The pathophysiology of Irritable bowel syndrome (IBS) is multifactorial involving complex interplay of altered intestinal permeability, mucosal immune activation, visceral hypersensitivity and gut dysbiosis. Although the exact triggers for these pathological changes in IBS are not clear but diet might play an important role. In fact, several studies have reported improvement in gastrointestinal symptoms on a diet low in FODMAPs (LFD) in patients with IBS, specifically in diarrhea predominant IBS (IBS-D). However, the mechanism of action of LFD is not well understood.

we aim to evaluate the following in patients with IBS-D. The effect of LFD on colonic permeability. 2.The effect of LFD on abundance of colonic mucosal immune cells specifically T-cell and mast cell abundance 3.The effect of LFD on colonic mucosal microbiome

Inclusion Criteria:

• aged 18-65 years at the time of screening
• normal serum studies including serum tissue-transglutaminase antibodies, thyroid stimulating hormone levels, C-reactive protein, complete blood count since the onset of symptoms
• normal stool studies including C diff testing, culture, ova and parasites since the onset of symptoms
• IBS-SSS score of ≥175 at the end of 7-day screening period

Exclusion criteria:

• individuals already on a LFD or other dietary restriction such as gluten free diet within the past 6 months
• individuals with any known food allergy or insulin-dependent diabetes
• known history of celiac disease, inflammatory bowel disease or microscopic colitis
• prior small bowel or colonic surgery or cholecystectomy
• pregnant patients
• antibiotics in the past 3 months
• those who regularly use mast cell stabilizers or anti-histaminic or non-steroidal anti-inflammatory agents (NSAIDs) excluding daily baby aspirin or steroids or bile-acid binder.