Timing of MVPA and psychosocial outcomes in adolescents with overweight/obesity.

BACKGROUND

Adolescents with overweight/obesity have an elevated risk of mental health and behavioral difficulties. Exercise has been shown to impart psychological benefits to these individuals; however, whether the effects of moderate-to-vigorous physical activity (MVPA) delivery differ between weekdays and weekends is limited. Therefore, this study aimed to examine the effects of weekday and weekend MVPA at age 14 on internalizing and externalizing problems at age 17 among adolescents with overweight/obesity.

METHODS

We analyzed data from two assessment waves of the UK Millennium Cohort Study (MSC): MCS6 (2015-2016; age ∼14) and MCS7 (2018-2019; age ∼17). Data were restricted to adolescents classified as overweight/obesity at MCS6 using the UK90 thresholds. Weekday and weekend MVPA were measured at age 14 using the wrist-worn GENEActiv accelerometer, including one pre-specified weekday and one weekend day. Outcomes at age 17 were parent-reported Strengths and Difficulties Questionnaire (SDQ) internalizing (emotional + peer problems) and externalizing (conduct + hyperactivity/inattention) composites. We estimated average treatment effects (ATEs) and conditional average treatment effect (CATEs, heterogeneous effects) using a causal forest framework (EconML) and adjusted for pre-exposure covariates (age, sex, body mass index, ethnicity, cognitive decision-making, household income, parental education, and parental mental health). Missing data were treated via K-Nearest Neighbors imputation.

RESULTS

The analytic sample included 1,238 adolescents (mean age 14.25 years). Mean MVPA was higher on weekdays than weekends (135.74 ± 62.08 vs 113.80 ± 64.37 min/day). Covariate-adjusted average treatment effects (ATEs; per 1 min/day MVPA) were small and not statistically significant for internalizing or externalizing problems. Weekday MVPA ATEs were -0.0025 (95% CI -0.0062 to 0.0012) for internalizing and 0.0003 (-0.0027 to 0.0033) for externalizing; weekend MVPA ATEs were -0.0008 (-0.0051 to 0.0035) and 0.0005 (-0.0026 to 0.0037), respectively. Heterogeneity was evident only for weekday MVPA effects on internalizing (22.98% with significant individual effects), with height as the strongest moderator (β = 0.0002; p < 0.001; R² = 0.519) and more negative CATEs among shorter adolescents (Q1 -0.004994 vs Q4 -0.001903; ANOVA F = 106.741, p < 0.001).

CONCLUSIONS

In adolescents with overweight/obesity, estimated average effects of weekday and weekend MVPA at the age of 14 on parent-reported internalizing and externalizing problems at age 17 were close to zero under a selection-on-observables framework. Any potential benefits may be subgroup-specific and context-dependent; the observed weekday-specific heterogeneity warrants replication with more reliable exposure measurement and a richer set of contextual covariates.