Association of Pulmonary Function, Gut Microbiota, and Amino Acid Metabolism in COPD Patients.

Chronic obstructive pulmonary disease (COPD) is characterized by progressive airflow limitation and systemic inflammation. Growing evidence suggests a link between gut microbiota alterations and metabolic dysregulation in COPD. Methods: A cross-sectional study included 20 COPD patients (mean age: 65.4 ± 8.3 years, 57% male). Pulmonary function was assessed using FEV₁, FEV₁/FVC, and residual volume. Gut microbiota composition was analyzed via 16S rRNA sequencing, and plasma citrulline and glycine levels were measured using LC-MS. Correlation analysis examined associations between lung function, microbiota composition, and amino acid metabolism. Results: Patients with higher GOLD grades (III–IV) exhibited significantly lower FEV₁ (%) (mean: 50.7 vs. 60.1, p = 0.02), lower citrulline levels (28.3 vs. 33.5 µmol/L, p = 0.03), and higher glycine levels (310.2 vs. 285.4 µmol/L, p = 0.01). COPD patients had a lower relative abundance of Bacteroides (20.5% vs. 25.3%) and a higher proportion of Firmicutes (55.7% vs. 50.2%) and Proteobacteria (15.3% vs. 10.1%) compared to controls. Higher Firmicutes abundance correlated with lower citrulline (r = -0.81, p < 0.05) and higher glycine (r = 0.84, p < 0.05). Additionally, Bacteroides positively correlated with FEV₁ (r = 0.79, p < 0.05), suggesting a protective role. Conclusion: COPD is associated with gut microbiota and amino acid metabolism dysregulation, with higher Firmicutes and lower Bacteroides linked to declining lung function and altered metabolic profiles. Citrulline and glycine may serve as potential biomarkers for COPD progression, providing insights into gut-lung interactions and new therapeutic targets.