To evaluate the effects of a gluten-free diet in irritable bowel syndrome.

Irritable bowel syndrome with diarrhoea (IBS-D) is commonly encountered in primary and secondary-care practice. There are various triggers including diet. There is a growing number of people taking a gluten-free diet (GFD) of their own volition even in the absence of coeliac disease (CD). The investigators aim to assess whether a GFD could be of benefit to patients with IBS-D, previously naive to the effects of gluten and in whom CD has been excluded as per normal duodenal biopsies. The investigators will also assess whether certain factors may predict a clinical responder such as the presence or absence of at-risk coeliac serology.

Inclusion Criteria:

1. Patients aged 16 years or over
2. Patients fulfilling ROME III symptom based criteria for IBS-D but do not have coeliac disease

Exclusion Criteria:

1. Patients diagnosed with coeliac disease
2. Patients already on a gluten-free diet
3. Patients initially referred with self-reported gluten sensitivity
4. Patients with organic conditions that can mimic IBS-D such as idiopathic bile acid diarrhoea, pancreatic insufficiency, microscopic colitis, and inflammatory bowel disease