Combined influence of depression severity and low-grade inflammation on incident hospitalization and mortality risk in Italian adults

Background: Depression and low-grade systemic inflammation are associated risk factors for hospitalizations and mortality, although the nature of this relationship is under-investigated. Methods: We performed multivariable Cox regressions of first hospitalization/mortality for all and specific causes vs depression severity, in an Italian population cohort (N=13,176; age≥35 years; 49.4% men), incrementally adjusting for sociodemographic, health and lifestyle factors. We tested potential mediation, additive and interactive effects of INFLA-score, a composite circulating inflammation index, and potential concurrent mediations of main lifestyles and chronic conditions. Results: Over 4,856 hospitalizations (median follow-up 7.28 years), we observed an increased incident risk of events by 24% (CI=17-32%) and 59% (30-90%) for moderate and severe depression, which also showed a 125% (33-281%) increased risk of all-cause mortality (over 471 deaths, 8.24 years). These remained stable after adjustment for lifestyles, health conditions and INFLA-score, which explained 2.1%, 7.6%, 16.3% and 8%, 14.9% and 12% of depression influence on hospitalizations and mortality risk, respectively. These proportions remained substantially stable after reciprocal adjustments. INFLA-score showed significant additive (but not interactive) effects on both hospitalizations and mortality risk. Limitations: Depression severity was defined using a sub-version of Patient Health Questionnaire 9, which was validated here. Directionality links among exposures could not be established since they were collected simultaneously. Conclusions: These findings suggest a combined influence of depression and low-grade inflammation on health, which is partly intertwined and dependent on lifestyles and chronic conditions. This suggests the existence of pathways other than inflammation through which depression may play its detrimental effect