In this study, the investigators will evaluate the FODMAP diet as an alternative treatment for functional dyspepsia and explore its effect on different aspects of the pathophysiology of FD.

The study design starts with a baseline period of 2 weeks, includes 6 weeks of a 'strict' FODMAP diet (no blinded), followed by a 9 week 'reintroduction' phase (single blind). Based on the patients' symptom results following each of these challenges, patients will then follow the 'moderate' FODMAP diet, where the FODMAPs that did not trigger any symptoms during the challenge tests can now be consumed. Finally patients follow 2 weeks of a diet 'moderate' in FODMAP levels (no blinded). Blood and stool samples will be collected during the complete trial at several timepoints. In addition, before and after the strict diet phase a gastroscopy will be performed as well as an intragastric pressure measurement and breath test.

Inclusion Criteria:

• patients with functional dyspepsia (postprandial distress syndrome)
• written informed consent
• 18-70 years

Exclusion Criteria:

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 irritable bowel syndrome (IBS) 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.

open label strict diet phase blinded participant during reintroduction