The purpose of this study is to evaluate if low-FODMAP (Fermented Oligo-, Di-, Monosaccharides And Polyols) rye bread is better tolerated in irritable bowel syndrome than commonly available traditional rye bread higher in FODMAP carbohydrates. The study also aims to investigate patients' compliance to rye bread regimen, potential changes is gut microbiota and hydrogen production during the test periods (a marker of large bowel fermentation of poorly absorbed carbohydrates).

Inclusion Criteria:

• IBS according to Rome III criteria. Accepted sub-types include IBS-M, IBS-D and IBS-U
• Age 18-65 years
• Willing to use rye bread daily during the study periods

Exclusion Criteria:

• IBS-C (constipation dominant)
• Celiac disease
• IBD
• Major gastrointestinal operations like bowel gastric resection
• Non-treated hypo- or hyperthyroidism
• Alcoholism, severe depression, dementia, cancer or other diseases likely to severly impair the participants ability to conclude the protocol
• Regular (almost daily) use of NSAIDs, antibiotics or lactulose
• Linaclotide and other prescription medicines targeted specifically to IBS
• Pregnancy and lactation
• Follows currently strict low-FODMAP diet