Using surveys conducted in middle and high schools, researchers also found increases in the number of days youth reported having breakfast each week and in how often they ate fruits and vegetables. Those trends have corresponded to a leveling off in obesity rates, but not a decline, the study showed.
"I would like to believe that all the public health efforts focusing on increasing physical activity and increasing fruit and vegetable consumption are having an effect, because that seems to be a pattern," Ronald Iannotti, the lead author on the study from the University of Massachusetts Boston, said.
"The fact that (obesity) is leveling off, that's a surprise and a major change from the steady increase that we've seen over time," Iannotti, who worked on the study while at the Eunice Kennedy Shriver National Institute of Child Health and Human Development in Bethesda, Maryland, said. "This is great news."
He and co-author Jing Wang analyzed surveys given to a nationally-representative sample of students in sixth through tenth grades in 2001-2002, 2005-2006 and 2009-2010 as part of the Health Behavior in School-aged Children study. Each survey period included responses from between 9,000 and 15,000 adolescents.
The researchers found "encouraging" trends on measures of most diet and lifestyle habits.
For example, the number of days each week that kids reported being physically active for at least 60 minutes increased from 4.3 in 2001-2002 to 4.5 in 2009-2010, with similar trends among boys and girls. Likewise, youth reported eating breakfast on three school days each week on the first survey and 3.3 days on the last.
The average number of hours students spent watching TV each day fell from 3.1 to 2.4, with drops in both weekday and weekend viewing.
Frequency of fruit and vegetable consumption also rose slightly over time - although it remained at less than one daily serving of each, on average - and consumption of sweets and soft drinks fell.
However, the proportion of survey participants who were overweight or obese, based on their own height and weight reports, did not decrease, the researchers wrote Monday in Pediatrics.
Rates of obesity - defined as body mass index, a measure of weight in relation to height, in the 95th percentile or higher - rose from 10.3 percent in 2001-2002 to 12.7 percent in 2005-2006, then held steady through the final survey.
"This is encouraging, because at least it looks like things have kind of stabilized, and at least they're not going in the wrong direction," Marian Huhman, who studies health communication and health campaigns at the University of Illinois at Urbana-Champaign, said.
"Maybe it just takes a few years for the outcomes of obesity changes to follow from the behavioral changes," she added.
Huhman, who wasn't involved in the study, pointed to the "effort on many, many fronts" that may have led to the positive changes in physical activity, sedentary behavior and diet - such as walk-to-school programs and campaigns targeting food marketing.
However, adolescents "are still largely not meeting the recommendations for amount of screen time, amount of physical activity (and) amount of fruit and vegetable consumption," she said.
Iannotti echoed that concern.
"Although they're increasing, the recommendation is five servings of fruits and vegetables per day. And we're looking at one or two," he said.
"There's still vast room for improvement."