Inclusion Criteria:  written informed consent patients with IBS who fulfil the criteria according to Rome IV patients with suspected IBS who undergo gastroduodenoscopy for diagnosis of IBS healthy controls with no suspicion of food intolerance and/or presented for screening with an indication for oesophago-gastrodudodenoscopy (e.g. medical preventive examination, exclusion of H. pylori or gastritis)  Exclusion Criteria:  lack of written consent patients with known allergy to fluorescein. Patients with carcinoma, liver disease, proven bacterial overgrowth Patients with diabetes mellitus Patients with severe cardiovascular disease or multiple concomitant drug therapy (beta-blockers) Patients with wheat allergy Pregnant women