Major depressive episodes and mortality in the Canadian household population.

Objective: To explore the association between major depressive episodes (MDE) and subsequent mortality in a representative sample of the general household population, with adjustment for other determinants of mortality.

Method: The analysis used four datasets from the Canadian Community Health Survey (CCHS); the CCHS 1.1 (conducted in 2000 and 2001), the CCHS 1.2 (conducted in 2002), the CCHS 2.1 (conducted in 2003 and 2004) and the CCHS 3.1 (conducted in 2005 and 2006). Each survey included an assessment of past-year major depressive episodes (MDEs) and was linked to mortality data from the Canadian Mortality Database for January 1, 2000 to December 31, 2011. The hazard ratio (HR) for all-cause mortality was estimated in each survey sample. Random effects, individual-level meta-analysis was used to pool estimates from the four survey data sets. Estimates were adjusted for other determinants of mortality prior to pooling in order to help quantify the independent contribution of MDE to all-cause mortality.

Results: The unadjusted HR was 0.77 (95% CI 0.63-0.95). A naïve interpretation of this HR suggests a protective effect of MDE, but the estimate was found to be strongly confounded by age (age adjusted HR for MDE: 1.61, 95% CI 1. 34-1.93) and by sex (sex adjusted HR for MDE: 1.15, 95% CI 0.75-1.77). The age and sex adjusted HR was: 1.70 (95% CI 1.45-2.00). No evidence of effect modification by any determinant of mortality was found, including sex. After adjustment for a set of mortality risk factors, the pooled HR was weakened, but remained statistically significant, HR = 1.29 (I-squared = < 1%, tau-squared < 0.001, 95% CI 1.10-1.51). Smoking was the strongest single confounding variable.

Conclusions: MDE is associated with elevated mortality. The elevated risk is partially attributable to psychosocial, behavioral and health-related determinants. Since MDE itself may have caused changes to these variables, these estimates cannot fully quantify the independent contribution of MDE to mortality. However, these results suggest that clinical and public health efforts to counteract the effect of MDE on mortality may benefit from attention to a broad set of mortality risk factors e.g. smoking, physical activity, management of medical conditions.