A new study sheds light on why many adolescent girls are gaining weight. A decline in activity in the transition from childhood to adulthood could be responsible, according to a study published in the July 23 issue of The Lancet (2005; 366 [9482], 301–7).

In collaboration with others, Sue Y.S. Kimm, MD, from the University of New Mexico in Albuquerque, looked at activity levels and changes in body mass index (BMI) and fatness in over 1,100 black and over 1,100 white girls. The investigators assessed BMI and skinfold thickness each year from the time the girls were 9 or 10 years old to when they were 18 or 19. The subjects answered questions on physical activity and food intake at years 1, 3, 5 and 7–10 of the study. The results? There was a pronounced decline in physical activity, whereas the rate of overweight and obesity doubled.

How do you stop this weight gain? The authors state that physical activity equivalent to 2.5 hours of brisk walking per week could potentially prevent the increases, which in this study ranged from 4 to 6 kilograms (kg) (8.8–13.2 pounds) in white girls and from 6 to 9 kg (13.2–19.8 pounds) in black girls.