Body Roundness Index Shows U-Shaped Link to Airflow Limitation in Chinese Adults

Central adiposity has repeatedly been linked to impaired lung function, but evidence connecting the body roundness index (BRI) to airflow limitation defined by percentage of predicted peak expiratory flow (PEF%) in Chinese adults has remained limited. A new cross-sectional study published in BMC Pulmonary Medicine addresses that gap using data from a large national survey, and its central finding is striking: the relationship between BRI and airflow limitation is not linear but U-shaped, with risk lowest at an intermediate body roundness value of approximately 4.9.

The researchers analyzed 15,573 participants aged 45 years or older from the 2015 wave of the China Health and Retirement Longitudinal Study (CHARLS). PEF% was calculated as the ratio of measured to predicted peak expiratory flow. Airflow limitation (AL) was defined as PEF% below 80%, and severe airflow limitation (SAL) as PEF% below 60%.

Methodologically, the team used multivariable logistic regression with sequential adjustment for potential confounders. To probe nonlinearity, they applied restricted cubic splines (RCS), and then took the spline-derived nadir as a fixed knot in segmented logistic regression, allowing the estimated association to differ on either side of the turning point. Effect modification and robustness were assessed through interaction and sensitivity analyses.

The continuous analyses showed U-shaped associations of BRI with both AL and SAL, with P values for nonlinearity below 0.001 for both outcomes. Both spline curves reached their lowest point at approximately BRI = 4.9. Below that threshold, each 1-unit increase in BRI was associated with roughly 14% lower odds of both airflow limitation and severe airflow limitation. At or above 4.9, each additional unit of BRI was associated with approximately 9% higher odds of AL and approximately 10% higher odds of SAL.

Quartile analyses told a broadly consistent story. Participants in the second through fourth BRI quartiles had lower odds of both AL and SAL than those in the lowest quartile. For airflow limitation, the odds ratios were essentially unchanged across Q2 through Q4, and for severe airflow limitation they showed modest variation without a progressive increase toward the highest quartile, echoing the flat-bottomed shape suggested by the spline curves.

The study also tested whether the association differed across subgroups. Significant interactions were observed for age, sex, and smoking status for airflow limitation, and for sex and smoking status for severe airflow limitation. The findings remained robust across the sensitivity analyses the authors conducted.

Several caveats deserve emphasis. This is a cross-sectional study of a single national survey wave, so it can establish associations, not causal effects; body roundness and lung function were measured at the same time, and reverse causation or unmeasured confounding cannot be excluded. Moreover, the outcomes are defined by PEF% rather than spirometric airflow obstruction, so the results speak specifically to PEF%-defined limitation in Chinese adults aged 45 and older, and the authors describe the evidence on BRI and PEF%-defined airflow limitation in this population as previously limited.

Within those limits, the study offers a quantitative anchor for a question that has often been treated in broad strokes: where, along the range of body roundness, does the association with reduced peak expiratory flow change direction? The fitted curves in this analysis place that point at approximately BRI = 4.9, with lower odds on either side of higher and lower values alike. For researchers studying adiposity and respiratory health in aging populations, the CHARLS-based results provide a detailed, nationally grounded description of that U-shaped pattern, and the authors report that the findings held up under the sensitivity checks they performed.

Subject of Research: The association between body roundness index and peak expiratory flow-defined airflow limitation in middle-aged and older Chinese adults

Article Title: U-shaped association between body roundness index and PEF%-defined airflow limitation among middle-aged and older adults in CHARLS: a cross-sectional study

Article References: Luo, X., & Xu, S. (2026). U-shaped association between body roundness index and PEF%-defined airflow limitation among middle-aged and older adults in CHARLS: a cross-sectional study. BMC Pulmonary Medicine. https://doi.org/10.1186/s12890-026-04870-3

Image Credits: AI Generated

DOI: 10.1186/s12890-026-04870-3

Keywords: body roundness index, airflow limitation, peak expiratory flow, CHARLS, central adiposity, cross-sectional study, lung function, China, obesity, epidemiology, U-shaped, association