Fast-food restaurants have taken the heat for this country's obesity epidemic as people complain that huge portions and fattening ingredients have led to the super-sizing of Americans. Three quarters of all adolescents eat fast food once a week, and consumption has been increasing since the 1970s, leading many to blame fast food for childhood obesity. But lean children also consume fast food, and researchers at Children's Hospital in Boston wondered why some teenagers gain weight while others do not.

What the researchers wanted to know: How does fast food affect the total calorie consumption of overweight versus lean teenagers?

What they did: The researchers studied the eating patterns of about 50 teenagers, half overweight and half lean, in two separate studies. Overweight teens were defined as those who had a body-mass index of higher than the 85th percentile for their age according to the Centers for Disease Control and Prevention weight charts. First, they fed the teenagers in groups of four who were the same sex and similar weights an hourlong lunch at a food court of chicken nuggets, fries, cookies, and cola and told them to eat as much as they wanted. In the second study, done completely separately from the first, the researchers contacted the same teenagers and asked them what they had eaten the day before. They continued calling each participant (an average of seven times) until they had information for each person on two days of eating a fast-food meal at one of the five major chains—McDonald's, Burger King, Wendy's, KFC, or Taco Bell—and two days of not eating fast food.

What they found: At the experimental fast-food meal, overweight teens consumed more than lean teens, averaging 1,860 calories versus 1,458. Still, even the lean teens were eating 57 percent of their recommended daily calorie intake in one sitting—way more than is healthy. But the biggest difference that the scientists found was when they looked at the total daily calories eaten by both groups. Overweight adolescents tended to eat more calories on days when they ate fast food, whereas lean teenagers compensated for additional calories from fast food by eating less at other meals, so their daily intake did not differ between fast-food and non-fast-food days.

What it means to you: The authors suggest that overweight teens are not as likely as other teens to compensate when they overeat fast food by consuming less at other meals. They do not condemn fast food but do say that it is important for teenagers (and others, for that matter) to watch how much they eat, or to compensate by eating less at other meals. Fast-food intake for one meal should not make up more than 30 percent of the daily intake of calories, about 800 calories for most people.

Caveats: This study does not prove that fast food causes obesity. From the results, it is impossible to say whether overweight teens eat more because they are overweight or if they are overweight because they eat more. In addition, the researchers relied on self-reporting for a good portion of their data, and teenagers, especially overweight teenagers, may have said they ate less than they actually did. The researchers tried to correct for this bias, but it still could have influenced the study.

Find out more: The KidsHealth website, a service of the Nemours Foundation, which was founded to improve children's health, includes a section for teens. That section has a great page on calculating body mass index and what to do if you are overweight.

For parents, the National Institute of Diabetes and Digestive and Kidney Disorders, part of the National Institutes of Health, has a page with information on how to help an overweight teen.

Read the article: Ebbeling, C.B. et al. "Compensation for Energy Intake From Fast Food Among Overweight and Lean Adolescents." Journal of the American Medical Association. June 16, 2004, Vol. 291, No. 23, pp. 2828–2833.

Abstract online: http://jama.ama-assn.org