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Features of the metabolic syndrome are becoming increasingly evident in children. Decreased physical activity is likely to be an important etiological factor, as shown previously for subjective measures of physical activity in selected groups. The purpose of this study was to examine the relationship between the metabolic syndrome and objectively measured physical activity and whether fitness modified this relationship.A total of 589 Danish children (310 girls, 279 boys, mean [±SD] age 9.6 ± 0.44 years, mean weight 33.6 ± 6.4 kg, mean height 1.39 ± 0.06 m) were randomly selected. Physical activity was measured with the uni-axial Computer Science & Applications accelerometer (MTI actigraph) worn at the hip for at least 3 days (≥10 h/day) and fitness with a maximal bike test. As outcomes, we measured sitting systolic and diastolic blood pressure, degree of adiposity (sum of four skinfolds), and, finally, insulin, glucose, triglicerides, and HDL cholesterol in fasting blood samples. The outcome variables were statistically normalized and expressed as the number of SDs from the mean. (i.e., Z scores). A metabolic syndrome risk score was computed as the mean of these Z scores. Multiple linear regression was used to test the association between physical activity and metabolic risk, adjusted primarily for age, sex, sexual maturation, ethnicity, parental smoking, socioeconomic factors, and the Computer Science & Applications unit, as well as for fitness. Robust SEs were computed by clustering on school.All children were in the nondiabetic range of fasting glucose. Metabolic risk was inversely related to physical activity (P = 0.008). The relationship was weakened after adjustment for fitness, but there was a significantly positive interaction between physical activity and fitness.Physical activity is inversely associated with metabolic risk, independently of potential confounders. The interaction between physical activity and fitness suggests that the potential beneficial effect of activity may be greatest in children with lower cardiorespiratory fitness.