Keeping your teenagers in shape depends on the calories they consume and their activity. And if they’re like the average American teen, they need all the help they can get.

Overweight teens consume 110 to 165 more calories than required each day, which translates into an extra 10 pounds around their waistlines that will only get bigger over time, nutritionists say.

In the past four decades, obesity rates have more than tripled for children 6 to 19. One-third of all American children either suffer from obesity or are overweight with high risk of becoming obese, according to federal health statistics. But Harvard researchers suggest that preventing excess weight gain in your kids during childhood may be more effective than trying to deal with the problems when they’re teenagers.

In fact:

•  Having class three times a week instead of once can mean a difference of 240 calories per week for a typical 9-year-old boy.

•  One hour of TV watching replaced by one hour of slow walking means burning 55 calories.

•  Reducing TV viewing by one hour is associated with 160 fewer calories consumed per day.

Support physical education classes

Here’s what you can do to keep your children, regardless of their ages, in shape:

•  Support longer and more frequent physical education classes that require them to remain physically active for at least half the class.

•  Cut their consumption of sugar-sweetened beverages in schools and at home. Switching one can of soda for water could mean slashing 150 calories.

•  Having them eat less fast food to save 120 calories a day.

•  Decrease the time they spend watching TV as a way to increase physical activity.

•  Limit exposure to advertising, which has been associated with increased energy intake.