There's a new reason to let your kids cruise the mall as often as possible. It may be one of the best ways to help them from becoming obese adults.

In addition to genetics, a key factor in predicting future risk of obesity is how folks spend their leisure time, say Norwegian researchers after studying a group of teens for 20 years through adulthood. They find that the participants who were least likely to later become fat were those who got the most leisure time physical activity -- that is, exercise in everyday activities such as brisk walking, taking the stairs, and other ways of burning calories outside of a gym or regimented workout.

The real message of this finding is that the foundation of adult body weight may be poured during the teenage years. But while genetics may be the cornerstone, habits and lifestyle play a key part. So get and keep your teens physically active to lower their health risks decades from now.

For their study, which is published in this month's Archives of Pediatrics and Adolescent Medicine, the researchers tracked 475 students from the time they were 15 to their mid-30s. In that time, they noted several key indicators of later obesity.

As expected, if the patients were heavy as teens or had overweight parents (especially a father), they were more likely to be obese adults. But the researchers also revealed a few surprises: Quitting smoking as a teen or young adult also raised risk of later being overweight -- although remaining a smoker did not. And being highly educated, and having parents who were well-degreed, lowered risk.

But when it comes to something everyone can control, maintaining or increasing leisure time physical activity appears to offer the greatest protection against later obesity risk.

"The message is try to get in some exercise as part of the everyday activities -- walk the stairs rather than taking the elevator and park the car a distance away from your destination in order to get a few minutes of walking," lead researcher Elisabeth Kvaavik, MSc, of the Institute for Nutrition Research at the University of Oslo, tells D4D.

And pass this message on, as soon as possible: "Obesity prevention has to start at a young age, as there is substantial stability in body weight from childhood into adolescence and adulthood."

While that may seem obvious, this study actually adds new insight to some longtime concerns, says another expert not involved in Kvaavik's study.

"This is a good study because they showed that adolescents who increased leisure time physical activity reduced their risk of gaining weight over time," says Myles S. Faith, PhD, of the Weight and Eating Disorders Program at the University of Pennsylvania School of Medicine.

That's important because while children are typically very active, exercise levels dip once they reach their teen years. "As they get to adolescence, they often become more sedentary, and this is especially problematic for girls," Faith tells D4D. "What this study does is speak to the importance of keeping up that activity during adolescence."

Faith adds that this study also helps answer some concerns over the real benefit of leisure exercise like that achieved through cruising the mall -- as opposed to more intense "power" walking or regimented workouts.

"There has been some debate as to the long-term effects of leisure time physical activity. Can 30 minutes of leisure time physical activity have long-term impact? This study shows that it can have a beneficial effect."