities.

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Vigorous Physical Activity Boosts Heart Health

The day’s exercise doesn’t have to take place all at once. Exercise can be spread throughout the day as long as the intensity is moderate to high. Computer time can be a reward for vigorous exercise. “It may be easier to encourage children to do an hour of exercise if they know they are allowed to spend a few hours playing computer games,” Ekelund says.

But, he cautions, specific types of sedentary behavior such as TV viewing may need to be reduced, as research has shown that it may lead to unhealthy snacking.

The study included information on more than 20,000 children and teens from 14 studies. Children were aged 4 to 18. Fully 75% of children were normal weight, 18% were overweight, and 7% were obese. Children spent an average of 30 minutes per day engaging in moderate to vigorous physical activity and 354 minutes per day being sedentary.

The more vigorous exercise the children got, the greater the potential health benefits, the study showed. For example, children who got the least amount of vigorous exercise had a waist circumference that was 4 to 5 centimeters greater than those who exercised the most, regardless of time spent being sedentary.

“If this difference persists into adulthood, it may confer a considerable health hazard,” Ekelund says.

20 Minutes More a Day Can Make a Big Difference

To reap these benefits, kids would have to add about 20 minutes of moderate to vigorous physical activity each day.

Eleanor Mackey, PhD, is a clinical psychologist at the Obesity Institute at Children's National Medical Center in Washington, D.C. She says healthy habits such as regular vigorous exercise only stick if they become a family affair. “Do it together to make it fun and not a chore,” she says. “Join the swim team or soccer team or something that is fun and structured and gets on the calendar so nobody misses it.” A brisk walk before or after dinner can also be fun for the whole family.

“Don’t nag kids about exercise,” she says. Adding 20 minutes of moderate to vigorous activity per day isn’t hard. Kids can do jumping jacks during the commercials of their favorite TV show.

Meet them where they are. “You can’t just pick an activity for your kids, because if they don’t want to do it, you will all be miserable.” she says. The earlier you start, the more likely these healthy habits will stick.