Eighteen obese adolescents, with a body mass index (BMI) above 30, were recruited to participate in the study and were individually matched by age and sex with 18 normal-weight (BMI < 27) adolescents from the same school. Free-living physical activity can be assessed in a more short-term fashion with the doubly labeled water (DLW) method, or over a longer term of continuous activity with a biomechanical device such as an accelerometer. In this study, both methods were employed. During the initial clinic visit, the rate of total energy expenditure (TEE) figure was determined using the DLW method. The teenagers were also instructed in how to wear an accerometer, which continuously registers both the intensity of activity and the duration of unbroken periods of a particular level of activity.
The TEE of the two groups was not significantly different, nor were their resting metabolic rates. However, accelerometry indicated that the intensity and duration of physical activity and total amount of physical activity in the obese group was significantly less than that of the normal-weight group throughout the entire day. The obese adolescent boys spent 29% less time each day engaged in moderate-to-high intensity activities than did normal-weight boys. Among adolescent girls, the number of minutes spent continuously engaged in moderate-to-high level activities was 73% lower in obese girls than in those of normal weight.
The results of the study suggest that the "energy cost" of physical activity is the same for both obese and nonobese persons, even after adjustments for such factors as the additional energy required to sustain movement in an individual with a larger body mass. The intensity and duration of physical activity on a long-term basis may be important to maintaining energy balance in a society where unrestricted food consumption is always available.
Ekelund, Ulf et al. Physical activity but not energy expenditure is reduced in obese adolescents: a case-control study. Am J Clin Nutr 2002;76:935-41.
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