The U-shape association between body roundness index and asthma: a cross-sectional study

Asthma, a common disease with high morbidity and mortality, results in a significant global health burden. Identifying modifiable risk factors, such as obesity, is crucial for effective asthma prevention. However, the association between body roundness index (BRI), a novel obesity index, and asthma is unclear. This study aimed to explore the association between BRI and asthma. A cross-sectional analysis of NHANES 2005–2018 data was performed. Baseline characteristics were compared, and the association between BRI and asthma risk was assessed using logistic regression with adjustments. Generalized additive models (GAM) with smooth splines were used to model the dose-response relationship. Threshold effect analysis and log-likelihood ratio (LLR) test were utilized to identify potential non-linear association. Among the 35,711 participants, 5,075 (14.21%) reported a history of asthma. Stratification by BRI quartiles (Q1–Q4) revealed the lowest asthma incidence in Q2 (11.43%). Using Q2 as the reference, asthma incidence exhibited a U-shaped relationship with BRI, increasing in both lower (Q1) and higher (Q3 and Q4) quartiles. Threshold effect analysis identified an inflection point at a BRI value of 4.35. Below this threshold, the adjusted asthma odds decreased 15% (OR = 0.85, 95% CI: 0.80–0.91, p < 0.0001) for each unit increment in BRI. Above this threshold, the adjusted asthma odds increased by 8% (OR = 1.08, 95% CI: 1.04–1.12, p < 0.0001) for each unit increment in BRI. This study revealed a U-shaped association between BRI and asthma, indicating that both deviations from an optimal BRI are associated with elevated asthma risk.