Three out of four American teens eat fast food at least once a week. So why is it that some teens can eat fast food and remain lean, while others who eat it gain weight?

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A study published in Wednesday's issue of the Journal of the American Medical Association suggests that food compensation is the key.

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Tara Searight, 16, eats fast food about once a week. She especially loves fries.

"Every time I eat fast food, I keep thinking, 'This is awful for me. I know this is awful for me and I just can't stop because it tastes so good,'" Searight said.

But she stays at a healthy weight, in part by compensating for the fast food she eats.

"If I have fast food for lunch, I'll definitely try to have something healthier for dinner like a salad, or put in some extra fruits and vegetables," Searight said.

That kind of compensating is one difference between lean teens and overweight ones, according to researchers at Children's Hospital in Boston and the University of Minnesota.

"When lean adolescents eat a large fast-food meal, they seem to compensate for the calories in the meal by eating less of other food throughout the day," said Dr. Cara Ebbeling, of Children's Hospital. "In contrast, obese adolescents do not seem to compensate."

In the study of 54 teenagers, researchers measured how many calories they consumed during a large fast-food meal. They also tracked how many calories were consumed during days when the teens ate fast food compared to days when they didn't. Both normal-weight and overweight teens participated in the study.

The study found that during a single large fast-food meal, with its high fat, starch and sugar and low fiber content, teens consumed 60 percent of their daily calorie needs.

"Obese adolescents had a particular problem compensating for the calories in a large fast-food meal, such that they consumed 400 more calories on days when they had a fast-food meal compared to days when they did not," Ebbeling said.

Ebbeling said it's not known if lack of compensation is why teens become obese or if teens lose the ability to compensate once they become obese. She also said there's no question that all teens should eat less fast food.

Study Abstract