Dual trajectories of intrinsic capacity and frailty: a joint predictor framework for dementia risk in middle-aged and older adults

BACKGROUND: While intrinsic capacity (IC) and frailty are established predictors of adverse health outcomes, their dynamic trajectories and synergistic impact on dementia risk remain underexplored. This study aimed to identify heterogeneous trajectories of IC and frailty across midlife and older age, examine their interrelationships, and determine their independent and joint associations with dementia incidence. METHODS: Data were obtained from the China Health and Retirement Longitudinal Study (CHARLS), with 5,472 participants aged ≥ 50 years followed between 2011 and 2020. We used group-based dual trajectory modeling on data from the 2011-2015 waves to identify distinct IC and frailty trajectories. Incident dementia cases were ascertained at the 2018 and 2020 follow-ups. Cox proportional hazards models evaluated the associations of IC trajectories, frailty trajectories, and their combinations with dementia risk. RESULTS: Three IC trajectories were identified: stable high, stable moderate, and stable low; and four frailty trajectories: stable robust, improving pre-frail, stable pre-frail, and stable frail. The two sets of trajectories showed strong interdependence. During a median 5-year follow-up, 472 incident dementia cases were recorded. Compared with the stable high IC group, the stable low IC group showed a significantly higher dementia risk (HR: 3.87, 95% CI: 2.62-5.72). Similarly, the stable frail group had an elevated risk (HR: 3.01, 95% CI: 2.15-4.21) relative to the stable robust group. The combination of stable low IC and stable frail trajectories was associated with a 7.31-fold higher risk (HR: 7.31, 95% CI: 3.93-13.59). Incorporating these two trajectories significantly improved the prediction of dementia risk. CONCLUSIONS: Longitudinal IC and frailty trajectories are strong and interdependent predictors of dementia. Dynamic monitoring these trajectories offers a valuable framework for identifying high-risk individuals and guiding targeted interventions to preserve functional capacity, reduce vulnerability, and mitigate dementia risk in aging populations.