Association of the combined atherosclerosis index of plasma and frailty index with arthritis in middle-aged and older adults: a cohort study

The Atherogenic Index of Plasma (AIP) is a marker of atherosclerosis, and the Frailty Index (FI) reflects physiological decline. The combined effect of AIP and FI remains insufficiently studied. This study aimed to investigate the association between a combined AIP-FI measure and the risk of arthritis. This study utilized data from the China Health and Retirement Longitudinal Study (CHARLS) database between 2011 and 2020. A total of 4437 participants aged ≥ 45 years without arthritis at baseline were included. Cox proportional hazards regression models and restricted cubic spline (RCS) models were employed to explore the relationship between the AIP-FI and the risk of incident arthritis. Over a 10-year follow-up period, a total of 1470 participants developed arthritis, with an incidence rate of 33.13%. Cox regression analysis revealed that each 1-unit increase in AIP-FI was associated with a 3% increase in arthritis risk (HR = 1.03; 95% CI: 1.02 to 1.04). Participants in the highest tertile (T3) of AIP-FI had a 55% higher risk compared to those in the lowest tertile (T1) (HR = 1.55; 95% CI: 1.35 to 1.78). The RCS curve demonstrated a significant nonlinear positive association between AIP-FI and arthritis risk (P for overall < 0.001, P for nonlinear < 0.001). This study indicates that a higher AIP-FI is significantly associated with an increased risk of arthritis. By integrating metabolic and frailty data, the AIP-FI was associated with arthritis risk and may help identify older Chinese adults at higher risk. However, these findings require further validation in independent cohorts before any clinical consideration.