Evaluating short-chain fatty acids in breath condensate as surrogate measurements for systemic levels and investigation into alternative respiratory sample matrices

Short-chain fatty acids (SCFAs) are metabolic by-products from microbial fermentation of complex carbohydrates and protein. They have gained clinical interest for their protective effects, including within the lung microenvironment. SCFAs are detectable in circulation and exhaled breath condensate (EBC), posing questions as to whether exhaled SCFAs originate from the gut and/or lung microbiota. Mapping SCFAs from the lung could improve our understanding of microbial activity in respiratory conditions. SCFA measurements in EBC were evaluated using a validated gas chromatography-mass spectrometry assay. Six healthy participants ingested sodium acetate, calcium propionate and sodium butyrate to acutely increase circulating SCFAs. EBC samples were collected alongside venous draws, with circulating and exhaled levels compared. A series of additional respiratory sample matrices from patient samples was investigated to gain novel insights into SCFAs within different respiratory biofluids. Serum SCFAs were increased in line with known responses. However, these increases were not observed in EBC, indicating a lack of correlation between circulating and exhaled SCFAs. SCFAs were detected in all additional respiratory biosamples, with EBC and sputum reporting the highest concentrations. Interestingly, branched-chain moieties were notably abundant in sputum, indicating the potential for their local production by bacterial fermentation of lung mucus proteins. SCFAs in EBC do not reflect circulatory levels and, therefore, are not a suitable surrogate measurement to inform on systemic load. These data suggest that exhaled SCFAs are potentially derived from lung microbial metabolism, supporting the need for further investigation into SCFA production, function and diagnostic utility in respiratory health.