Lack of exercise may not explain teen obesity

NEW YORK (Reuters Health) - Most U.S. teenagers are not as active as they should be, but a lack of exercise does not seem to account for rising rates of teen obesity, a new study finds.

Using government survey data collected between 1991 and 2007, researchers found that in recent years, U.S. teens have averaged more time in gym class and less time in front of the TV.

Moreover, there was no evidence that teens' exercise levels changed appreciably at any time during the study period -- even though those years saw an increase in teen obesity. It's estimated that nearly one-third of U.S. children and teens are now overweight or obese.

The findings suggest that waning exercise levels are "not likely the major explanation of the recent increase in obesity among U.S. adolescents," the investigators report in the journal Obesity Reviews.

That does not mean, however, that it's fine for teenagers to be sedentary. Children and teens still need to develop regular exercise habits for the sake of their overall health, according to the researchers.

"Our study suggests that more vigorous efforts are needed to help young Americans engage in adequate regular physical activity and reduce sedentary behaviors, which will help promote good health," senior researcher Dr. Youfa Wang, of Johns Hopkins University in Baltimore, said in a news release from the university.

For the study, Wang and colleagues used data from an annual government survey that tracks the health and lifestyle of U.S. high school students.

Overall, they found, only 35 percent of teenagers surveyed in 2007 met the current recommendations for physical activity -- performing any activity that gets the heart rate up at least one hour per day, five or more days out of the week.

But there was no evidence that teenagers' exercise habits shifted significantly during the study period.