Physical Activity Intensity and Adiposity in Canadian Youth

Abstract

PURPOSE: The purpose of this study was to compare the strength of the association between various intensities of physical activity PA and adiposity in a population-based sample of Canadian youth. HYPOTHESIS: We hypothesized that among youth 10-15 years of age, the strength of the association between PA and measures of adiposity (waist circumference (WC) and BMI Z score) would be significantly greater for moderate (MPA) and vigorous (VPA) physical activity than for light (LPA) or sedentary time. METHODS: Data was collected in 630 youth between grades 5 to 9 participating in the 2008 wave of the Healthy Hearts Prospective cohort study. The primary exposure variable, PA intensities were assessed objectively using a hip-mounted accelerometer worn for seven consecutive days. Thresholds (counts/min) to classify PA intensity were based on a regression formula established by Puyau (MSSE 2004). Primary outcome variables were BMI Z score (CDC growth charts) and WC (measured at the top of the iliac crest). Additional measurements included cardiorespiratory fitness (Leger 20m shuttle run), and self-reported dietary intake (WebSpan, 24-hr recall). RESULTS: In multivariate analyses the B-coefficient for the association with BMI Z score was negative and significant (p <0.03) for MPA (-0.13) and VPA (-0.11) but was not significant for sedentary time or LPA. MPA and VPA were also negatively associated with WC (B = -0.89, p=.048 and B = -0.10, p=.022 respectively). Sedentary time and LPA were not negatively associated with WC. Compared to youth in the highest tertile of VPA, the odds of being overweight were ∼2-fold higher for youth in the middle (OR = 1.7, CI 1.0 - 2.) and lowest tertile (OR=2.3, CI 1.5- 3.7). A similar trend was observed across tertiles of MPA. No effect was seen across tertiles of sedentary or light minutes. CONCLUSION: In contrast to LPA or sedentary time, greater daily VPA and MPA are associated with lower levels of adiposity. Intervention studies are needed to confirm the observation that higher intensity PA is more effective than light PA for reducing the risk of overweight in youth.