Effect of Body Mass Index on Lung Function in Chinese Patients with Chronic Obstructive Pulmonary Disease: A Multicenter Cross-Sectional Study

Objective The aim of this study was to explain "obesity paradox" in chronic obstructive pulmonary disease (COPD) by evaluating the effect of body mass index (BMI) on lung function in Chinese patients with COPD. Methods A total of 1644 patients diagnosed with COPD were recruited from four Chinese tertiary hospitals and were divided into four groups including underweight, normal weight, overweight and obese according to BMI classification standard. The medical data of these patients were collected and used for the multiple linear regression analyses. Results After adjustment for age, sex, educational level, economic status, smoking status, alcohol consumption, duration of COPD history, events of acute exacerbation in previous year, hypertension, diabetes mellitus, cardiovascular disease, cerebrovascular disease and osteoporosis, BMI had a curvilinear correlation with the forced expiratory volume in the first second (FEV1) in patients with Global Initiative for Obstructive Lung Disease (GOLD) 1-2 grade (first-order coefficient β, 0.09; 95% CI, 0.03-0.16; second-order coefficient β, -0.002; 95% CI, -0.003--0.001; P<0.01). However, BMI had a positive correlation with FEV1 in patients with GOLD 3-4 grade (β, 0.01; 95% CI, 0.008-0.017; P<0.01) when BMI was used as a quantitative variable. When BMI was used as a qualitative variable, only FEV1 in overweight group with GOLD 1-2 grade was significantly higher than that of normal weight group (P<0.01). Interestingly, both overweight and obese groups had higher FEV1 in GOLD 3-4 grade compared with normal weight group (β, 0.06; 95% CI, 0.02-0.11; β, 0.11; 95% CI, 0.04-0.18; P<0.01). The effect of BMI on predicted percentage of FEV1 (FEV1%) was similar to that of FEV1 in different GOLD grades. Conclusion Obesity only had a protective effect on lung function in COPD patients with GOLD 3-4 grade rather than GOLD 1-2 grade. Trial Registry ClinicalTrials.gov, No.: NCT03182309, URL: www.clinicaltrials.gov.