Mediating role of physical fitness and fat mass on the associations between physical activity and bone health in youth.

We aimed to assess if the relationship between VPA and bone health is simultaneously mediated by PF and fat mass in adolescents. Bone health was assessed by quantitative ultrasound (QUS) in 412 participants (221 girls) aged 10-18 years. VPA was assessed by accelerometry and PF was measured using specific protocols from FITescola®. Fat mass (%) was assessed using two skinfolds (triceps and calf). Parallel mediation analysis was performed by Hayes' PROCESS (V.3.3-model 4) for SPSS. We observed that in boys, handgrip mediated the associations of VPA with speed of sound on the third distal radius (R-SoS). While, speed at 20 m and handgrip mediated the relationship of VPA with speed of sound on the tibial midshaft (T-SoS). Body fat (%) only acted as a mediator when handgrip integrates the mediation model. For girls, the only mediating variable for the relationship between VPA and R-SoS or T-SoS was the PACER test. Handgrip, speed and fat mass (%) in boys, and cardiorespiratory fitness in girls mediates the relationships between VPA and bone health assessed by QUS. Promoting muscular fitness and cardiorespiratory fitness and decrease of fat mass through VPA in adolescents may be an important strategy to improve bone health.