The role of frailty in the association between depression and somatic comorbidity: results from baseline data of an ongoing prospective cohort study.

BACKGROUND: Depression and physical frailty in older persons are both associated with somatic diseases, but are hardly examined in concert. OBJECTIVES: To examine whether depression and physical frailty act independently and/or synergistically in their association with somatic diseases. DESIGN: Baseline data of an ongoing observational cohort study including depressed cases and non-depressed comparison subjects. SETTINGS: Netherlands Study of Depression in Older persons (NESDO). PARTICIPANTS: 378 depressed older persons confirmed by the Composite International Diagnostic Interview (CIDI), version 2.1, and 132 non-depressed comparison subjects. METHODS: Multiple linear regression analyses adjusted for socio-demographic and life-style characteristics were conducted with the number of somatic diseases as the dependent variable and depression and physical frailty as independent variables. Physical frailty was defined as ≥3 of the following characteristics, slowness, low physical activity, weight loss, exhaustion, and weakness. RESULTS: Depression and physical frailty did not interact in explaining variance in the number of somatic diseases (p=.57). Physical frailty, however, partly mediated the association between depression and somatic diseases, as the strength of this association decreased by over 10% when frailty was added to the model (B=0.47, p=.003, versus B=0.41, p=.01). The mediation effect was primarily driven by the frailty criterion exhaustion. Of the remaining frailty components, only slowness was associated with the number of somatic diseases; but this association was fully independent of depression. CONCLUSIONS: Our results suggest that depression and physical frailty have common pathways towards somatic diseases, as well as unique pathways. As no high-risk group was identified (no significant interaction), mental health nurses should regularly monitor for physical frailty within their caseload of depressed patients.