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People who take little exercise tend to become overweight. In a longitudinal US study reported in 2002 the prevalence of overweight and obesity in girls doubled during adolescence (between the ages of 9 and 19 years), primarily because of reduced activity rather than increased food intake. Now further data from the same study (the National Heart, Lung, and Blood Institute’s Growth and Health study) have underlined the importance of adolescent inactivity in the development of obesity (
At three sites (in San Francisco, Cincinnati, and Washington DC) between 1987 and 1997 a total of 2287 girls were followed up from age 9–10 years to age 18–19 years. Body mass index (BMI) and skinfold thicknesses were recorded annually and activity was assessed at baseline and at 3, 5, and 7–10 years. Throughout the study black girls (n = 1152) had significantly higher BMI, skinfold thicknesses, and energy intake than white girls. White girls were more likely to remain active, more likely to smoke, and less likely to give birth to a child. For every reduction in physical activity of 10 metabolic equivalent (MET) –times per week, BMI increased by 0.14 kg/m2 in black girls and 0.09 kg/m2 in white girls and sum of skinfold thicknesses by 0.62 and 0.63 mm. At ages 18–19 years BMI was 2–3 kg/m2 greater in inactive girls than in active girls. In both races habitual activity scores declined considerably between the ages of 9 and 19 years and the decline was more marked among girls who were relatively inactive at the start of the study. Energy intake increased over time among black girls but not white.
The authors of this paper conclude that encouraging continued activity among adolescents could be an important method of preventing obesity.