The Investigators will consecutively enroll adult patients referred to outpatients' Clinics of 21 Centres because of persistence of relevant intestinal and extraintestinal symptoms believed by them to be caused by the ingestion of food containing even low doses of gluten. In their clinical management patients in gluten-containing diet since at least two months will undergo ad hoc tests including serum determination of IgA anti-tissue transglutaminase and anti-endomysial antibodies, IgG anti-gliadin antibodies (AGA), total IgA and IgE, wheat-specific IgE, upper endoscopy with collection of multiple duodenal biopsies, HLA genotyping, fecal calprotectin and lactose breath test. The Investigators will investigate serum AGA IgG since they have been detected in some patients with "suspected" nonceliac gluten sensitivity. Patients with celiac disease, wheat allergy, intolerance to further food source of fermentable oligo- and disaccharides, monosaccharides and polyols, lactose intolerance, Helicobacter pylori infection, giardiasis, inflammatory bowel disease, cirrhosis, excessive alcohol intake, intake of nonsteroidal anti-inflammatory agents, use of systemic immunosuppressant medication, poorly controlled psychiatric disease, pregnancy and those unable to give written informed consent will be excluded. The Investigators will also recruit as control group 420 age- and sex-matched healthy volunteers, undergoing IgG AGA and all negative for anti-tissue transglutaminase antibodies. Healthy volunteers will not be affected by any comorbidity and food intolerance.