Higher remnant cholesterol inflammatory index is associated with increased frailty risk in the UK Biobank.

Background: Frailty is a major public health concern associated with adverse outcomes. The remnant cholesterol inflammatory index (RCII), a novel biomarker integrating lipid metabolism and systemic inflammation, has been proposed as an indicator of adverse health outcomes. However, little is known about its relationship with frailty. The study aimed to investigate the longitudinal association between RCII and frailty risk in the UK Biobank. Methods: A total of 402,850 participants from the UK Biobank were included at baseline. RCII was calculated as remnant cholesterol (RC, mg/dL) × C-reactive protein (CRP, mg/L)/10, and frailty was assessed using the Fried frailty phenotype. Cox proportional hazards regression models were applied to evaluate the association between baseline RCII and incident frailty. Restricted cubic spline (RCS) analyses were used to explore potential nonlinear relationships. To capture cumulative exposure, we additionally analyzed 12,895 participants with the same measurements to examine the relationship between cumulative RCII and frailty risk. Results: During a median follow-up of 15.58 years, 2,327 participants (0.58%) developed frailty. Frailty incidence increased progressively across RCII quartiles, from 0.3% in Q1 to 0.9% in Q4 ( Conclusion: Higher RCII levels, both at baseline and cumulatively, are associated with an increased risk of frailty. RCII may be a promising biomarker for frailty risk stratification and a potential target for early prevention in aging populations.