Grip strength in the association between frailty and new-onset depression in older adults: a multinational longitudinal study

Background Frailty is a multidimensional geriatric syndrome characterized by cumulative physiological decline. While associated with various adverse outcomes, the longitudinal relationship between frailty and new-onset depression across diverse national populations remains insufficiently explored. This study investigated the association between the Frailty Index (FI) and new-onset depression in three large aging cohorts. Methods This multinational longitudinal cohort study included community-dwelling adults aged ≥50 years from three nationally representative aging cohorts: the Health and Retirement Study (HRS), the English Longitudinal Study of Aging (ELSA), and the Survey of Health, Aging and Retirement in Europe (SHARE). Frailty was assessed using a deficit accumulation–based frailty index (FI). New-onset depression was defined by validated CES-D 8 or EURO-D scores among participants free of depression at baseline. Associations were estimated using Cox proportional hazards, Fine–Gray competing risk, and discrete-time logistic regression models. Dose–response patterns were examined using restricted cubic splines, and mediation analysis evaluated the role of grip strength. Results Over median follow-ups of 8 (HRS), 2 (ELSA), and 4 (SHARE) years, 749, 973, and 7,821 participants developed new-onset depression, respectively. Cohort-specific incidence rates were 34.5% in HRS (749/2, 171), 18.6% in ELSA (973/5, 223), and 30.4% in SHARE (7, 821/25, 721). Adjusted analyses showed the FI was consistently associated with depression risk across all cohorts. Each one-unit FI increase was associated with an approximately 4% higher risk (HR = 1.04, p < 0.001 for all cohorts). Participants in the highest FI quartile had significantly elevated risk compared to the lowest. Results were robust across all statistical models. Restricted cubic splines indicated a nonlinear association, which was partially mediated by grip strength. Conclusion Frailty is an independent predictor of new-onset depression in older adults across diverse international populations. Early frailty identification and targeted multidimensional interventions—including grip strength enhancement, structured physical activity, nutritional optimization, and psychosocial support—may help reduce depression incidence and promote healthy aging.