There are only a few studies that have examined the interaction between physical activity (PA), cardiorespiratory fitness (CRF), and overweightness/obesity in adolescents, and these studies have shown some limitations. The objective of this study was to examine whether the association between PA (moderate-vigorous PA [MVPA], moderate PA [MPA], and vigorous PA [VPA]) and obesity is mediated by CRF. This cross-sectional study was conducted in six middle and high-schools in the Porto district (Portugal), comprising 632 children and adolescents. Fat mass (FM), CRF, MVPA, MPA, and VPA were assessed. The Process macro for SPSS was used. CRF was significantly associated with PA for both children and adolescents. Likewise, FM was negatively and significantly associated with CRF for both children (β = -0.337) and adolescents (β = -0.255). FM was associated with PA in children (MVPA: β = -0.102; MPA: β = -0.118; VPA: β = -0.305) and adolescents (MVPA: β: -0.103; MPA: β = -0.082; VPA: β = -0.204). The mediation analysis showed that the association between PA (MVPA//MPA/VPA) and obesity was mediated by CRF, in both children and adolescents, with a significant value in the Sobel test. Obesity is associated with CRF and MVPA, MPA, and VPA. However, CRF acted as a complete mediator between the association of obesity and PA.
Keywords: body mass index; exercise; fat mass; mediation; physical fitness; schools.