Remnant cholesterol inflammatory index in relation to frailty burden and longitudinal trajectories: Findings from CHARLS and ELSA.

BACKGROUND: Frailty represents a critical geriatric syndrome characterized by increased vulnerability to stressors, underpinned by chronic inflammation and metabolic dysregulation. This study aimed to investigate the prospective association between baseline Remnant Cholesterol Inflammatory Index (RCII) and both baseline frailty and long-term progression trajectories in two large, ethnically diverse aging cohorts. METHODS: This study utilized data from the China Health and Retirement Longitudinal Study (CHARLS; n = 8916) and the English Longitudinal Study of Aging (ELSA; n = 5732). RCII was calculated from baseline remnant cholesterol (RC) and high-sensitivity C-reactive protein (hs-CRP). Frailty was assessed using a 32-item frailty index (FI). Linear mixed-effect models were used to examine associations between RCII tertiles and FI trajectories. RESULTS: In cross-sectional analyses, the highest RCII tertile was associated with a significantly increased baseline FI in both CHARLS and ELSA after full adjustment. In longitudinal analyses, results for RC alone indicated that while high levels were associated with consistently elevated frailty, they were not consistently associated with an accelerated rate of progression, characterized by parallel trajectories. A significant interaction between the highest RCII tertile and time was observed in CHARLS (β = 0.197, 95% CI: 0.060 to 0.335, P = 0.020) and ELSA (β = 0.055, 95% CI: 0.042 to 0.069, P < 0.001). CONCLUSION: Higher baseline RCII is consistently associated with both greater baseline frailty and is also associated with steeper frailty trajectories in older adults. These findings suggest that RCII may serve as a valuable biomarker to identify older adults at high risk for functional decline.