All teenagers seem prone to overeat fast food but overweight ones are less likely to cut back in their other meals when they do, a new U.S. study suggests.

The researchers noted fast food consumption by children has increased in tandem with an obesity epidemic.

Canadian overweight, obesity statistics

The 2003 Canadian Community Health Survey found 19.5 per cent of young Canadians are overweight and an additional 7.8 per cent are obese.

Almost 15 per cent of Canadian adults are considered obese, and another 33 per cent are considered overweight. Both figures are up one per cent since the last survey in 2001.

Those aged 45 to 64 had the highest obesity levels: 37.9 per cent are overweight and 18.5 per cent obese, according to the study released June 15.

The survey used the Body Mass Index, which determines total body fat based on a person's reported weight and height.

Source: Health Indicators June 2004: Statistics Canada

Assuming fast food contributes to obesity, the team wanted to test if overweight teens are more susceptible to the harmful effects of fast food than lean teens.

To find out, Cara Ebbeling of Children's Hospital in Boston and her colleagues fed 54 adolescents aged 13 to 17 extra large fast food meals at a food court. Participants could eat as much as they wanted in one hour.

In a second study, the researchers looked at how much food 51 of the unsupervised participants consumed in two days when they ate fast food and two days without fast food.

The scientists found average energy intake from the fast food meal in the first study made up almost 62 per cent of the estimate daily energy requirements for all the adolescents.

In the second study, overweight teens consumed significantly more total energy on the fast food days than non-fast food days, but the lean participants consumed virtually the same amount on both days.

"This observation suggests that overweight individuals do not compensate completely for the massive portion sizes characteristic of fast food today," the study's authors wrote in Wednesday's issue of The Journal of the American Medical Association.

"These findings suggest that, at least, fast food consumption serves to maintain or exacerbate obesity in susceptible individuals."

It's estimated 75 per cent of adolescents eat fast food at least once per week, the study said.