During an online assessment participants will fill in questionnaires evaluating demographic data, psychological variables, the degree of self-assessed physical activity and symptoms of irritable bowel. Dietary intake will be assessed by a newly developed food frequency questionnaire (FFQ) and a 4 days food diary. In order to validate the FFQ in different populations, participants will be recruited from university students, staff of UZ Brussel and VUB, and from the community by advertisement (including social media). IBS patients will be recruited from the gastro-enterology outpatient clinic.

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.

Healthy subjects, IBS patients