Objectively measured physical activity and longitudinal changes in adolescent body fatness: an observational cohort study

BackgroundThe data regarding prospective associations between physical activity (PA) and adiposity in youth are inconsistent. ObjectiveThe objective of this study was to investigate associations between baseline levels of objectively measured PA and changes in adiposity over 2.5 years from mid-to-late adolescence. MethodsThis was an observational cohort study in 728 school students (43% boys) from Cambridgeshire, United Kingdom. Fat mass index (FMI, kgm(-2)) was estimated at baseline (meanstandard deviation age: 150.3 years) and follow-up (17.5 +/- 0.3 years) by anthropometry and bioelectrical impedance. Habitual PA was assessed at baseline by 3d combined heart rate and movement sensing. Average daily PA energy expenditure (PAEE) and the time (mind(-1)) spent in light, moderate and vigorous intensity PA (LPA, MPA and VPA, respectively) was estimated. Multilevel models were used to investigate associations between baseline PA and change in FMI (FMI). Adjustment for baseline age, sex, follow-up duration, area-level socioeconomic status, season of PA assessment, sedentary time, energy intake and sleep duration was made; baseline FMI was also added in a second model. ResultsFMI increased significantly over follow-up (0.6 +/- 1.2kgm(-2), P<0.001). Baseline PAEE and LPA positively predicted FMI in overfat participants (P0.030), as did VPA in initially normal fat participants (P0.044). There were further positive associations between PAEE and FMI in normal fat participants, and between MPA and FMI in both fat groups, when adjusted for baseline FMI (P0.024). ConclusionsBaseline PAEE and its subcomponents were positively associated with small and unlikely clinically relevant increases in FMI. These counter-intuitive findings may be explained by behavioural changes during the course of study follow-up.