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The effect of screen time during secondary school on percent body fat was examined in a cohort of 744 Canadian adolescents aged 12-13 years at baseline. Participants completed self-reported questionnaires on television viewing and computer use in 19 survey cycles over 57 months from 1999 to 2005. Triceps skinfold thickness and subscapular skinfold thickness were measured in survey cycles 1 and 19. Four screen-time trajectory groups identified in growth mixture modeling included steady-low screen time (73% of the sample), steady-high (10%), increasers (9%), and decreasers (8%). The effect of screen-time trajectory on percent body fat at survey cycle 19 was modeled in boys and girls separately by using linear regression, adjusting for baseline percent body fat and physical activity. Relative to that of steady-low screen-time trajectory group boys, percent body fat was 2.9 (95% confidence interval: 0.7, 5.0) and 2.4 (95% confidence interval: 0.5, 4.2) percentage units higher on average among "increasers" and "steady-high" trajectory group boys, respectively. There was no evidence that screen time has an effect on percent body fat in girls overall, although physical activity modified the association between screen time and percent body fat in both sexes. Efforts to prevent obesity in youth should emphasize reducing screen time.

The authors examined the potential association between changes in the number of moderate-to-vigorous physical activity (MVPA) sessions per week, adiposity, and systolic blood pressure (SBP) during adolescence. SBP and anthropometric factors were assessed biannually (1999/2000, 2002, and 2004) in a cohort of 1,293 Canadian adolescents aged 12–13 years in 1999. Self-reported 7-day recall data on MVPA sessions ≥5 minutes in duration were collected every 3 months over the 5-year period. Estimates of initial level and rate of decline in number of MVPA sessions per week from individual growth models were used as predictors of SBP in linear regression models. A decline of 1 MVPA session per week with each year of age was associated with 0.29-mm Hg and 0.19-mm Hg higher SBPs in girls and boys, respectively, in early adolescence (ages 12.8–15.1 years) and 0.40-mm Hg and 0.18-mm Hg higher SBPs, respectively, in late adolescence (ages 15.2–17.0 years). The associations were not attenuated by changes in body mass index, waist circumference, or skinfold thickness in girls during late adolescence. Although weaker, associations were evident in boys during late adolescence, as well as in both girls and boys during early adolescence. These results support prevention of declines in MVPA during adolescence to prevent higher blood pressure in youth. [ABSTRACT FROM PUBLISHER]

Longitudinal studies are needed to increase understanding of the causes of childhood obesity. To identify 1- and 2-year predictors of excess weight gain among preadolescents, the authors conducted a prospective cohort study of fourth- and fifth-grade students in 16 elementary schools located in multiethnic, low-income neighborhoods in Montreal, Quebec, Canada, that were participating in the evaluation of a school-based heart health promotion program. Subjects included 2,318 children aged 9–12 years with baseline and 1-year follow-up data and 633 children aged 9–11 years with baseline and 2-year follow-up data. One-year predictors of highest decile of change in body mass index (BMI) identified in logistic regression analyses included baseline BMI of 90th percentile or more (odds ratio (OR) = 2.66, 95% confidence interval: 1.80, 3.94) in boys and baseline BMI of 90th percentile or more (OR = 2.34, 95% confidence interval: 1.46, 3.76), no sports outside school (OR = 1.90, 95% confidence interval: 1.18, 3.06), and playing video games everyday (OR = 2.48, 95% confidence interval: 1.04, 5.92) in girls. Two-year predictors included baseline BMI of 90th percentile or more (OR = 3.26, 95% confidence interval: 1.52, 7.01), no sports outside school (OR = 2.14, 95% confidence interval: 0.96, 4.77), and least active (OR = 2.18, 95% confidence interval: 1.01, 4.71) in boys; only baseline BMI of 90th percentile or more (OR = 2.22, 95% confidence interval: 1.02, 4.81) was significant in girls. Results suggest the need for interventions to promote increased physical activity in children. Am J Epidemiol 2000;152:739–46. [ABSTRACT FROM PUBLISHER]