Exhaled breath analysis in hepatology: State-of-the-art and perspectives

Table 1 Summary of the main volatile organic compounds found in the exhaled breath of patients with liver cirrhosis

Compounds and molecules	Metabolic disturbances
Nitrogen derivatives
Ammonia[14]	Altered urea cycle
Trimethylamine[29]	Reduced hepatic catabolism of trimethylamine and/or increased degradation and absorption of dietary phosphatidylcholine and choline mediated by altered intestinal microbiome
Ketones[16]	Increased insulin resistance, hepatic glycogen exhaustion, hepatic gluconeogenesis impairment
Sulfur derivatives[8]	Incomplete metabolism of sulfur-containing amino acids in the transamination pathway
Dimethylsulfide[12]	Main responsible for fetor hepaticus
Alkanes, alkenes, terpenes and aliphatic acids[29]	Lipid peroxidation mediated by oxygen radical produced by hepatic CYP activity
Ethane and pentane[30]	More typical of alcohol induced liver injury
Limonene[13]	Impaired biotransformation by CYP2C (partially dependent on dietary intake of citrus fruits and vegetable)
Acetic and propionic acid[9]	Reduced hepatic metabolism of short chain fatty acids produced by gut microbiome
Alcohols
Methanol[22]	Pectin degradation; its levels are partially dependent on dietary intake of fruits and on a variable role of intestinal microbiome
Ethanol[31]	More typical of NAFLD, even in complete absence of alcohol consumption; possible role of intestinal microbiome in the production of ethanol in obese patients

CYP: Cytochrome P450 enzyme activity; NAFLD: Nonalcoholic fatty liver disease.