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

Figure 3 Tryptophan metabolism. Tryptophan is converted to 5-hydroxytryptophan by tryptophan hydroxylase (TPH) and this is the rate limiting step in the pathway. Aromatic amino acid decarboxylase (AAAD) subsequently converts 5-HTP into serotonin (5-HT). These reactions occur both in the central nervous system (CNS) (where 5-HT regulates a myriad of functions including emotion, cognition, stress and visceral perception) and in the enteric nervous system (gastrointestinal motility and secretion).