Does moderate to vigorous physical activity mediate the association between depression and physical function in midlife: Evidence from two British birth cohort studies

Background: Mental health and physical health are intrinsically linked, yet the mechanisms are not well un-derstood. We investigated whether moderate-vigorous physical activity (MVPA) mediated the association be-tween depression and physical function (PF) in midlife.Methods: Individuals from two UK birth cohorts born within one week in 1958 (n = 7278) and 1970 (n = 6097) with data on depression (ages 33/34; Malaise Inventory), MVPA (age 42; self-reported) and PF (ages 50/56; Short Form-36 subscale). Covariates included sex, childhood and adulthood social class, maternal mental health, childhood mood, alcohol consumption, smoking habits, sleep, marital status, BMI and long-standing illness/ disability. Linear or multinomial logistic regression models examined associations between depression, MVPA and PF. We used a parametric g-computation mediation analysis approach to estimate percent differences in PF.Results: Depression was associated with less frequent MVPA and poorer PF. Lower MVPA was associated with worse PF. The direct effect - randomised analogue not operating via MVPA - of depression on PF was-18.8 % (95%CI:-25.8,-11.8) and-15.8 % (20.6,-11.0) in the 1958 and 1970 cohorts, respectively. The indirect effect - operating via MVPA - was-0.5 % (-1.0,-0.03) and-0.2 % (-0.6, 0.3), resulting in a total proportion mediated of 3.1 % (0.1, 6.0) and 0.9 % (-1.6, 3.4). Limitations: MVPA was self-reported. Intermediate confounders and mediators were measured at the same age, however associations did not change in sensitivity analysis considering age 46 MVPA (1958 cohort) Conclusions: Although higher MVPA was protective against poor PF, there was only minor evidence that it mediated the association between depression and PF. Further investigation into other potential mediators of pathways from mental to physical health is needed.