Cross-Sectional Associations of Objectively-Measured Physical Activity and Sedentary Time with Body Composition and Cardiorespiratory Fitness in Mid-Childhood: The PANIC Study

Abstract

Background: The minimum intensity of physical activity (PA) that is associated with favourable body composition and cardiorespiratory fitness (CRF) remains unknown.

Objective: To investigate cross-sectional associations of PA and sedentary time (ST) with body composition and CRF in mid-childhood.

Methods: PA, ST, body composition and CRF were measured in a population-based sample of 410 children (aged 7.6 ± 0.4 years). Combined heart-rate and movement sensing provided estimates of PA energy expenditure (PAEE, kJ/kg/day) and time (min/day) at multiple fine-grained metabolic equivalent (MET) levels, which were also collapsed to ST and light PA (LPA), moderate PA (MPA) and vigorous PA (VPA). Fat mass index (FMI, kg/m²), trunk fat mass index (TFMI, kg/m²) and fat-free mass index (FFMI, kg/m^2.5) were derived from dual-energy X-ray absorptiometry. Maximal workload from a cycle ergometer test provided a measure of CRF (W/kg FFM). Linear regression and isotemporal substitution models were used to investigate associations.

Results: The cumulative time above 2 METs (221 J/min/kg) was inversely associated with FMI and TFMI in both sexes (p < 0.001) whereas time spent above 3 METs was positively associated with CRF (p ≤ 0.002); CRF increased and adiposity decreased dose-dependently with increasing MET levels. ST was positively associated with FMI and TFMI (p < 0.001) but there were inverse associations between all PA categories (including LPA) and adiposity (p ≤ 0.002); the magnitude of these associations depended on the activity being displaced in isotemporal substitution models but were consistently stronger for VPA. PAEE, MPA and to a greater extent VPA, were all positively related to CRF (p ≤ 0.001).

Conclusions: PA exceeding 2 METs is associated with lower adiposity in mid-childhood, whereas PA of 3 METs is required to benefit CRF. VPA was most beneficial for fitness and fatness, from a time-for-time perspective, but displacing any lower-for-higher intensity may be an important first-order public health strategy. Clinical trial registry number (website): NCT01803776 ( https://clinicaltrials.gov/ct2/show/NCT01803776 ).

Fig. 1

Daily cumulative awake time spent above single-MET categories. Data are mean values and error bars represent ± standard deviation. Sedentary time corresponds to the region: ≤1.5 METs; light physical activity: 1.5–3 METs; moderate physical activity: >3–6 METs; vigorous physical activity: >6 METs. Inset shows magnified plot for >4 METs. METs metabolic equivalents

Fig. 2

Associations between the cumulative awake time above single MET categories and a FMI, b TFMI, c FFMI and d CRF. Statistical analyses performed using multiple imputed datasets and linear regression adjusted for physical activity monitor wear characteristics (proportion of weekend data and season of measurement), demographics (age, sex, household income), behaviours (sleep duration, energy intake, frequency of breakfast consumption, number of meals per day, snacking), birth weight, maternal and paternal BMI. Adjustment for CRF was further made when FMI, TFMI and FFMI were outcomes and CRF was adjusted for FMI. School clustering was accounted for by using robust standard errors. FMI and TFMI were skewed and natural log-transformed prior to analyses, their data have been back-transformed to represent the percentage difference (95 % CI) in variables per 10 min spent above a MET level. All results are scaled to represent the association between exposures and outcomes per 10 min difference in exposures. METs metabolic equivalents, FMI fat mass index, TFMI trunk fat mass index, FFMI fat-free mass index, CRF cardiorespiratory fitness