Objective To investigate the association between cardiometabolic index (CMI) and new-onset arthritis among middle-aged and older adults in China, providing a novel metabolic perspective for early risk identification.

Methods Based on the prospective cohort data from China Health and Retirement Longitudinal Study (CHARLS), participants aged ≥ 45 years without arthritis at baseline were included. Associations were assessed using multivariable logistic regression, restricted cubic spline (RCS) regression models, piecewise regression models and subgroup analyses.

Results A total of 4, 642 participants were included, with a median age of 58.0 (51.0-65.0) years, among whom 1, 475 participants were identified to be new-onset arthritis cases during the follow-up. After multivariable adjustment, compared with the lowest quartile (Q1) of CMI, the Q2 group (OR=1.25, 95% CI: 1.05-1.50) and the Q3 group (OR=1.29, 95% CI: 1.08-1.56, P=0.007) showed a significantly increased arthritis risk, and the Q4 group also exhibited an elevated trend (OR=1.29, 95% CI: 1.06-1.58, P=0.013). RCS analysis revealed a nonlinear "inverted J-shaped" association between CMI and arthritis risk (nonlinearity P=0.001), and piecewise regression identified a breakpoint of 1.709 (95% CI: 1.632-1.786). No significant interactions were observed in subgroup analyses (P > 0.05).

Conclusion Elevated CMI levels are associated with an increased risk of new-onset arthritis among middle-aged and older adults in China, exhibiting a nonlinear "inverted J-shaped" association. CMI may serve as a simple metabolic biomarker for assessing arthritis risk.