Correlation of BMI With Dynamic Pulmonary Function Indices in Nonobese Young Adults: A Cross-Sectional Study

Background and aim Pulmonary function reflects the functional capacity of the lungs, airways, thoracic cage, and respiratory muscles. BMI, a widely used indicator of nutritional status, has been extensively studied for its association with pulmonary function. However, this relationship remains inconsistent in nonobese individuals, in whom BMI may have only a limited influence. This study aimed to evaluate the relationship between BMI and dynamic pulmonary function indices in healthy nonobese young adults. Methods This cross-sectional analytical study was conducted in the Department of Physiology, Rajendra Institute of Medical Sciences, Ranchi, India, from December 2024 to December 2025. A total of 162 apparently healthy young adults aged 18-25 years with BMI <25 kg/m² were included. Pulmonary function tests were performed using a computerized spirometer in accordance with the American Thoracic Society guidelines. Pulmonary function parameters were compared across BMI categories. Pearson's correlation analysis was used to assess the relationship between anthropometric parameters and pulmonary function indices. Multivariable linear regression analyses were performed to identify independent predictors of forced vital capacity (FVC) and forced expiratory volume in one second (FEV₁). Results No statistically significant differences in pulmonary function parameters were observed across BMI categories (all p > 0.05). Height demonstrated a strong positive correlation with FVC (r = 0.608) and FEV₁ (r = 0.637), while body weight showed moderate positive correlations with FVC (r = 0.308) and FEV₁ (r = 0.287) (all p < 0.01). BMI was not significantly correlated with any spirometric parameter. In multivariable regression analyses, height and sex were significant independent predictors of both FVC and FEV₁, whereas BMI and age were not independently associated with either outcome. Conclusions Among healthy nonobese young adults, pulmonary function parameters did not differ significantly across BMI categories, and BMI was not independently associated with spirometric indices. Height showed the strongest independent association with FVC and FEV₁, while sex also contributed significantly after adjustment for age and BMI. These findings suggest that body size, particularly height, has a greater influence on lung volumes than BMI within the nonobese range.