There is extensive pre-clinical evidence for potential health effects of blueberries. These are related, but not exclusively due to their high polyphenol content. This translational, clinical, randomized, cross-over, double-blind, placebo-controlled study will investigate the effects of blueberries in freeze-dried powder form on gastrointestinal and extra-gastrointestinal symptoms and function, as well as specific potential underlying mechanisms, in patients with the overlapping functional gastrointestinal disorders, irritable bowel syndrome and functional dyspepsia.

Inclusion Criteria:

• Fifty-five successive male or female patients with Functional Gastrointestinal Disorders FGID) enrolled from our Gastroenterology Group Practice (GGP) in Bern, Switzerland, classified as having either Irritable Bowel Syndrome (IBS), Functional Dyspepsia (FD) or both according to Rome 4 criteria (Drossman et al 2016; Schmulson et al. 2017)
• Age between 18 and 50 years
• Body mass index 18.5-29.9 kg/m2
• European / Caucasian ethnicity
• Able to give informed consent in German as documented by signature

Exclusion Criteria:

• Evidence of clinically significant disease, as assessed by usual clinical practice (history, blood and stool tests, imaging and endoscopy, as clinically required) and by investigator.
• Colonoscopy, antibiotics or probiotics within 2 weeks before or during the study.
• Planned dietary modifications (including polyphenol-rich fruit or vegetable smoothies, drinks or diets) or initiation of new treatments during the study period, besides the study interventions.
• Inability or contraindications to undergo the investigated intervention
• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc.

Double-blind