Physical Activity and Adolescent Brain Structure, Mental Health, and Academic Outcomes

Importance Urbanization and digitalization are contributing to reductions in the physical activity of adolescent individuals. Daily step count is an increasingly accessible and widely used metric for quantifying physical activity, yet little is known about how it is associated with mental health, academic performance, and brain health in adolescents. Objective To investigate the association of daily step count and future mental health and academic performance in adolescents, and whether differences in brain structure mediate these associations. Design, Setting, and Participants This cohort study used data from participants in the Adolescent Brain Cognitive Development (ABCD) Study (release 5.1), a large-scale, longitudinal investigation designed to comprehensively characterize psychological and neurobiological development from late childhood to adolescence. The ABCD Study was conducted across 21 sites across the US, providing a diverse and geographically distributed sample. Participants were adolescent individuals with valid daily step count data and usable neuroimaging data collected between November 2018 to November 2020. Data were analyzed from May 2025 to August 2026. Exposure Pedometer-based measurements of average daily step count over the previous 3 weeks. Main Outcomes and Measures Linear mixed models examined associations of daily step count with mental health, academic performance, and brain structure, and assessed how these associations varied between different step count categories. Using structural equation modeling, step-count–associated brain morphology was examined to determine whether it statistically mediated the association between physical activity and mental health, and/or academic outcomes 1 year later. Dose-response associations were also examined. Results This study included 5515 adolescent participants (2640 [47.9%] female; mean [SD] age 11.93 [0.65] years) using step counters. Higher step count was associated with lower total mental health problems (B [SE], −0.080 [0.015]; t = −5.497; P < .001) and greater cortical thickness in the paracentral (B [SE], 0.057 [0.016]; t = 3.621; P < .001) and rostral anterior cingulate (B[SE], 0.052 [0.016]; t = 3.319; P = .001) regions. Further analyses showed a significant dose-response association, with progressively lower mental health problems across higher step categories compared with the under 5000-step group. Mediation analyses revealed that greater paracentral thickness statistically mediated the association between higher daily step count and fewer mental health problems 1 year later. Conclusions and Relevance In this cohort study, higher daily step counts were associated with structural differences in the brains of adolescents, which were in turn associated with better mental outcomes prospectively. Beneficial associations for mental health appeared to accrue continuously across the observed range, with no evidence of a threshold or plateau, highlighting that every step counts and the potential value of promoting daily steps in adolescents.