Joint associations of the triglyceride–HDL cholesterol–glucose–body mass index composite index and C-reactive protein with incident cardiometabolic multimorbidity in older Chinese adults: a nationwide prospective cohort study

Background Cardiometabolic multimorbidity (CMM), defined as the coexistence of diabetes, heart disease, and stroke, is a major health burden in ageing populations. The triglyceride–high-density lipoprotein cholesterol–glucose–body mass index composite index (TyHGB) integrates lipid, glycaemic, and adiposity-related metabolic burden, whereas C-reactive protein (CRP) reflects systemic inflammation. We examined the individual and joint associations of TyHGB and CRP with incident CMM among Chinese adults aged 60 years or older. Methods This prospective cohort study included 3,857 participants from the China Health and Retirement Longitudinal Study who were free of CMM at baseline. TyHGB was calculated as TG/HDL-C + 0.7 × fasting blood glucose + 0.1 × body mass index. TyHGB and CRP were dichotomized by their baseline medians and combined into four exposure groups. Incident CMM was defined as the development of two or more cardiometabolic diseases. Cox models with multiple imputation estimated hazard ratios (HRs) and 95% confidence intervals (CIs). Restricted cubic splines, interaction, subgroup, competing-risk, sensitivity, and prediction analyses were performed. Results During follow-up, 372 participants developed CMM. In the fully adjusted model, high TyHGB was strongly associated with incident CMM (HR 2.44, 95% CI 1.93–3.09), whereas high CRP showed a weaker association (HR 1.24, 95% CI 1.01–1.53). Compared with low TyHGB plus low CRP, high TyHGB plus low CRP (HR 2.91, 95% CI 2.09–4.07) and high TyHGB plus high CRP (HR 2.85, 95% CI 2.06–3.93) were associated with higher CMM risk, while low TyHGB plus high CRP was not significant. Spline analyses showed a nonlinear positive association for TyHGB but no significant dose–response association for log-transformed CRP. No additive or multiplicative interaction was detected. The joint classification modestly improved time-dependent AUCs, and findings were generally consistent across subgroup and sensitivity analyses. Conclusions Among Chinese adults aged 60 years or older, TyHGB was robustly associated with incident CMM, while CRP provided weaker independent risk information. Joint TyHGB–CRP classification helped distinguish CMM risk gradients and may support metabolic–inflammatory risk stratification in older adults.