Validation of the 13C-labelled breath test to diagnose sucrase-isomaltase deficiency (n=40)  Inclusion Criteria:  Signed informed consent BMI 18-30 kg/m2 Referred for gastroscopic examination with suspected GI disorder  Exclusion Criteria:  • Unwilling or not capable of signing the informed consent  Sucrase-isomaltase deficiency as a cause of symptoms in patients with irritable bowel syndrome (n=80)  Inclusion Criteria:  Signed informed consent BMI 18-30 kg/m2 IBS diagnosis according to Rome IV criteria  Exclusion Criteria:  Inflammatory bowel disease, celiac disease or diabetes mellitus Chronic immune diseases affecting the GI-system Unwilling/unable to maintain a stable diet throughout the study period Use of antibiotic treatment for the last 4 weeks Currently on a restrictive diet