Serotonin, 5-hydroxytryptamin (5-HT), plays an important role in regulating the gastrointestinal functions. In this study we will modulate the serotonin system with acute tryptophan depletion (ATD) and administration of 5-hydroxytryptophan (5-HTP). ATD is based on ingestion of an amino acid drink devoid of the precursor of serotonin and hence caused a decreased serotonin level. 5-hydroxy-tryptophan is the direct precursor of serotonin and its administration, an increased serotonin level is expected. During these interventions, gut permeability and visceral sensitivity will be measured in healthy individuals and patients with irritable bowel syndrome, which is characterized by altered permeability and visceral hypersensitivity.