Background: Prevalence of obesity increases worldwide and it is associated with increased respiratory diseases. Body Mass Index (BMI) is a simple index of obesity. Lung function tests are important in the diagnosis of respiratory diseases. Therefore, association between BMI and lung function parameters can assess the effect of obesity on respiratory system. There are limited studies on association between BMI and lung function among healthy Sri Lankans.

Objective: To find the association between lung function parameters and BMI in healthy Sri Lankans aged 14 years and above.

Methodology: This was a descriptive cross-sectional study carried out among non-smoking healthy Sri Lankan Tamils living in Northern Sri Lanka. Lung function parameters were measured by Cosmed Micro Quark electronic spirometer (Italy) as per ATS guidelines. Height and weight were measured using standard laboratory scales (Seca, Germany) and BMI was calculated. Data was analyzed in SPSS version 20. Analysis was done separately for males and females and also for participants lesser than 20 years of age and above. Association between lung function parameters and BMI was derived using Pearson correlations. Participants over 20 years were categorized into underweight, normal, over weight and obese based on BMI cut off values for Asian adults. Lung function parameters between groups were compared. 

Results & discussion: In participants under 20 years of age (406 males & 337 females), BMI showed statistically significant (p<0.05) mild positive correlations with Vital Capacity, Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), Forced Vital Capacity (FVC), Forced Expiratory Volume in the first second (FEV1), Peak Expiratory Flow Rate (PEFR) and Mid Expiratory Flow rate (FEF 25-75%). The correlation coefficients varied between 0.163 and 0.463. In over 20 years population (369 males & 350 females), correlations between most of the lung function parameters showed negative correlations with BMI. However, the correlation coefficients were significant only with ERV (-0.348 in males, -0.449 in females). Comparison of underweight versus normal, normal versus overweight and overweight versus obese groups revealed that ERV and IRV had significant changes between all groups. In addition, FVC and FEV1 of females reduced significantly in overweight females than normal weight females. Positive correlations in under 20 years of age could be due higher BMI due to increase muscle mass during growth. But, in adults increasing BMI could be mainly due to increased fat percentage which adversely affect the lung function. However, the location of fat deposition varies between individuals which could explain the observation in different lung function parameters in over 20 years population.

Conclusion: This study found that association between BMI and lung function parameters varies with age. 

Limitation & recommendation: Number of participants in overweight group is comparatively lower in this study. Further studies including higher number of obese population and using other indicators of BMI can give more information on association between lung function and obesity.

Key words: Association, Body Mass Index, Lung Function Parameters