Teen Obesity: Lack of Exercise May Not Be to Blame

[The Straight Story]

Teen Obesity: Lack of Exercise May Not Be to Blame

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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. (See nine kid foods to avoid.)

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.

http://news.yahoo.com/s/time/20091110/hl_time/08599193677700