Dietary tryptophane is metabolised by the gut microbiome to indole and IPA which reaches the human blood system. Indole is further metabolized into indoxyl sulfate which also reached the blood. d3f classic metabolic pathway PW:0000002 Pathway Ontology 19234110 PubMed Metabolomics analysis reveals large effects of gut microflora on mammalian blood metabolites. Proc Natl Acad Sci U S A 2009 Wikoff WR Anfora AT Liu J Schultz PG Lesley SA Peters EC Siuzdak G