Reliability of a Food Frequency Questionnaire in the Assessment of Dietary Intake Including FODMAPs in Different Populations, and Relationship With IBS Symptoms and the Degree of Self-assessed Physical Activity

Inclusion Criteria: * 18 - 65 years; * Provide written informed consent; * Living in Belgium Exclusion Criteria: * Diet \< 8 weeks prior to participation; * Known or suspected eating disorder; * Major psychiatric disorder; * Known gastro-intestinal disease (IBS is allowed); * Any malignancy in the past 3 years (basocellular carcinoma is allowed); * Chemotherapy in the past 6 months; * Infectious gastro-enteritis in the past 6 months (infectious gastro-enteritis defined as the concurrent presence of one of the following signs during 2 or more consecutive days: fever, vomiting, sudden onset of diarrhea, hospitalization because of these symptoms); * Previous abdominal surgery (appendectomy and cholecystectomy are allowed); * Alcohol abuse defined as \> 14 U per week; * Use of illicit drugs; * Intake of drugs with known major gastro-intestinal side effects. * Initiation of neuromodulators less than 3 months before participation. Neuromodulators are allowed when taken at a stable dose for at least 3 months; * Pregnancy.