Slim teenagers who fill up on fast food consume less food throughout the rest of the day, while their overweight peers don't adjust their eating habits to compensate for the extra calories, according to a study in the Journal of the American Medical Association.
Fat or thin, all the teens in the study over-ate when offered unlimited fast food, suggesting how the high-fat, high-calorie meals popular at restaurants such as McDonald's and Burger King, the world's two biggest fast-food chains, contribute to rising obesity rates, researchers said.
Children's fast-food consumption has ballooned sixfold since the 1970s, and three of every four now eat it at least weekly, said David Ludwig, senior author of the paper at Boston's Children's Hospital.
The study sheds light on why some teenagers who regularly eat fast-food meals aren't overweight even as others gain kilos.
"This study, together with a half-dozen others, raises concern that fast food could be an important cause of obesity," said Ludwig, a paediatric endocrinologist.
"We know more about zinc, which is a trace nutrient, than we do about the effect of fast food on health, which is the dominant dietary pattern among children."
Fast-food restaurants have been adding healthier food choices, offering everything from salads to fruit and yoghurt.
More options may be in the works, some said.
"We continue to provide a wide range of options to meet our customers' varied diet and lifestyle needs," Wendy's spokesman Bob Bertini said.
"We have been actively testing now for some time some alternative menu options."
In a separate study in the same journal, Government researchers said two out of every three American adults were overweight at the start of the 21st century, a plateau in the nation's obesity epidemic showing recent efforts to promote healthy eating and exercise were not yet paying off,
The first part of the fast-food study took place in a suburban Boston food court, where the 54 volunteers were given an unlimited fast-food lunch.
Overweight teens ate substantially more than their lighter counterparts, consuming 1860 calories on average compared with 1458 calories.
That was more than half the daily food needs for both groups, the researchers said.
The second part of the study compared how much the teens ate on a regular day that included a fast-food meal to a day without fast food.
The information was gathered during surprise telephone interviews to reflect normal eating patterns.
The overweight teens ate significantly more on days when they had fast food, taking in 2703 calories on average compared with 2295 calories on days without McDonald's, Burger King, KFC, Wendy's and Taco Bell, the study found.
The average-weight teens, whose body mass ranked in the 65th percentile for children their age, had an uncanny ability to consume the same amount of calories regardless of food type.
They consumed 2575 calories on average when eating fast food, versus 2622 when they didn't have the high-fat foods.
"When the lean kids have fast food, they decrease their consumption of other foods by a precise amount, so their calorie intake on days with or without fast food are virtually identical," Ludwig said.
"The obese kids don't seem to have that ability, suggesting they have difficulty compensating for the massive portion sizes characteristic of fast food today."