Researcher Lisa Sutherland of the University of North Carolina at Chapel Hill analyzed federal data on the diet, weight and physical activity of teens, ages 12 to 19. From 1980 to 2000, calories eaten rose 1 percent and obesity rose 10 percent, while physical activity dropped 13 percent.
Those percentages show that teenagers must have been getting fat primarily because they burned fewer calories. "If caloric intake is flat and physical activity is declining, there is a cause and effect relationship there," Sutherland said.
She presented her findings last month in San Diego at a scientific conference of the Federation of American Societies for Experimental Biology. However, although other experts accept the idea that teens have become less active, the experts find it hard to swallow the conclusion that teens have not been overeating as well.
Sutherland looked at three large federal surveys. Data on weight came from the National Health and Nutrition Examination Survey, and data on physical activity was from the Youth Risk Behavior Survey, both maintained by the Centers for Disease Control and Prevention. Data on caloric intake was from the Nationwide Food Consumption Survey maintained by the Agriculture Department.
The study said that teenagers ate an average of 2,290 calories a day over the 20 years. It also said that while 42 percent of teens reported doing at least 30 minutes of physical activity on a typical day at the start of the study, only 29 percent did at the end.
The study was funded by an unrestricted grant from the National Soft Drink Association. But Sutherland said that in keeping with university rules, the association had no control over any aspect of the research.
"I was trained as a nutritionist," Sutherland said. "The data kept coming out that caloric intake was basically flat, but there was a huge drive to look at diet. I said, 'Let's look at physical activity."'
It's not surprising that teens have become less physically active, Sutherland said. Today's kids have more and better computers and video games, and less school physical education or after-school play, she said.
"I remember wanting to go outside the minute the sun came up, and my parents dragging me to go inside at sunset," said Sutherland, who is 35.
She noted that her study was limited because the three surveys had differing methodologies, and the decline in physical activity was based on students' self-reports.
While they accept Sutherland's idea that teenagers are burning too few calories, some outside observers think the report underestimates the damage also done by bad diet.
"I would take exception to that 1 percent (increase in calories)," said Dr. Reginald Washington, of Denver, who chairs the sports medicine and fitness committee of the American Academy of Pediatrics. "I think it's much higher than that."
Fast food calories are a big part of teens' eating patterns, and supersizing is making the portions grow, Washington said.
"We are pretty sure they are eating too much, no matter what the data say," said Dr. Nancy Krebs of the University of Colorado Health Sciences Center in Denver, who chairs the pediatricians' group's committee on nutrition. "There is quite a consensus that it is due to a combination of factors."
"Our view is that it is a complex issue," said clinical nutritionist JoAnn Hattner of Stanford University, a spokeswoman for the American Dietetic Association. "It may well be their activity is down, and for some it may be a combination of increased caloric intake and decreased activity."
Accepting the conclusion that food is not a big part of the problem could take pressure off food companies to cut the calories they feed the nation, Hattner said.
"There is enough clamor throughout the country that we are getting corporations to change," Hattner said. "We need to continue that clamor."
By Ira Dreyfuss