"Decreasing physical activity during adolescence seems to play a major role in weight gain among girls as they transition from children to women. Inactive girls gained an average of 10 pounds to 15 pounds more than girls who were active between the ages of 9 and 19, according to a study appearing in the July 16 issue of The Lancet. The number of calories consumed increased marginally and did not appear to be associated with the weight gain." (reference)