RCII-frailty-sarcopenia burden and incident chronic lung disease: a prospective cohort study of Chinese middle-aged and older adults based on CHARLS

Abstract Background The remnant cholesterol inflammatory index (RCII) integrates remnant cholesterol with systemic inflammation. Whether RCII combined with frailty and sarcopenia is associated with subsequent reported chronic lung disease (CLD) remains unclear. Methods We analysed a selected CHARLS cohort aged ≥45 years without reported CLD at baseline in 2011, with follow-up through 2020. The exploratory, equal-weight RCII-Frailty-Sarcopenia (RCII-FS) burden counted high RCII, frailty (frailty index [FI] ≥0.25), and sarcopenia. We fitted Cox models, assessed nonlinear and interaction terms, performed sensitivity analyses, and evaluated discrimination using bootstrap and 70:30 split validation. We estimated Aalen–Johansen cumulative incidence with death as a competing event in a supplementary analysis. Results Among 7,443 participants, 1,156 reported CLD during a median 9.0 years of follow-up. Relative to zero components, fully adjusted HRs (95% CIs) for one, two and three components were 1.19 (1.04–1.37), 1.71 (1.41–2.08) and 3.07 (2.06–4.58); the per-component HR was 1.33 (1.21–1.45). In separate adjusted models, HRs were 1.17 (1.04–1.32) for high RCII, 1.79 (1.51–2.12) for frailty and 1.20 (1.01–1.42) for sarcopenia. Exploratory restricted cubic splines suggested a nonlinear RCII pattern (P overall=0.032; P nonlinear=0.014), a positive FI association (both P < 0.001), and an inverse association for pSMI (P overall=0.003; P nonlinear=0.097). In a supplementary Aalen–Johansen analysis with death as a competing event, 9-year cumulative incidence of first reported CLD was 13.70% (95% CI 12.44-15.07) with zero components and 34.75% (26.05-45.33) with three components. Adding burden to conventional predictors provided a very small increment in discrimination. Conclusions Higher RCII-FS burden was associated with subsequently reported CLD in this CHARLS-derived cohort, with frailty showing the clearest component association. RCII-FS remains an exploratory, equal-weight construct. Its main value appears to lie in characterising the epidemiologic association; predictive and clinical utility remain unestablished. Clinical trial registration Not a clinical trial.