Background: Nutritional status is recognized as a prognostic factor in patients with chronic obstructive pulmonary disease (COPD) while the effects of overweight and obesity may differ across COPD severity stages. The precise relationship between body mass index (BMI) and rate of lung function decline in COPD remains uncertain.

Methods: In pooled data of 10,759 COPD patients from 7 clinical trials (median follow up of 5 years), we examined the non-linear relationships of BMI with the rate of decline in Forced Expiratory Volume in One Second (FEV1) using restricted cubic spline regressions on 1,000 bootstrap samples, stratified by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) classifications of COPD severity at baseline, and adjusting for age, sex, smoking status and cohort effects.

Results: This analysis included 3,674 patients in GOLD 1 (mild COPD), 5,012 in GOLD 2 (moderate COPD) and 2,073 in GOLD 3 and 4 (severe and very severe COPD). Median baseline BMI was approximately 25 in all three subgroups. In overweight and obese men (BMI=25–40), unit increase in BMI was associated with a decelerated FEV1 decline in both smokers and non-smokers across all GOLD stages, whereas in overweight and obese women, increase in BMI affected lung function to a much lesser degree and was even associated with a slightly accelerated FEV1 decline in non-smokers in GOLD 1 and 2. In underweight men and women (BMI=17–21), unit increase in BMI was associated with a decelerated FEV1 decline in smokers in GOLD 1 and GOLD 3 and 4, but GOLD 2 smokers were less affected. In normal weight men and women (BMI=22–24), BMI increase was associated with an accelerated FEV1 decline in non-smokers, whereas it was the opposite for smokers in GOLD 1 and 2.

Conclusion: This study discovered a non-linear relationship between BMI and the rate of FEV1 decline in COPD patients, in which BMI increase generally slowed down the decline of FEV1 in overweight and obese patients and in normal and underweight smokers with mild or severe COPD. The nature of such relationship merits further investigation.