Beyond the Threshold: Non-Linear Impact of Body Mass Index on Pulmonary Function in Severe Obesity

INTRODUCTION: The conflicting impact of obesity on pulmonary function has been observed. Our study aimed at exploring the relationship between BMI and pulmonary function parameters in individuals with obesity. METHODS: This cross-sectional study included a consecutive cohort of bariatric surgery candidates with pulmonary function tests. BMI, analysed as both quartile-stratified groups and a continuous variable, was evaluated for associations with pulmonary function parameters and the risk of pulmonary impairment. RESULTS: A total of 1834 patients with obesity (BMI range: 27.5-76.4 kg/m²) were included in our study. BMI exhibited a significantly positive correlation with FVC, FEV1, TLC, and FRC (P<0.001) and a negative correlation with FVC%pred, FEV1%pred, FEV1/FVC%, and RV/TLC (P<0.05). Non-linear relationships were found between BMI and both FVC%pred and FEV1%pred (P for non-linearity<0.01) after adjusting for covariables. Beyond the threshold BMI of 38.2 kg/m², FVC%pred and FEV1%pred declined significantly (FVC%pred: β=-0.522, 95% CI: -0.756 to -0.289; FEV1%pred: β=-0.435, 95% CI: -0.869 to -0.199), and the risk of pulmonary function impairment increased (OR=1.008, 95% CI:1.002 to 1.014). CONCLUSION: BMI exhibited a non-linear relationship with pulmonary function parameters. Beyond the critical threshold of 38.2 kg/m2, increasing BMI led to a significant decline in ventilatory capacity, concomitant with an amplified risk of pulmonary function impairment.