When researchers observed adolescent girls, they noted that there was a doubling in the rate of excess weight and obesity in girls who had a pronounced decline in physical activity during the transition from childhood to adolescence.

Over 9 million young people in the United States aged 6-19, or 16 percent, are considered overweight, according to the Centers for Disease Control (CDC) in Atlanta, Georgia.

"Preventing the steep decline in activity during adolescence is an important method to reduce obesity," Sue YS Kimm, the lead researcher, said in a study published online by The Lancet medical journal.

Research by scientists at the University of New Mexico in Albuquerque showed that increasing exercise to the equivalent of 2-5 hours of brisk walking each week could help to prevent girls from gaining 4-9 kilos (9-20 lbs) during their teens.