Background: Psychosocial work stressors may lead to low back pain (LBP) through depressive symptoms or to depression through LBP. Depressive symptoms or LBP may also modify these associations.

Methods: We examined prospective interrelationships between job demands, LBP and depsive symptoms. We used comparable data from three consecutive biennial surveys in 2010-2016, from the Swedish Longitudinal Occupational Survey of Health (SLOSH) and the Work Environment and Health in Denmark (WEHD) cohorts, broadly representative of the working populations in Sweden and Denmark. We conducted multivariate counterfactual based mediation analyses allowing for four-way decomposition of the total effect of job demands, on incident LBP (N=2813, 2701) and incident major depression (N=3707, 5496). The four components estimated direct and indirect effects through mediation and/or interaction.

Results: We observed no association between job demands and incident LBP four years later, but job demands was associated with later major depression (relative risks=1.88, 95% confidence interval=1.45-2.31 in SLOSH and 1.64, 1.18-2.11 in WEHD, adjusted for age, sex, panel (SLOSH data), education, cohabitation, physically strenuous work and chronic diseases. About 37% of the association was attributed to interaction between job demands and LBP in SLOSH. No interaction was found in WEHD. LBP partly mediated the relationship, by 14% in SLOSH and 2%, while statistically insignificant in WEHD.

Limitations: Possible limitations include lack of comparable data on disabling low back pain, different scales for depressive symptoms, misclassification and residual confounding.

Conclusions: This suggests mainly a direct effect of job demands on major depression, or through other pathways than LBP.