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.

In 1993, for example, 66 percent of teens got enough short bursts of vigorous exercise - 20 minutes of running, biking or other heart-pumping activity at least three days per week. That figure was 64 percent in 2005.

When it came to moderate exercise - which should, according to guidelines, be performed at least 30 minutes per day, on five or more days per week - only 27 percent met that goal in 1999 (the first year in which the survey asked this question). That figure was unchanged in 2005.

On the other hand, there were some encouraging trends, according to Wang’s team. One was the recent increase in time spent in school physical education classes: in 2007, 30 percent of high schoolers were taking a daily physical education class, versus 25 percent in 1995.

The researchers also found a decline in teenagers’ TV time. In 1999, 43 percent of students spent three or more hours watching TV on school days - a figure that dipped to 35 percent in 2007.

As a whole, the researchers write, the findings suggest that exercise habits have not been the main factor in the nation’s teen obesity problem.

“Although only one third of U.S. adolescents met the recommended levels of physical activity,” Wang said, “there is no clear evidence they had become less active over the past decade while the prevalence of obesity continued to rise.”

Instead, he said, other factors - like unhealthy diets - may be the driving force.

More studies are needed to better understand the factors feeding the nation’s obesity trends, and how to best combat them, the researchers conclude.

SOURCE: Obesity Reviews, online October 30, 2009.