Long-term high-altitude residence is associated with a progressively elevated risk of self-reported multimorbidity:...

High-altitude residence affects approximately 503 million people worldwide, yet its longitudinal impact on multimorbidity (coexistence of two or more self-reported, physician-diagnosed chronic conditions) remains poorly understood. Using four waves (2011-2018) of the China Health and Retirement Longitudinal Study (CHARLS), we examined 8,121 adults aged ≥45 years from prefectures classified as high altitude (≥1,500 m) or low altitude (<1,500 m). Generalized estimating equations (GEE) revealed a significant altitude-by-time interaction (odds ratio [OR]=1.033, 95% confidence interval [CI]: 1.006-1.060, P = 0.014), indicating a progressively faster rise in multimorbidity prevalence at high altitude over follow-up. Among the 3,852 baseline multimorbidity-free participants after 1:8 propensity score matching, Cox regression showed that high-altitude residence independently increased incident multimorbidity risk by 53.8% (hazard ratio [HR]=1.538, 95% CI: 1.343-1.761, P < 0.001). The association was consistent across all eight subgroups and multiple sensitivity analyses (HR range: 1.432-1.523). High altitude should be incorporated into chronic-disease risk assessment, and prevention policies in high-altitude regions should prioritize multimorbidity, emphasizing sustained monitoring of long-term residents.