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Teenagers can significantly lower their likelihood of being overweight or obese by walking or biking to school and playing on at least one high school sports team, but preferably two or more, a new study suggests.

Though the spread of childhood obesity in the last decade has spurred health authorities to ramp up their efforts to promote youth activity, the new findings are among the first to demonstrate that walking or riding a bike to school actually has an impact on weight gain among high school students. The study also found that while school-based exercise can reduce or stem weight gain, it is sports participation in particular that makes a difference. Physical education classes, the researchers found, did not reduce or prevent weight gain, likely because they do not offer students the same level of regular, challenging exercise as competitive sports.

“I think being a part of some kind of team or organization gives kids the opportunity to have moderate to vigorous activity consistently,” said Keith M. Drake, an author of the study and a postdoctoral research fellow at the Hood Center for Children and Families at the Geisel School of Medicine at Dartmouth. “I think a lot of time physical education requirements are not that strict. Kids are not in P.E. that often, and when they are, the physical activity is not that strenuous.”

In their study, which was published in the journal Pediatrics, Dr. Drake and his colleagues tried to quantify the impact that greater sports participation and less reliance on mass transit might have on childhood obesity. They calculated that if all adolescents played on at least two sports teams per year — in other words, one team per season — obesity rates would plunge 26 percent and the prevalence of overweight adolescents would fall by 11 percent. And if all adolescents walked or biked to school at least four days a week, they found, the number who were obese would drop by 22 percent.

Nationwide, about a third of high school students are overweight or obese, a figure that has prompted federal efforts to boost physical activity of any kind among teenagers. Health officials recommend that children get an average of about one hour a day of moderate to vigorous activity, but most fall far short of that.

One of the government’s most recent efforts to boost adolescent exercise was a $100 million-a-year program called Safe Routes to School, which targets the way students commute. In 2001, less than 16 percent of children between 5 and 15 walked, biked or “actively” commuted to school, compared with about 50 percent of youngsters in that age range 40 years ago. Created in 2005, Safe Routes aims to boost those numbers by helping communities build more sidewalks, bike paths and other “pedestrian friendly” routes.

Yet past studies have generally found that while walking and biking to school increases physical activity levels, it has little or no impact on body weight. Dr. Drake and his colleagues noted that the sample sizes in those studies were small — in many cases, just a few hundred students — so recruited over 1,700 high school students and their parents to examine the relationship between commutes to school, levels of sports participation and body weight.

Over all, only 10 percent of students in the study were “active” commuters, walking or biking to school more than three-and-a-half days a week; most, about 70 percent, said they never did either. Active commuting, the study found, lowered the risk of obesity by 33 percent, though it did not seem to protect against being overweight.

“We found that it’s only affecting kids who are near the obesity cutoff,” Dr. Drake said. “We think that’s because walking and biking to school are fairly leisurely exercise and kids don’t tend to do it unless they live close to school. So we think it’s only affecting kids who are very overweight, whereas for most normal kids it wouldn’t affect them.”

As for more organized activities, about 17 percent of the students played on one sports team, 18 percent played on two, and 35 percent were on three or more. The study revealed a dose response relationship: the greater the participation level in organized sports, the lower the risk of being overweight or obese. The trend became most significant when students played on at least two teams, representative of year-round participation in sports.

“Playing on one sports team is definitely better than playing on zero, but I would say at least two or three is probably ideal,” Dr. Drake said.

Dr. William Stratbucker, the medical director of the Helen DeVos Children’s Hospital Healthy Weight Center in Grand Rapids, Mich., who was not involved in the research, said the findings were intriguing because they showed a sort of threshold for effective physical activity. But he said the idea that every child should join a sports team “is not achievable in real life.” Some children may face economic barriers, like the costs associated with team sports, or health issues, like asthma, that limit their participation.

Another way to encourage vigorous exercise among teenagers, Dr. Stratbucker said, is for parents to make it a part of their lives. “If the parents are enjoying an active lifestyle themselves, that’s modeling behavior that leads to children participating in activity to the levels that they need to be,” he said.

Not every student can make the varsity baseball or basketball team, Dr. Drake said, but there are other ways — like playing intramural or club sports, or joining a dance or gymnastics team — for children to get strenuous exercise.

“I don’t think it has to be a sports team,” he said. “There are other athletic opportunities, even for the noncompetitive kids, that can achieve the same benefit.”