 Gastroduodenoscopy will be performed by an experienced endoscopist Jan Tack. Hereby, 12 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, 4 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 fluorescein will then be measured using a fluorescence plate reader. Also, 2 biopsies will be used for mRNA extraction and subsequent cDNA synthesis. This cDNA will be used to measure the gene expression of cell-to-cell adhesion proteins and acid-sensing receptors by means of real-time PCR. In addition, 2 biopsies will be prepared for immunofluorescence and immunohistochemistry and 2 will be used for western blot to measure changes in distributionexpression of the cell-to-cell adhesion proteins and of bile acid-sensitive receptors. Two biopsies will be obtained to study ultrastructural alterations by transmission electron microscopy. 