Association between TyG-FI and depressive symptom trajectories in middle-aged and older adults: interaction with cognitive function and mediation analysis of sleep quality

BACKGROUND: Metabolic disturbance and frailty have both been linked to depression, but their combined association with long-term depressive symptom patterns remains unclear. This study examined the association between the triglyceride-glucose frailty index and depressive symptom trajectories in middle-aged and older adults. METHODS: We used data from 10,684 participants in the China Health and Retirement Longitudinal Study. Depressive symptom trajectories across five waves were identified using group-based trajectory modeling. Logistic regression was used to examine the association between the triglyceride-glucose frailty index and depressive symptom trajectories. Restricted cubic spline and two-piecewise regression models were applied to assess nonlinearity and threshold effects. Interaction and mediation analyses were conducted to explore the roles of total cognition and sleep quality. RESULTS: Two depressive symptom trajectories were identified: a low depressive symptoms group (80.16%) and a high depressive symptoms group (19.84%). Higher triglyceride-glucose frailty index levels were significantly associated with greater odds of belonging to the high depressive symptom trajectory group. In the fully adjusted model, the odds ratios (95% confidence intervals) for the middle and highest triglyceride-glucose frailty index tertiles were 3.99 (3.18-5.00) and 12.68 (10.08-15.94), respectively, compared with the lowest tertile. A nonlinear positive association was observed, with an inflection point at 47.55. Total cognition significantly modified this association, and sleep quality mediated 16.9% of it. CONCLUSIONS: A higher triglyceride-glucose frailty index was associated with an increased likelihood of belonging to the high depressive symptom trajectory group in middle-aged and older adults. The association was nonlinear, modified by total cognition, and partly mediated by sleep quality.