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/m2 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/m2 and 29.9 kg/m2. For BMI < 19.9 kg/m2, 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/m2, 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/m2. For BMI ≥ 21.6 kg/m2, 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.
Keywords: Asthma; Body mass index; Cross-cultural study; Global burden of disease; Nonlinear relationship.
© 2025 The Author(s).