Figure 1.

Metabolism of amino acids and carbohydrates by gut microbiota. Complex carbohydrates are converted to monosaccharides and oligosaccharides, and then fermented to hydrogen (H₂), carbon dioxide (CO₂), ethanol, and short-chain fatty acids (SCFAs). SCFAs serve as a major source of energy for colonocytes and regulatory T lymphocytes (Tregs), or are converted to acetyl coenzyme-A (Acetyl-CoA), H₂, and CO₂. The deamination and decarboxylation of amino acids leads to the formation of ammonia, SCFAs, phenolic compounds, nitrosamines and hydrogen sulfide (H₂S).