Are sitting, standing and stepping associated with adiposity markers in pre, primary and secondary school children and adolescents? A pooled data cross-sectional compositional approach.

Evidence on the associations between device-measured sitting and health outcomes in children and adolescents is inconclusive; however, understanding this relationship is important given the high prevalence of sitting during these life stages and its potential implications for long-term health. This study examined associations between compositions of sitting, standing, and stepping time with adiposity markers - waist circumference (WC) and body mass index (BMI) - in 2-18-year-olds. Data were pooled from eight Australian studies (2009-2014), including 1992 participants aged 2-18 years. Postural behaviours were measured using activPAL monitors and analysed using compositional data analysis. Linear regression models, adjusted for study and wear time, were conducted (n = 1,120 for zBMI; n = 1,071 for zWC), stratified by school setting (preschool, primary, secondary). Postural compositions were significantly associated with zBMI and zWC in primary and secondary school-aged children, but not in preschoolers. In primary school children, more sitting time was associated with lower zWC. Among adolescents, more sitting was linked to higher zBMI, more standing with lower zBMI and zWC, and more stepping with higher zWC. Findings in older children and adolescents suggest that the impact of postural behaviours on adiposity may become more pronounced with age. Longitudinal studies are needed to clarify these relationships.