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The key to maintaining normal body weight lies in regular exercise and getting weight under control before puberty.

Even though some teenagers may eat less than their thinner peers, fat cells laid down in the body at an early age never go away.

Lack of exercise and an abundance of fat cells alter body chemistry and create an environment where fewer calories in cause more weight gain.

And obesity expert Matthew Gillman of the Harvard School of Public Health says the amount of physical activity kids participate in is key. “When you’re less physically active, you actually need fewer calories to maintain your weight,” he explains.

But if that weight is already higher than it should be, that could signal the beginning of a long-term problem.

“Once you become overweight, there are changes in your body that make you different from someone who’s not [overweight],” explains Sophia Yen of Stanford School of Medicine. “You have extra fat cells, and you have different insulin levels,” which can make it feel like you’re eating less than you are.

Being fit at at early age is crucial and tackling weight gain before it begins can provide a lifetime of good health.