Sex-specific associations between weight-adjusted waist index and peak expiratory flow in middle-aged and older Chinese adults: a cross-sectional study from CHARLS

Weight-­adjusted waist index (WWI), an anthropometric measure reflecting central obesity, has been associated with obesity-­related health outcomes. Peak expiratory flow (PEF) and its predicted percentage (PEF%pred) are simple and reliable indicators of expiratory flow. This study aimed to investigate sex­-specific associations between WWI and PEF in middle-­aged and older Chinese adults. Data were sourced from the 2015 wave of the China Health and Retirement Longitudinal Study (CHARLS), involving 14,676 participants. WWI was calculated as waist circumference divided by the square root of body weight. PEF was measured in L/min; PEF%pred was calculated as the percentage of measured PEF relative to the predicted value based on validated equations for Chinese adults. Participants were categorized into sex-specific WWI quartiles (Q1-­Q4), with Q1 as the reference. Multivariable linear regression and generalized additive models with restricted cubic splines (RCS) were employed to examine the associations of WWI with PEF and PEF%pred, stratified by sex. In the fully adjusted models, each one-unit increase in WWI was associated with lower PEF (β = −24.10 L/min, 95% CI: −29.01 to − 19.19) and lower PEF%pred (β = −5.01%, 95% CI: −6.15% to − 3.87%) in males, with weaker associations in females (β = −10.99 L/min, 95% CI: −13.57 to − 8.41 for PEF and β = −3.21%, 95% CI: −4.02% to − 2.41% for PEF%pred; all P < 0.001). The WWI × sex interaction was statistically significant for PEF ( P < 0.001) and PEF%pred ( P = 0.016). Further, RCS analyses revealed inverted L-­shaped dose-response relationships in both sexes, with similar exploratory inflection points at approximately 11.03 cm/√kg in males and 11.14–11.15 cm/√kg in females; PEF and PEF%pred were significantly lower above these exploratory inflection points, and this inverse association was substantially stronger in males. Subgroup analyses demonstrated significant effect modification by BMI in males ( P for interaction < 0.001), with stronger associations in overweight/obese participants (BMI ≥ 24 kg/m 2 ).Stratified analyses showed that high WWI was associated with lower PEF even within the normal BMI and normal WC strata. High WWI is independently associated with lower PEF in middle-aged and older Chinese adults, extending previous evidence by demonstrating sex-­specific nonlinear patterns with similar exploratory inflection points but substantially stronger inverse associations in males. These findings highlight the potential of WWI as an associative marker of lower PEF; future longitudinal and interventional studies are needed to determine whether WWI-based strategies can help preserve respiratory health in the context of the global obesity epidemic.