Fig. 4

Short-chain fatty acid levels in fecal samples from healthy, chronic obstructive pulmonary disease (COPD) I–II subjects, and COPD III–IV subjects. a The levels of total short-chain fatty acids were significantly lower in COPD III–IV subjects. b–d Acetic acid, isobutyric acid, and isovaleric acid levels were also lower in COPD III–IV subjects. e–h Reductions in the levels of propionic acid, butyric acid, valeric acid, and caproic acid were observed in COPD patients, although this trend was not statistically significant. Significance was determined by the ANOVA test, and p values were corrected using the Bonferroni method. Each dot corresponds to a sample. Healthy, n = 60; COPD I–II, n = 63; COPD III–IV, n = 30. *p < 0.05, **p < 0.01