Association of Remnant Cholesterol Inflammation Index as potential marker of metabolic syndrome: A cross-sectional study from NHANES and CHARLS

Remnant cholesterol (RC) and high-sensitivity C-reactive protein are metabolic syndrome (MetS) risk factors. The Remnant Cholesterol Inflammation Index (RCII), combining both, reflects metabolic and inflammatory risk. This study assessed RCII's association with long-term MetS risk in US (National Health and Nutrition Examination Survey [NHANES]) and Chinese (China Health and Retirement Longitudinal Study [CHARLS]) populations. We analyzed 3743 NHANES and 9616 CHARLS participants. RCII quartiles were created. Associations with MetS risk were evaluated using multivariable logistic regression (adjusting for covariates) and restricted cubic spline (RCS) regression for dose-response. Predictive performance was assessed via the receiver operating characteristic and decision curve analysis curves. MetS prevalence was 40.34% (NHANES) and 59.31% (CHARLS). NHANES (US): each 1-unit RCII increase was associated with a 3% higher MetS risk (odds ratio [OR] = 1.03, 95% confidence interval [CI]: 1.02-1.03, P < .0001). Participants in the highest RCII quartile (Q4) had a 6.98-fold increased risk compared to Q1 (OR = 6.98, 95% CI: 5.18-9.41, P < .0001). RCS revealed a nonlinear inverse "J"-shaped relationship; risk increased significantly below an inflection point (RCII = 11.175) (OR = 1.164, P < .0001) but plateaued above it (P = .9307). Race and hypertension status significantly modified the association. CHARLS (China): each 1-unit RCII increase was associated with a 0.3% higher MetS risk (OR = 1.003, 95% CI: 1.003-1.004, P < .0001). Q4 participants had a 5.73-fold higher risk versus Q1 (OR = 5.73, 95% CI: 4.90-6.69, P < .0001). RCS also showed an inverse "J"-shaped relationship with an inflection point (RCII = 59.107). Risk increased significantly below 59.107 (OR = 1.029, P < .0001) and weakened above it (P = .0979). Gender, age, nationality, marital status, BMI, diabetes, smoking, stroke, and chronic kidney disease were significant interaction factors. Elevated RCII levels are significantly associated with increased MetS risk in both US and Chinese populations, demonstrating a nonlinear relationship. By integrating metabolic (RC) and inflammatory (high-sensitivity C-reactive protein) risk, RCII serves as a valuable tool for MetS risk stratification and clinical decision-making. However, the cross-sectional design limits causal inference, and further prospective studies are warranted to verify this association.