Background Greater moderate-to-vigorous physical activity (MVPA) is associated with lower risk of some child mental health problems (CMHP). However, there is no evidence showing the potential population impact of increasing MVPA on CMHP (prevalence and inequalities). We used data from the UK Millennium Cohort Study (∼18 000 children born 2000–2002), to model a hypothetical MVPA intervention scenario, simulating universal achievement of the government’s MVPA target of 60 min (m) MVPA per day.
Methods 6344 children had MVPA (measured using accelerometers at 7 years[y]); of these, 4590 had data on outcome (CMHP), exposure (socio-economic circumstances) and potential confounders. CMHP at 11 y were measured using parent-reported Strengths and Difficulties Questionnaire (SDQ) total score, dichotomised using an established cut-off.
Predicted probabilities of CMHP were estimated in logistic marginal structural models, weighted for attrition, adjusted for MVPAand baseline and intermediate confounding (including externalising behaviours at 7[y], to account for potential reverse causality between MVPA and some aspects of CMHP, such as hyperactivity).Inequalities were assessed using Risk ratios (RRs) and differences (RDs) [95% CIs], according to household income quintiles. Intervention was simulated by re-estimating predicted probabilities after modifying theMVPA variable.
Results 49% of children achieved the 60 m MVPA target, with greater activity levels observed in lowest (65 m) compared to highest (62 m) income quintile. Greater MVPA was associated with increased risk of CMHP (RR 1.003 [0.999–1.007]), and with externalising problems in particular (RR 1.009 [1.005–1.013]).
Overall prevalence of CMHP was 12.2%, with relative and absolute inequalities between lowest and highest income quintiles (RR 2.6 [1.5–3.7]; RD 11.7 [6.0–17.4]).
Simulation of the intervention led to 96% achievement of the 60 m MVPA target (30 m average increase for all children, assuming 100% uptake). CMHP prevalence increased to 14.1%. Relative inequality decreased slightly (RR 2.5 [1.5, 3.6]), and absolute inequality increased (RD 13.3%; [6.6, 20.0]). In sensitivity analyses with internalising problems as the outcome, greater MVPA decreased absolute inequality in CMHP, but not relative risk or prevalence.
Conclusion Findings based on a UK-representative sample of children with objective MVPA data and a validated measure of CMHP, imply that universal achievement of the national MVPA target may not reduce prevalence in CMHP or absolute inequalities. However, these findings are likely subject to reverse causation (despite adjustment for earlier externalising behaviours). Further analyses will examine these relationships in detail, including differentiating aspects of CMHP (emotional, peer, conduct and hyperactivity subscales), using teacher-reported SDQ, and positive mental health outcomes.
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