Association between BMI and asthma in adults over 45 years of age: analysis of Global Burden of Disease 2021, China Health and Retirement Longitudinal Study, and National Health and Nutrition Examination Survey data

Background Asthma is a major global health concern, and body mass index (BMI) is a key risk factor. This study aims to investigate the potential nonlinear relationship between BMI and asthma risk in populations over 45 years of age using large-scale, cross-national data. Methods This cross-sectional study utilised three databases: GBD 2021, China Health and Retirement Longitudinal Study (CHARLS; cross-sectional data from baseline survey, January 01, 2011 to December 31, 2011), and National Health and Nutrition Examination Survey (NHANES; cross-sectional data from 2011 to 2012 cycle). Participants aged ≥45 years after excluding those with missing data on BMI, asthma history, smoking history, age, sex, and BMI outside 10–80 kg/m 2 were included. Asthma was defined by self-report in CHARLS and by physician diagnosis plus recent symptoms in NHANES. Smooth curve fitting was performed to visualise the BMI-asthma relationship, adjusting for multiple confounders. We applied segmented regression models to identify potential threshold effects, used likelihood ratio tests to compare linear and non-linear models, and employed bootstrap resampling for confidence intervals. Findings High BMI was the primary risk factor for asthma-related DALYs globally (14.93% in 2021). From CHARLS, we included 13,393 participants, comprising 6267 males (46.79%) and 7126 females (53.21%). From NHANES, we included 2925 participants, comprising 46.6% males and 53.4% females. CHARLS data revealed a U-shaped relationship between BMI and asthma risk, with critical points at 19.9 kg/m 2 and 29.9 kg/m 2 . For BMI < 19.9 kg/m 2 , asthma risk increased by 28% with each unit decrease in BMI (OR = 1.28, 95% CI: 1.15–1.43). For BMI ≥ 29.9 kg/m 2 , asthma risk increased by 25% with each unit increase in BMI (OR = 1.25, 95% CI: 1.05–1.49). NHANES data showed a non-linear relationship with a turning point at 21.6 kg/m 2 . For BMI ≥ 21.6 kg/m 2 , asthma risk increased by 5% with each unit increase in BMI (OR = 1.05, 95% CI: 1.03–1.06). Interpretation This study elucidates a significant non-linear relationship between BMI and asthma risk in populations aged 45 years and older, providing insights for tailored asthma prevention strategies, although the cross-sectional design limits causal inference. Future studies should focus on collecting and stratifying longitudinal data and adjusting for asthma diagnosis timing to obtain more accurate results. Funding National Nature Science Foundation of China.