Does positive affect mediate the association of multimorbidity on depressive symptoms?

Objectives Multimorbidity poses various challenges, and previous research has indicated a causal relation with depression. As multimorbidity is not curable, the underlying mechanisms are of great interest. Positive affect is a major resource for coping with chronic conditions and for the prevention of depression. Long-term multimorbidity, however, may deplete positive affect. The purpose of this paper is to investigate the role of positive affect in the association between multimorbidity and depressive symptoms. Method We used four consecutive waves (2008, 2011, 2014, 2017) of the nationally representative German Ageing Survey (DEAS) with a total of 1,558 older adults aged 40 and over. To account for time-varying confounding, exposure-induced mediator-outcome confounding, and reciprocities, we applied the mediational g-formula with inverse-probability weighting techniques. We also tested for exposure-mediator interaction to adjust for differences in mediation across the duration of multimorbidity. Results We confirmed a positive longitudinal relation between multimorbidity and depressive symptoms, both of which were negatively associated with while positive affect. The model without interaction indicated a share mediated of ca. 18.3% on the total effect of multimorbidity on depressive symptoms. Addition of interaction led to substantial differences for multimorbidity duration and levels of positive affect. Associations for long-term multimorbidity (at least two survey waves) were more substantial, and the share mediated doubled (>40%). Additionally, the direct effect of multimorbidity on depressive symptoms diminished for short-term multimorbidity. Conclusion Strengthening positive affect could reduce depressive symptoms in those facing multimorbidity. This study also discusses methodological challenges in performing longitudinal mediation analysis. We advise researchers to consider the mediational g-formula and exposure-mediator interaction.