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You don't have to spend much time with teenagers to know that the average adolescent would rather devote an afternoon to sitting in front of the TV, computer or video-game console than working out in a gym. And in recent years, as physical-education classes have been progressively cut from cash-strapped public-school curriculums, teens have had even more time to lounge, slouch, hang out or do anything but break a sweat.

It's no surprise, then, that obesity rates among U.S. youngsters have skyrocketed, tripling from 1976 to 2004. Public-health experts and obesity researchers attribute the trend in part to kids' increasingly sedentary lifestyles. As teens spend more and more time anchored before a screen — burning fewer and fewer calories each day — they're storing more of that unused energy as fat. Hence, the ballooning rates of obesity.

That's precisely why the findings of a new study by researchers at the Johns Hopkins Bloomberg School of Public Health came as such a surprise. The report, published last week in the journal Obesity Reviews, finds that the amount of physical activity among U.S. teens has not in fact changed significantly over the past two decades, even while that population has gotten heavier. "On the one hand, we have seen the obesity-prevalence increase, but we don't see a decrease in physical activity," says Dr. Youfa Wang, an associate professor at the Center for Human Nutrition at Hopkins and lead author of the study. "This suggests that physical activity is not a good explanation for the increase in prevalence of obesity."

In simple terms, body weight is a reflection of the balance between two variables: the calories a body takes in and the calories it burns off. As far as the average U.S. teen is concerned, the study suggests, the culprit behind weight gain is not a decrease in exercise but an increase in consumption. Of course, that doesn't mean teens are getting adequate exercise: Wang analyzed data from nearly 16,000 high school students between the ages of 15 and 18, who took part in the Centers for Disease Control and Prevention's longitudinal Youth Risk Behavior Surveillance Survey, about their physical activity. He and his team found that in 2007, only 34.7% of teens met federal physical activity recommendations, which call for activity strenuous enough to cause heavy breathing for a total of an hour a day for five or more days a week.

But the survey also found that teens' overall rate of daily exercise had not changed much since 1991, when the study sample was first asked to report their participation in gym classes in school and their level of physical activity at home. The percentage of teens attending daily gym class has stayed relatively steady since 1991; on average, the yearly change in the proportion of students participating was less than 1%. The percentage of ninth- through 12th-graders getting adequate levels of moderate physical activity — exercise such as slow bicycling, fast walking or pushing a lawn mower, which did not make participants break a sweat — also changed very little, from 26.7% in 1999 to 26.5% in 2005, the latest year for which the data was available. Yet obesity rates continued to rise.

So does this mean that exercise isn't important in controlling weight? As tempting as that conclusion might be, Wang and other health experts say that's not exactly what the new data show. The findings may say less about the role of exercise by itself than about the other variable in the weight equation — diet — and the interaction of the two. While exercise may not contribute directly to weight loss, it is critical for maintaining a healthy weight, since it helps calibrate the balance between energy taken in and energy burned off. "The data is too gross, and too general to assume that [exercise doesn't count]," warns Dr. Janet Walberg Rankin, a professor in the department of human nutrition, foods and exercise at Virginia Tech. "We need to have a dual approach to weight involving both activity and diet. I would hate for people to take away from this study that activity has nothing to do with weight."

Rankin points out that even small changes in a person's energy balance can have a significant effect on weight. Studies have shown that eating just 10 to 20 extra calories per day — that's one peanut M&M or one tortilla chip — that don't get burned through activity can result in a 2-lb. gain on average over the course of a year. "But none of the methods we have now are accurate enough to pick that up," says Rankin.

She advises people to take the new data with, well, a grain of salt. The information was collected by asking participants to self-report their exercise habits, which is a notoriously unreliable method — people are not very good at gauging their activity accurately. Add to that the fact that questionnaires are not refined enough to pick up small changes in people's energy intake and expenditure, and it's obvious why the findings are informative but not game-changing. "These data are useful in highlighting who should be targeted — the most difficult cases," says Rankin. In the new study, that group includes African-American girls, who got the least amount of exercise among all adolescent groups.

Still, the study highlighted some encouraging trends. For instance, the percentage of teens who spent more than three hours a day in front of the TV dropped from 1999 to 2007, from 43% to 35%. While Wang acknowledges that students may simply be substituting computer or other sedentary screen time for television-viewing, he notes that it's still a trend in the right direction. Far from being an excuse not to exercise, Wang sees the data as a wake-up call for parents and teens. "The important message is that compared to the recommendations for physical activity, the physical activity of American adolescents is still at a very low level," says Wang. "We still need to make a greater effort to promote physical activity. Even if it does not explain obesity, it has many other beneficial effects."

• Alice Park
• New data from a long-term survey suggests that U.S. teens are no less active today than they were in 1991. Yet they've gotten increasingly heavier over the same period