 During week four and week twelve, the patients will come to the hospital for one day after an overnight fast. First, they are asked to fill in a bundle of questionnaires concerning physical complaints, depression, anxiety disturbances, paindisease, bodyinteroceptive awareness, traumaabuse and personality and to give a fecal sample that theyve collected the day before. The fecal sample will be used to evaluate the gastrointestinal microbiota in FD patients. Then, two blood samples, to evaluate liver function parameters and bile acid synthesis, will be taken and an endoscopy will be performed. All endoscopies will be performed by an experienced endoscopist Jan Tack and nine duodenal biopsies 2 biopsies at a time Radial JawTM3 with needle; outside diameter 2.2mm; Boston Scientific, 302 Parkway, Global Park, Heredia, Costa Rica will be obtained. To measure the in vitro transepithelial resistance, 3 biopsies will be examined using an adapted mini-Ussing chambers system. After equilibration, the mucosal side of the tissue will be exposed to 4kDa FITC-dextran as a measure of paracellular permeability. A sample will be taken from the serosal side during 2h at 30min interval. The concentration of fluoresceïn will then be measured using a fluorescence plate reader. Also, two biopsies will be used for mRNA extraction and subsequent cDNA synthesis. This cDNA will be used to measure the gene expression of bile acid receptors by means of real-time RT-PCR. In addition, one biopsy will be prepared for immunohistochemistry and two will be used for western blot to measure changes in distributionexpression of the bile acid receptor proteins. One biopsy will be obtained to study ultrastructural alterations by transmission electron microscopy. 