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Can adding 75 minutes of brisk walking to the average teenage girl's week prevent her from growing into an overweight adult? About 10 minutes a day? Against an epidemic of overweight children and teens?

The authors of a fascinating new report suggest it would be a good place to start. Their study is one of two important new looks at children's activity levels and their health—particularly the growing problem of overweight kids and teens who will have a heightened risk for cancer, heart disease, and other serious illnesses if they carry that excess weight into adulthood.

Lifestyle, Body Weight, and Cancer Evidence now links about a third of cancer deaths in US to poor diet, a sedentary lifestyle, and excess weight. (Learn more). 16%—Overweight or obese children in 1999-2002 6%—Overweight or obese children in 1976-1980 Source: National Health and Nutrition Examination Survey

The research with girls, called the Health and Growth Study, found a steep drop in physical activity caused many teenage girls to gain weight—an extra 10-15 pounds by age 19 for the more sedentary girls. Researchers tracked 2,287 girls in San Francisco, Cincinnati, and the Washington, DC area beginning at ages 9 or 10. Many had slipped into an inactive, 'couch potato' lifestyle over the next 10 years, with their weekly physical activities equal to only 2.5 brisk, 30 minute walks or less.

The sedentary girls' weight gain was not linked to eating more calories, according to study authors, led by Sue Kimm, MD, MPH, of the University of New Mexico at Albuquerque. Full results were published in The Lancet medical journal.

Active Girls Morph into Sluggish Teens

"These results show that girls are at a literal standstill when it comes to exercise and physical activity in their pre-teen years," said Elizabeth Nable, MD, director of the National Heart, Lung, and Blood Institute (NIHLB), which funded the research.

Study findings document how far activity levels drop as girls become teens and how common a sedentary lifestyle has become in this age group.

• Weekly leisure-time physical activity for all girls dropped by an average of 7.5 brisk, 30-minute walks between ages 9 and 19.
• By age 19, 68% of white girls were inactive.
• By age 19, 89% of black girls were inactive.
• Girls who remained "active" did at least 5 brisk, 30-minute walks or equally vigorous activities per week.

Between the bad news, experts at the NIHLB have pinpointed a practical way for teenage girls to avoid gaining too much weight.

"While 2.5 or more brisk walks per week is considered a modest level of activity, increasing exercise by that small amount could potentially prevent weight gain," said Eva Obarzanek, Ph.D., research nutritionist at NIHLB .

And potentially save lives, too, according to Colleen Doyle, MS, RD, ACSÂ’s director of nutrition and physical activity. Kids who are overweight tend to become adults who are overweight, which increases the risk of cancer, heart disease, diabetes, and other conditions. Preventing excess weight gain as kids get older is key for them to grow into a healthy adulthood.

Bursts of Activity Add up to Big Health Benefits

The good news is that this study—and others—demonstrate that little bits of activity can add up to big benefits overtime. Parents can play a tremendous role in getting their kids more active—at home, at school, and in the community:

At Home

• Be a role model. Get out and be active with your kids. Take a walk, ride a bike, throw a ball.
• Turn off the TV. Do something active, instead.
• Walk when you can—to the store, library, bank . . . etc.

At School

• Be an advocate for daily physical education requirements within your school district.
• Take advantage of physical activity-based after school activities and/or intramural sports.

In the Community

• Advocate for sidewalks, bike lanes and more parks for your children to play in.
• Participate in community-based walks, runs or bike rides.

Exercise Boosts Children's Health, but Exercise Time Has Dwindled

Another key report confirms previous recommendations for children to get 60 minutes or more of moderate to vigorous physical activity every day. The authors of "Evidence Based Physical Activity for School-Age Youth" were asked by the US Centers for Disease Control to comb through more than 850 scientific articles. They found substantial proof that regular physical activity boosts health. Well-designed activities build up bone density, help teens with high blood pressure control it, and help heavy kids lose weight.

The portion of children ages 6-19 who are overweight now stands at 16%—nearly triple what it was 25 years ago.

"The evidence is quite clear for overweight children that a program of physical activity works. It helps them lose weight. It has to be moderately intense exercise for 30-60 minutes, on three to seven days per week," said Robert Malina, PhD. an expert in children's growth and development at Tarleton State University in Texas. The report was published in the Journal of Pediatrics.

But the opportunities for children to be active every day have changed drastically in the last generation, according to Malina: "How many sedentary activities fill children's days now?"

"In some places they're eliminating recess . . . . Sitting in school is a socially-sanctioned form of sedentary activity," he added. "There's school time, homework, TV time, p.c. time—parents send their kids to classes to learn computer skills. And we want kids to be quiet all the time, too."

The Case for Schools to Set Students in Motion

Surveys show only about a quarter of high school students get daily P.E. classes, more than a third get no regular vigorous activity at all, and on average, high school students take just one year of PE before graduation. Malina says, "Parents need to force the community's hand to provide more physical activities for children."

"We need to modify our view of school sports, so it's not just for the elite athletes. Reinstitute intramurals. If you make programs available and attractive to all kids, they will be there," he added. "Beginning in middle school, schools should emphasize organized physical activities for youngsters.

Malina, William Strong, MD, and more than a dozen other experts who worked on the report also found evidence that exercise programs done on school time will not make kids' grades drop.

What's a Parent to Do?

"Parents should be active with their children," said Malina. "For children in the primary grades the evidence is very clear that they need their parents to be active with them," he continued.

And that is time well spent. Not only do parents and kids get more activity, it’s a way to spend additional time together as a family. Going for a walk after dinner, playing catch or Frisbee, even gardening, raking leaves or shoveling snow can help burn calories and increase energy levels—and are great opportunities to spend some quality time together.

Other recommendations from the evidence-based report include:

• Children can accumulate the 60 minute exercise goal during an entire day, by walking to school, in recess, gym class, intramural sports, and playing soccer with a parent, for example.
• Activities should be moderate to vigorous, such as brisk walking, bicycling, and active outdoor play.
• Parents should limit sedentary activities (outside of school) to two hours or less per day.
• Sedentary activities include telephone conversations, TV viewing, computer use, and video games.
• Organized exercise should be developmentally appropriate, enjoyable, and involve a variety of different activities.

"As parents, we want the best for our children," noted Doyle. "Encouraging your kids to be active, being active yourself, and working to make our schools and communities more activity-friendly can help give the start your kids need for a lifetime of good health."

Citations: "Relation between the changes in physical activity and body-mass index during adolescence: a multicentre longitudinal study." Published in The Lancet (Vol. 366, No. 9482: 301-307). First author: Sue Y.S. Kimm of the University of New Mexico Health Sciences Center.

Evidence Based Physical Activity for School-Age Youth." Published in The Journal of Pediatrics (Vol. 146, No. 6: 732-737). First author: William B. Strong, MD, Medical College of Georgia.

"Physical Activity Recommendations: Where do we go from here?" Published in The Journal of Pediatrics (Vol. 146, No. 6: 719-720). First author: William H. Dietz, MD, PhD, National Center for Chronic Disease Prevention and Health Promotion.