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Background: Sedentary time (ST) has been reported to have a range of negative health effects in adults, however, the evidence for such effects among children and adolescents is sparse. The primary aim of the study was to examine associations between changes in sedentary behavior (time and fragmentation) and changes in adiposity across childhood and adolescence.Methods: Participants were recruited as part of the Gateshead Millennium Study. Measures were taken at age 7 (n=502), 9 (n=506), 12 (n=420) and 15 years (n=306). Participants wore an ActiGraph GT1M and accelerometer epochs were ‘sedentary’ when recorded counts were less than or equal to25 counts per 15 s. ST was calculated and fragmentation (SF) was assessed by calculating the number of sedentary bouts per sedentary hour. Associations of changes in ST and SF with changes in adiposity (body mass index (BMI), and fat mass index (FMI)) were examined using bivariate linear spline models.Results: Increasing ST by 1% per year was associated with an increase in BMI of 0.08 kg m−2 per year (95% CI: 0.06–0.10; P<0.001) and FMI of 0.15 kg m−2 per year (0.11–0.19; P<0.001). Change in SF was associated with BMI and FMI (P<0.001). An increase of 1 bout per sedentary hour per year (that is, sedentary time becoming more fragmented) was associated with an increase in BMI of 0.07 kg m−2 per year (0.06–0.09; P<0.001) and an increase in FMI of 0.14 kg m−2 per year (0.10–0.18; P<0.001) over the 8 years period. However, an increase in SF between 9–12 years was associated with a 0.09 kg m−2 per year decrease in BMI (−0.18–0.00; P=0.046) and 0.11 kg m−2 per year decrease in FMI (−0.22–0.00; P=0.049).Conclusions: Increased ST and increased SF from 7–15 years were associated with increased adiposity. This is the first study to show age-specific associations between change in objectively measured sedentary behavior and adiposity after adjustment of moderate-to-vigorous-intensity physical activity in children and adolescents. The study suggests that, targeting sedentary behavior for obesity prevention may be most effective during periods in which we see large increases in ST.
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