Background: Low body mass index (BMI) is associated with chronic obstructive pulmonary disease (COPD) in populations where many are overweight. Substantial uncertainty remains about the relationship in populations with lower mean BMI levels, and about the relevance to it of the effects of smoking or of reverse causality.
Methods: A nationally representative prospective cohort study included 221,194 Chinese men aged 40-79 years in 1990-91, who were followed up for 15 years or to the age of 80 years. Hazard ratios for COPD-related mortality vs baseline BMI were adjusted for age, smoking, drinking and other factors. To reduce reverse causality, main analyses excluded all men with prior history of any respiratory diseases or abnormal lung function at baseline, leaving 2960 COPD-related deaths (16% of all deaths).
Results: The mean baseline BMI was 21.7 kg/m(2). There was a highly significant inverse association between BMI and COPD-related mortality among men without any apparent impairment of lung function. Approximately 90% of men had a baseline BMI <25 kg/m(2), and among them, 5 kg/m(2) lower BMI was associated with 31% (95% confidence interval 18-45%) higher COPD-related mortality. The excess risk persisted after restricting the analysis to never-smokers or excluding the first 5 years of follow-up.
Conclusions: Low BMI is associated with increased COPD mortality in a relatively lean adult male population in China where COPD is one of the most common causes of death.