Joint association of Body Roundness Index and Atherogenic Index of Plasma with new-onset stroke in middle-aged and older adults: first evidence from the China Health and Retirement Longitudinal Study

Background Visceral obesity and dyslipidemia are independent stroke risk factors. The Body Roundness Index (BRI) indicates visceral fat, while the Atherogenic Index of Plasma (AIP) reflects an atherogenic lipid profile. This study investigates the joint association of BRI and AIP with stroke risk in middle-aged and older adults in China. Methods A total of 7,349 participants without a stroke history at baseline (2011) were included from the China Health and Retirement Longitudinal Study (CHARLS), with follow-up until 2018. Kaplan–Meier survival curves, multivariable Cox proportional hazards regression, subgroup analysis and mediation analysis were performed. Results During the 7-year follow-up, 358 new-onset stroke cases (4.87%) were recorded. Independently, participants in the highest quartile of both BRI (HR = 1.54, 95% CI: 1.10–2.17) and AIP (HR = 1.66, 95% CI: 1.20–2.32) faced significantly higher stroke risks compared to the lowest quartile. Joint association showed that the combined High AIP & High BRI group had the highest risk (HR = 1.88, 95% CI: 1.39–2.56), with P interaction < 0.001, especially in males (HR = 2.17; 95% CI: 1.41–3.33) or those aged 45–59 years (HR = 2.02; 95% CI: 1.27–3.21). Moreover, AIP partially mediated the association between BRI and stroke (β = 0.0012, P  = 0.046), accounting for 17.8% of the total effect. Conclusions A significant interaction was found between BRI and AIP levels in the association with stroke risk, our findings help formulate early screening and targeted prevention strategies for stroke.