Bidirectional relationships among physical activity, cardiorespiratory fitness, BMI z-score, and percent body fat in adolescents: a longitudinal exploratory study

Introduction Although associations among cardiorespiratory fitness, moderate-to-vigorous physical activity (MVPA), body mass index (BMI) z-score, and percent body fat are established, their longitudinal and potentially bidirectional relationships remain unclear. Clarifying these relationships may inform more effective obesity-prevention strategies.Theory The study was informed by a bidirectional developmental perspective proposing that MVPA, cardiorespiratory fitness, and adiposity may reciprocally or sequentially influence one another during adolescence.Methods Participants were 450 adolescents in grades 5–8 (Mage = 12.64) from 16 urban middle schools who participated in the control arm of a larger trial. Cardiorespiratory fitness, MVPA, BMI z-score, and percent body fat were objectively assessed at baseline and 17 weeks later. Hierarchical linear regression models adjusted for demographics were performed.Results Lower baseline percent body fat predicted greater improvements in cardiorespiratory fitness (B = −0.07, p = .002), and higher baseline cardiorespiratory fitness predicted subsequent increases in MVPA (B = 0.81, p = .005). Lower baseline cardiorespiratory fitness (B = −0.57, p < .001), MVPA (B = −0.81, p < .001), and percent body fat (B = −0.17, p < .001) each predicted greater increases in their respective outcomes. Higher MVPA unexpectedly predicted greater increases in BMI z-score (B = 0.002, p = .012), and higher baseline BMI z-score predicted increases in percent body fat (B = 1.19, p < .001).Discussion Findings suggest sequential relationships among adiposity, fitness, and MVPA. Multicomponent obesity-prevention interventions simultaneously targeting all three domains may better support healthy development during adolescence. Longer longitudinal studies accounting for puberty and sex-specific development are warranted.