 Patients will visit the outpatient clinic five times; one visit before the start of the study, a visit during and at the end of the first period, and a visit during and at the end of the third period. During three of the visits, spike-biopsies are taken from the duodenum by oesophago-gastro-duodenoscopy. Blood samples are taken during all of the five visits. Patients will also fill in a quality of life questionnaire at the start and the end of the first and third period. 