A physical activity and sitting time balance index and psychological distress in adolescents: a prospective cohort study

This study introduces a Physical Activity and Sitting Time Balance Index (PASTBI) to address the interplay between physical activity (PA) and sitting time, and to examine its prospective association with psychological distress in adolescents across weight statuses. This prospective cohort study used data from 8753 adolescents in the UK Millennium Cohort Study. PASTBI was calculated as objectively accelerometer-measured moderate-to-vigorous PA (min/day) divided by self-reported sitting time (h/day), both measured at age 14. Psychological distress was assessed at age 17 with the Kessler K6 scale. Linear regression models adjusted for sociodemographics, baseline depressive symptoms, diet quality, and obesity status. Among normal-weight adolescents, PASTBI showed a linear inverse association with distress (β=−0.014, p = 0.030). Among adolescents with overweight/obesity, no linear association was found ( p = 0.996); however, exploratory quadratic models suggested a possible non-linear pattern (quadratic β=− 0.0013, p = 0.003). This pattern was attenuated after Winsorization and should be interpreted cautiously; the estimated turning point should be regarded as exploratory rather than as a validated behavioral threshold. Decomposition analyses showed that active sitting (e.g., homework) was more strongly associated with distress than passive sitting (e.g., TV) (β = 0.465 vs. β = 0.042). The utility of PASTBI in characterizing prospective associations with psychological distress may differ by weight status. Among adolescents with overweight/obesity, exploratory analyses suggested a possible non-linear association, although this pattern was sensitive to Winsorization. Active sitting showed a stronger positive association with psychological distress than passive sitting. These findings are hypothesis-generating and warrant future trials to test whether altering the PA-to-sitting balance or introducing movement breaks influences adolescent mental health.