BackgroundPeak expiratory flow (PEF) serves as a direct indicator of the functional status of the respiratory system. Higher body fat content, especially abdominal obesity, may relate to a deterioration in long-term respiratory function. The "A Body Shape Index" (ABSI) better assesses abdominal obesity, but its association with PEF is poorly understood.MethodsThe analysis demonstrated data from 14,386 middle-aged and older adults from the 2015 China Health and Retirement Longitudinal Study (CHARLS). ABSI, a sex-specific metric integrating waist circumference, weight, and height via allometric modeling derived from Chinese anthropometrics, was analyzed against PEF/PEF prediction using multivariable linear and spline regressions to characterize nonlinear associations. Threshold effects, subgroup, and sensitivity analyses ensured robustness.ResultsThis research showed a negative relationship between ABSI and both PEF and PEF predictions. An inverted L-shaped curve in the spline analysis characterized the association between ABSI and PEF/PEF prediction across the sexes. The ABSI threshold was 0.0782 and 0.0691 in males and females, respectively.ConclusionsAbdominal obesity negatively affects respiratory function, with ABSI thresholds varying by sex. Therefore, weight management should focus on a healthy ABSI to reduce abdominal obesity and safeguard respiratory health.