Tertiary care IBS patients will be recruited for a prospective low FODMAP intervention study, hereby focusing on the reintroduction phase of the diet, where patients are blindly challenged with different FODMAP groups administered as powder, to be added to the strict FODMAP exclusion phase. During the screening phase of two weeks, baseline data regarding psychological, nutritional, microbial, and genetics will be collected. Predictors of treatment response (a 50-point drop on the IBS-severity scoring system (IBS-SSS) during the strict FODMAP exclusion phase of 6 weeks) will be studied with linear mixed models.

Inclusion Criteria:

• Patients are eligible for the study if all of the following criteria are met:

  1. Patients with irritable bowel syndrome as per Rome IV diagnostic criteria

• Symptom characteristics of IBS according to Rome IV criteria 2. Patients must provide witnessed written informed consent prior to any study procedures being performed 3. Patients aged between 18 and 70 years inclusive 4. Male or female patients

Exclusion Criteria:

• Patients are excluded from the study if any of the following criteria are met:

  1. Patients with any condition which, in the opinion of the investigator, makes the patient unsuitable for entry into the study
  2. Patients with any major psychiatric disorders (including those with a major psychosomatic element to their gastrointestinal disease), depression, alcohol or substance abuse in the last 2 years
  3. Patients presenting with predominant symptoms of functional dyspepsia (FD) and of gastro-oesophageal reflux disease (GERD)
  4. Patients who changed their diet over the last 3 months or have previously tried the low FODMAP diet are excluded from the study.
  5. Females who are pregnant or lactating are excluded from the study.

Patients are blinded during reintroduction.