A Physical Activity and Sitting Time Balance Index and Adolescent Psychological Distress: A Prospective Cohort Study

While "move more, sit less" remains a cornerstone of public health guidelines, the incidence of mental disorders among adolescents—particularly those with obesity—continues to rise despite substantial intervention resources. This paradox suggests that traditional linear intervention models may be insufficient. To investigate the underlying mechanisms, this study utilized objective accelerometer data from 8,753 adolescents in the UK Millennium Cohort Study (MCS) to construct an integrated "Physical Activity and Sitting Time Balance Index" (PASTBI). Using a prospective design, we examined the impact of PASTBI on psychological distress at age 17. Our results indicate that while PASTBI generally improves mental health, it exhibits a non-linear threshold effect in adolescents with obesity. Specifically, the psychological protection mechanism is activated only when the index exceeds a specific threshold (≈ 12.8); low-dose improvements fail to yield significant benefits. Furthermore, mechanism analysis reveals that the nature of sedentary behavior matters: academic-related "active sitting" exerts a negative impact on psychological distress that is over ten times greater than that of passive sitting (e.g., television viewing). These findings suggest that interventions for adolescents with obesity require a "high-dose" strategy to overcome the behavioral change threshold. Additionally, educators should be vigilant regarding high-intensity academic sitting and mitigate cognitive depletion through fragmented physical activity.