American girls become less active as they become teenagers.  A new study being published in the July 23, 2005, print edition of the Lancet shows that as they age, overall activity levels during the transition between childhood and young adulthood are strong predictors of whether or not they will experience unhealthy weight gains.  

The results are from the ten-year NHLBI (National Heart, Lung and Blood Institute) Growth and Health Study of obesity development, funded by NHLBI of the National Institutes of Health and reported by Sue Y.S. Kimm, M.D.,M.P.H., Research Professor of Epidemiology in the Department of Internal Medicine at  the UNM Health Sciences Center School of Medicine. 

In this landmark multicenter longitudinal study, 1,213 black girls and 1,166 white girls from 3 different areas of the U.S. (the greater Washington, D. C. area, Cincinnati, and outside San Francisco) were followed annually from ages 9 or 10 until ages 18 or 19.

At ages 9 or 10, there were only small differences in body mass index (BMI, a measure of body weight adjusted for height) equivalent to about 4 to 5 pounds— between girls who were categorized 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 than did active girls   approximately 15 pounds more for black girls and 10 pounds for white  girls.

While overall levels of physical activity plummeted, 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 accrual of body fat and an increase of body mass index (BMI),.

The study also found differences between the black and white participants in BMI, food intake and activity levels with black girls being consistently heavier than white girls and their caloric intakes were 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.

In investigating possible root causes of the precipitous decline in physical activity, Dr. Kimm said that parental education more than household income seemed to play  a greater role in predicting the decline in activity, but this finding was seen only in white girls and in older black girls.  Single-parent homes were also associated with a greater drop in activity in older white girls but not in black girls. The reason for these racial differences is not clear. These findings were previously published in the New England Journal of Medicine. 

Kimm stressed however that activity while obviously important is probably  not solely responsible for all of adolescent weight gain.   "It would be nice to think that there is a smoking gun and only one variable involved in weight gain but we know that it is more complex than that," she said.  For instance, we need to moderate our food intake to avoid excessive caloric intake, relative to the sedentary lifestyle in America today.

Her recommendation is that we need to search for all possible means of fostering activity levels in youth, however small, and also make a concerted effort toward identifying and making available teen-friendly physical activities. 

"Too often people want big solutions to problems but it is hard to make big permanent changes all at once," she said.  "But even small changes add up over time.  Our recommendations are for finding more teen friendly and teen accessible activities that could be incorporated into their daily living.  We also need to ensure that our recommendations are within reach of all teens regardless of their socioeconomic status"

For example, one can encourage teens to help with household chores such as vacuuming or mowing the lawn.  Even clearing the dinner table by carrying dishes back to the kitchen does burn up calories, albeit relatively limited.  Carrying grocery bags or carrying a small child burns calories.  No matter how small, when we expend energy, it still adds up to calories burned.  Another venue may be the opportunities offered by large malls.  For instance, teens love the mall.  Kimm suggests that when teens go shopping, they go from one end to the other with a brisk walk.  This will avoid lingering at the show windows and thereby bypassing temptation to buy.  In addition, Kimm would like to see mall store owners coming together in programs to encourage teens to walk even more while out at a shopping center  possibly holding competitions for who could clock in the most mileage in a given afternoon and handing out store coupons as prizes.  This could be a "win win" situation for all increased physical activity for teens and greater volume of shoppers to the mall.

Kimm concludes in the Lancet paper that "our study findings suggest that habitual activity plays an important role in weight gain, with no parallel evidence that energy intake had a similar role… Since the steep decline in habitual activity occurs during adolescence, programs to moderate and prevent this decline during the teen years might be useful as part of the armamentarium for battling the current epidemic of obesity."


Contact: Cindy Foster, 272-3322