Low Body Mass Index Is Associated with Higher Odds of COPD and Lower Lung Function in Low- and Middle-Income Countries

Abstract

The relationship of body mass index (BMI) with lung function and COPD has been previously described in several high-income settings. However, few studies have examined this relationship in resource-limited settings where being underweight is more common. We evaluated the association between BMI and lung function outcomes across 14 diverse low- and middle-income countries. We included data from 12,396 participants aged 35-95 years and used multivariable regressions to assess the relationship between BMI with either COPD and lung function while adjusting for known risk factors. An inflection point was observed at a BMI of 19.8 kg/m². Participants with BMI < 19.8 kg/m² had a 2.28 greater odds (95% CI 1.83-2.86) of having COPD and had a 0.21 (0.13-0.30) lower FEV₁ and 0.34 (0.27-0.41) lower FEV₁/FVC z-score compared to those with BMI ≥ 19.8 kg/m². The association with lung function remained even after excluding participants with COPD. Individuals with lower BMI were more likely to have COPD and had lower lung function compared to those in higher BMI. The association with lung function remained positive even after excluding participants with COPD, suggesting that being underweight may also play a role in having worse lung function.

Keywords: COPD; body mass index; low and middle income countries; lung function.

Figure 1:. Sex-, site- and category-stratified prevelances of Body Mass Index (BMI).

The prevalence of BMI was stratified by sex (women on the left, men on the right) and WHO classification for underweight, normal, overweight, and obese individuals (BMI between 0–18.5, 18.5–25, 25–30, and over 30 kg/m², respectively) across the 13 LMIC sites. Sites were ordered according to the overall prevalence of Obesity in Men from highest (top) to lowest (bottom).

Figure 2:. Body mass index vs. the probability of having COPD in our sample.

We plotted probabilities from a multivariable adjusted model (solid line), single-variable non-adjusted model (broken line), and binned probabilities with corresponding 95% confidence intervals. The multivariable model was adjusted for age, sex, daily cigarette smoking, level of education completed, and post-treatment pulmonary tuberculosis.

Figure 3:. Associations between low body mass index (BMI < 19.8 kg/m²) and COPD outcomes obtained from multivariable regression models, and interaction effects with sex, smoking status, biomass use, age, and educational attainment.

Panel A shows estimates using data from all sites, while panel B shows site-specific estimates. In panel A, odds ratios and the corresponding 95% confidence intervals are represented by diamonds and lines, respectively. We also tabulated numerical values for the odds ratios and the corresponding 95% confidence intervals. In panel B, site-specific odds ratios are presented by triangles. In the overall model, we evaluated the association between having a BMI < 19.8 kg/m² and COPD prevalence adjusted for age, sex, biomass use, daily cigarette smoking, post-treatment pulmonary tuberculosis, and secondary education. We then evaluated for interaction effects between having a BMI < 19.8 kg/m² and either sex, smoking status, biomass use, age, or educational attainment on COPD outcomes. Models stratified by sex were adjusted for age, daily cigarette smoking, biomass, post-treatment pulmonary tuberculosis, and secondary education. Models stratified by smoking status were adjusted for age, sex, biomass, post-treatment pulmonary tuberculosis, and secondary education. Models stratified by biomass use were adjusted for sex, daily cigarette smoking, post-treatment pulmonary tuberculosis, and secondary education. Models stratified by age were adjusted for sex, daily cigarette smoking, biomass, post-treatment pulmonary tuberculosis, and secondary education. Models stratified by educational attainment were adjusted for age, sex, daily cigarette smoking, biomass, and post-treatment pulmonary tuberculosis. Models with severity and symptom status of COPD as outcomes were adjusted for age, sex, daily cigarette smoking, biomass, post-treatment pulmonary tuberculosis, and secondary education.

Figure 4:. Body mass index vs. pre-bronchodilator FEV₁/FVC z-scores.

Crude lung function values were plotted behind boxplots. Each boxplot encompasses values within one-unit of BMI (i.e., 1 kg/m²). We included a broken red line at a pre-bronchodilator FEV₁/FVC z-score of −1.64 to help visualize which values are below the lower limit of normal (bottom 5^th percentile).

Figure 5:. Body mass index vs. fitted values for FEV₁, FVC, and FEV₁/FVC, stratified by site.

All models were adjusted for age, sex, daily cigarette smoking, level of education completed, and post-treatment pulmonary tuberculosis. We used linear mixed-effects models with random intercepts by site.