Decline in physical activity plays key role in weight gain among girls

Girls who were inactive during adolescence gained an average of 10 to 15 pounds more than active girls, according to results of a 10-year observational study of obesity. Total calorie intake increased only slightly and was not associated with the weight gains. These new results show that a previously reported steep decline in physical activity among adolescent girls is directly associated with increased fatness and an increase of body mass index (BMI), a measure of body weight adjusted for height.

The results of the Health and Growth Study, funded by the National Heart, Lung, and Blood Institute of the National Institutes of Health, are published online in the the Lancet.

Study investigators previously found that girls’ leisure-time physical activity declined between the ages of 9 and 19 by an average of 7.5 brisk, 30-minute walks per week.

At ages nine and ten, there were only small differences in BMI - about 4 to 5 pounds - between girls who were evaluated as "active" (doing the equivalent of 5 or more brisk 30-minute walks per week) and those who were "inactive" (doing the equivalent of 2.5 or less brisk 30-minute walks per week). However, in the subsequent nine years of follow-up, the differences widened, so that inactive girls had three times greater gains in BMI and were approximately 10 to15 pounds heavier in the tenth year of the study.

"These results show that many girls are at a literal standstill when it comes to exercise and physical activity in their pre-teen and teen years. As parents, educators, and health care providers, we can do a lot to encourage girls to continue physical activity throughout their adolescence, a step that has been shown to help them maintain a healthy weight," said NHLBI Director Elizabeth G. Nabel, M.D.

The study is a multi-center, longitudinal study of obesity development in 1,213 black and 1,166 white girls who were followed up annually from ages 9 or 10 to ages 18 or 19. The study took place between 1987 and 1998 in San Francisco, Cincinnati, and the greater Washington, D.C., area.

Differences were noted between the black and white participants in BMI, food intake and activity levels. Girls who self-reported their race as black were consistently heavier than those who reported their race as white, their calorie intake was higher, and increased with age. Thirty-two percent of white participants maintained "active" physical activity status, compared with 11 percent of black girls. Conversely, 58 percent of black girls remained "inactive" compared with 28 percent of white girls.

At each annual study visit, BMI was derived from measures of height and weight and skinfold measurements were taken to evaluate total body fat. Data on physical activity and diet were collected from questionnaires and a three-day food diary, recorded under the supervision of a nutritionist.

Study authors acknowledge that food intake is generally underreported, especially among white girls and women. Dr. Sue Kimm, of the University of New Mexico School of Medicine and the study’s lead author, theorized that "the phenomenon of under-reporting calories may have increased with the age of the girls and may account for some of the largely unchanged dietary patterns, especially among the white girls in the study."

Both black and white participants who maintained "inactive" status had 20 percent higher gains in BMI and an average of 20-40 percent increase in skinfold thickness - a measure of total body fat than girls who maintained an "active" status.

"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 and serve as a goal for public health programs and schools, " said Eva Obarzanek, Ph.D. NHLBI research nutritionist. "Just preventing the decline in physical activity that currently occurs among adolescent girls may be enough to prevent obesity."