Irritable bowel syndrome: A microbiome-gut-brain axis disorder?

Figure 4 Impact of altered tryptophan metabolism in irritable bowel syndrome. In addition to serotonin, tryptophan can also be metabolised along the kynurenine pathway to generate neurotoxic and neuroprotective metabolites. The enzymes responsible for degradation along this pathway are immune (indoleamine-2,3-dioxygenase, IDO) and stress (tryptophan-2,3-dioxygenase, TDO) responsive. In IBS, this pathway is activated leading to a potential serotonergic deficiency and/or altered enteric nervous system (ENS) and central nervous system (CNS) availability of kynurenine and its metabolites. The microbiota appears to directly or indirectly regulate enzyme activity.