Urinary 3-indoxyl sulfate generation. Orally ingested proteins are enzymatically cleaved to produce tryptophan, which is then metabolized by tryptophanase-expressing intestinal bacteria into indole and absorbed by the intestinal tract. Microsomal enzymes in the liver then process indole into 3-indoxyl sulfate, which is excreted into the urine and can be quantified by liquid chromatography–mass spectrometry (LC-MS). Professional illustration by Ken Probst, Xavier Studio.