Patients with non-celiac gluten sensitivity (NCGS) complain about gastrointestinal and extraintestinal symptoms while consuming gluten-containing cereals. The symptoms strictly correlate with ingestion of gluten-containing cereals and disappear quickly under elimination diets. Till now, there are no specific markers to diagnose NCGS. This study aims to determine the effect of a low FODMAP diet (fermentable oligo-, di- and monosaccharides and polyols) and a glutenfree diet on the mucosal immunity, clinical symptoms, and psychological well-being. Furthermore, the influence of the diets on the microbiome will be analyzed.