Figure 1.

End products of the intestinal bacterial metabolism (I, IAA, pC, TMA, H₂S) are transported as such or after detoxification (sulfatation and glucuronidation) towards the liver via the portal vein where the remaining pC and I are conjugated and TMA is oxygenated. The end products of the bacterial and human metabolism are taken up into the circulation (IxS, IAA, pCS, pCG, TMAO, H₂S) where some bind to albumin (IxS, IAA, pCS and to a lesser extend pCG). H₂S is also generated by different tissues throughout the body. Finally, these compounds are excreted into the urine. Trp: tryptophan; Tyr: tyrosine; Cys: cysteine; I: indole; IAA: indole-3-acetic acid; pC: p-cresol; TMA: trimethylamine; TMAO: trimethylamine N-oxide; pCS: p-cresyl sulfate; pCG: p-cresyl glucuronide; IxS: indoxyl sulfate.