Impaired lung function observed in underweight Pakistani individuals aged ≤40 years: a cross-sectional analysis

<bold>Objective:</bold> There has been less focus on the association between underweight and pulmonary health. This study explored the relationship between underweight and lung function impairment in Pakistani individuals aged ≤40 years. <bold>Methods:</bold> This retrospective study evaluated eligible subjects who had undergone pulmonary function testing between 2011 and 2021 at the Aga Khan University Hospital (AKUH), Karachi, Pakistan. <bold>Results:</bold> A total of 1662 subjects were analyzed. Mean age was 33.18±5.05 years, and 1359 (81.8%) were male. 2.9% (n=48) were classified as underweight. Mean FEV1 (3.18±0.83, p=0.188), predicted % FEV1 (87.33±14.78, p=0.051), FVC (3.75±0.94, p=0.008), predicted % FVC (86.69±13.86, p=0.011), and PEF (3.60±1.38, p=0.919) were lower in the underweight group. After adjusting for sex, smoking history, current smoking status, and having at least one disease, adjusted mean values along with 95% CI remained lower in underweight group: predicted % FEV1 (87.37, 95% CI 83.81–90.94, p=0.050), FVC (3.81, 95% CI 3.63–3.99, p=0.040), and predicted % FVC (86.70, 95% CI 83.21–90.18, p=0.011). Compared to normal and overweight/obese groups, high number of underweight individuals ≤40 years presented with predicted % FEV1 <80% (n=12, 25.0%, p=0.035) and predicted % FVC <80% (n=12, 25.0%, p=0.049). <bold>Conclusion:</bold> This study demonstrated diminished lung function among underweight individuals ≤40 years in Pakistan. These results could be further investigated to help identify at-risk individuals who may be susceptible to developing pulmonary disease in later years.