Prospective associations between physical activity and obesity among adolescent girls: racial differences and implications for prevention.

OBJECTIVE To test for differences in prospective associations between physical activity and obesity among black and white adolescent girls. DESIGN Prospective cohort study using data from the National Heart, Lung, and Blood Institute Growth and Health Study. SETTING Multicenter study at the University of California (Berkeley), Children's Medical Center at the University of Cincinnati (Cincinnati, Ohio), and Westat, Inc, and Group Health Association (Rockville, Maryland). PARTICIPANTS A total of 1148 adolescent girls (538 black and 610 white) who provided valid data on levels of physical activity and obesity at ages 12 and 14 years. INTERVENTION Physical activity, assessed as accelerometer counts per day. MAIN OUTCOME MEASURES Three measurements of obesity were obtained using the Centers for Disease Control and Prevention definition of obesity (at or above the age-specific 95th percentile of body mass index), the International Obesity Task Force reference body mass index cut points for obesity in children, and the sums of skinfold thickness (with the cohort ≥90th percentile as indicative of obesity). RESULTS We found a strong negative dose-response association between quartiles of accelerometer counts per day at age 12 years and obesity at age 14 years (using all 3 measurements of obesity) in white but not black girls (P < .001 for body mass index interaction and P = .06 for sums of skinfold thickness interaction). The odds ratios for obesity (using the cohort ≥90th percentile for sums of skinfold thickness) in adjusted models between the top and the bottom quartiles of accelerometer counts per day were 0.15 (95% CI, 0.04-0.63; P = .03 for trend) in white girls and 0.85 (95% CI, 0.32-2.26; P = .93 for trend) in black girls. CONCLUSIONS Higher levels of physical activity are prospectively associated with lower levels of obesity in white adolescent girls but not in black adolescent girls. Obesity prevention interventions may need to be adapted to account for the finding that black girls are less sensitive to the effects of physical activity.