Association between BMI and knee osteoarthritis in Chinese adults aged 45 years and older: evidence from the 2021 Global Burden of Disease analysis and the China Health and Retirement Longitudinal Study

Background Amidst the dual challenges of rapid population aging and rising obesity prevalence, knee osteoarthritis (KOA) constitutes a major public health burden in China. This study examined the association between body mass index (BMI) and KOA in Chinese adults, and aimed to identify clinically meaningful BMI thresholds for weight management interventions. Methods Using data from the Global Burden of Disease 2021 Study (1990–2021), we quantified the KOA burden attributable to high BMI and assessed age-specific temporal trends. We further analyzed data from the China Health and Retirement Longitudinal Study (CHARLS), including participants aged ≥45 years after excluding those with missing BMI, KOA, or covariate information and those with BMI values outside 15–40 kg/m 2 . Restricted cubic spline modelling combined with piecewise linear regression was used to evaluate the non-linear relationship between BMI and KOA risk and to identify potential threshold effects. Results In 2021, China recorded 1.11 million disability-adjusted life years (DALYs) attributable to high BMI among individuals with KOA, a 369.28% increase from 1990. After age standardization, the DALYs rate increased by 91.68%, with an average annual increase of 2.13% ( p < 0.05). The crude DALYs rate in adults aged ≥45 years rose markedly from 1990 to 2021 and continued to increase with age. Among 12,769 CHARLS participants, 1,254 (9.82%) were diagnosed with KOA. The proportion of overweight and obesity were significantly higher in the KOA group than in the non-KOA group ( p < 0.001). After adjustment for demographic and health-related covariates, KOA prevalence increased monotonically with rising BMI. Piecewise regression analyses detected a non-linear association between BMI and KOA prevalence, with a threshold at 26.89 kg/m 2 . Above this threshold, each 1-unit BMI increase corresponded to a 10% rise in KOA prevalence [odds ratio (OR) = 1.10, 95% confidence interval (CI): 1.07–1.13, p < 0.001]. Conclusion A BMI of 26.89 kg/m 2 ...