Body mass index across adulthood and the development of airflow obstruction and emphysema

Abstract

Background: There is a known association between low body mass index (BMI) and COPD, but the temporal relationship with structural and physiological lung changes is unclear. We related longitudinal changes in BMI throughout adulthood to development of airflow obstruction (AO) and emphysema. Methods: BMI trajectories were modelled in participants of the Framingham Offspring Cohort from Exam 2 (n=3863, mean age: 44 years), to Exam 9 (mean age: 71 years), firstly in those who developed AO with spirometry (n=1036,), and secondly had emphysema on quantitative Computed Tomography (Exam 7-8, n=225), using fractional polynomial curves and piece-wise linear splines. The trajectories were compared to those of smokers and non-smokers who did not develop the conditions. In addition, rate of decline in spirometry measures with age, was compared by BMI tertiles, based on BMI below age 40, using multilevel linear models. Results: BMI was lower at all ages in AO and emphysema groups compared to smokers and non-smokers without these conditions. Predicted mean BMI age 45 in men was: AO 26.7; non-AO-non-smokers: 27.3; non-AO-smokers: 27.1; and in women was: AO: 24.6; non-AO-non-smokers; 25.5; non-AO-smokers: 25.5. Decline in FEV1:FVC ratio was steepest in the lowest BMI tertiles (age coefficients in men tertile 1(standard error of the estimate): -0.0033 (0.0001) tertile 3: -0.0029 (0.0001), women tertile 1: -0.0036 (0.0013) tertile 3: -0.0027 (0.0001)). Conclusion: The findings of a lower BMI across adulthood in those who develop airflow obstruction and emphysema, independent of smoking status, as well as a steeper decline in FEV1:FVC ratio in those with a lower BMI, suggest body mass may have a role in COPD development.