OBJECTIVE -To determine whether measures of physical activity are related to an insulin sensitivity index ([ISI] 10 4 /fasting insulin ϫ glucose) independent of weight or adiposity in children. RESEARCH DESIGN AND METHODS -We conducted a longitudinal study of 90 Pima Indian children (39 boys and 51 girls) at 5 and 10 years of age measuring adiposity (dual-energy X-ray absorptiometry), physical activity behavior (questionnaire: number of activities per week [ACT], average hours per week [TIME]),

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... and energy expenditure (doubly labeled water: physical activity level [PAL]). RESULTS -In cross-sectional analyses, ACT was correlated with ISI at 5 years of age (r ϭ 0.24, P ϭ 0.02) and at 10 years of age (r ϭ 0.21, P ϭ 0.05), but these relationships were not independent of weight or adiposity. PAL was correlated with ISI at 10 years of age (r ϭ 0.39, P ϭ 0.03) but was not independent of weight or adiposity. Longitudinally, ISI decreased from 5 to 10 years of age, and increases in weight and adiposity were associated with decreases in ISI (r ϭ Ϫ0.51 and Ϫ0.41, respectively; both P Ͻ 0.0001). ACT decreased from 5 to 10 years of age, but children who had smaller decreases in ACT had smaller decreases in ISI, independent of increases in weight or adiposity (partial r ϭ 0.22, P ϭ 0.04 adjusted for either weight or adiposity). CONCLUSIONS -These data suggest that early establishment and maintenance of an active lifestyle can have a beneficial effect on ISI that is partially independent of changes in weight or adiposity. This is particularly relevant considering the current epidemics of both obesity and type 2 diabetes in children. Abbreviations: ACT, activities per week; AEE, activity energy expenditure; DLW, doubly labeled water; GLM, general linear model; ISI, insulin sensitivity index; PAL, physical activity level; RMR, resting metabolic rate; TEE, total energy expenditure; TIME, average hours per week. A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.