Exclude or Expose in Irritable Bowel Syndrome: What Works for Whom and How?

This research project will assess how two treatments for irritable bowel syndrome (one dietary and one behavioral) work and for whom. This will be done by assessing moderators (what treatment works for who and in what context) and mediators (how treatment works). Investigators will also assess how the diet and behavioral treatments affect IBS symptoms during treatment. Participants will be randomized to either: i. A FODMAP diet online program that focuses on modifying the consumption of foods high or moderate in fermentable carbohydrates (FODMAPS) to manage IBS symptoms. or, ii. An Exposure-based Cognitive Behavioral Therapy (E-CBT) online program for IBS that focuses on changing symptom-related behaviors which are known to worsen IBS symptoms.

This research uses a randomized controlled trial to investigate the treatment of irritable bowel syndrome (IBS). A range of treatments for IBS are available, but each work for only a portion of patients, and it is unclear why. This research project will assess how two treatments (one dietary and one behavioral) work and for whom. Investigators will do this by assessing moderators (what treatment works for who and in what context) and mediators (how treatment works). This project will also assess how the diet and behavioral treatments affect IBS symptoms during treatment. Participants will be randomized to either: i. A FODMAP diet online program that focuses on modifying the consumption of foods high or moderate in fermentable carbohydrates (FODMAPS) to manage IBS symptoms. This treatment involves 3-phases: FODMAP restriction (phase 1), FODMAP re-introduction (phase 2), and FODMAP personalization (phase 3), or, ii. An Exposure-based Cognitive Behavioral Therapy (E-CBT) online program for IBS that focuses on symptom-related behaviors such as avoidance of activities and foods, excessive toilet visits, and behaviors to control symptoms which are known to worsen IBS symptoms. E-CBT uses daily self-observation and exposure exercises to change these behaviors to relieve IBS symptoms. Investigators believe that both treatments, FODMAP diet and E-CBT, will lead to an improvement in IBS gastrointestinal symptoms. Primary moderators and mediators to be assessed include gastrointestinal symptom anxiety, food avoidance behaviour and FODMAP intake.

Eligibility Criteria:
Inclusion Criteria: Each participant must meet all of the following criteria to be enrolled in this trial:

* Living in Australia or the United States
* Aged 18 years or older
* Body Mass Index (BMI) \[Equation\]18.5kg/m2 and \[Equation\] 34.9kg/m2
* Diagnosed with IBS by a General Practitioner, Family Physician, or Gastroenterologist, or Advanced Specialist Dietitian
* Currently fulfill Rome IV criteria for IBS
* Symptomatic at the time of recruitment (IBS-SSS \> 175)
* Access to a computer and internet
* Sufficient English language and computer skills to complete a text-based online treatment
* Willing to make dietary or behavioral changes in line with the allocated treatment program
* Ability to provide informed consent

Exclusion Criteria:Patients meeting any of the following criteria will be excluded from the trial:

* Presence or known history of other GI disease (e.g. coeliac disease, Inflammatory Bowel Disease) or history of gastrointestinal cancer
* History of major gastrointestinal surgery (not appendectomy, cholecystectomy or hemorrhoidectomy)
* Individuals who report alarm symptoms (e.g., blood in stool, recent unexplained/unintentional weight loss \>5% body weight, a recent change in bowel habits if \>50 years, family history of gastrointestinal cancer or gastrointestinal diseases, large volumes of diarrhea occurring at night, fever associated with gut symptoms, recurrent vomiting, persistent unexplained iron deficiency) will be excluded if appropriate medical investigations have not been conducted, unless written medical approval is provided from a general practitioner, family physical, or gastroenterologist.
* Diagnosis of major disease that could explain current gastrointestinal symptoms such as severe diabetic, cardiac, liver, neurological, neuropathy disease
* History or current diagnosis of psychotic disorder or bipolar disorder
* Current diagnosis of substance abuse disorder or major depressive disorder or active suicidal ideation
* History or current diagnosis of an eating disorder
* Current enteral/parenteral feeding or use of supplemental feeds (e.g. Ensure)
* Pregnant or lactating or planning to become pregnant during the 12-week intervention period
* Commenced or change in dose of antibiotics and medications that potentially affect the gastrointestinal transit (e.g. anti-diarrheals and laxatives) in the 3 months preceding study commencement.
* Commenced taking or changed dose of probiotics, prebiotics, fibre supplements and digestive enzymes in the 3 months preceding study commencement
* Commenced or change dose in psychotropic medication in the 3 months preceding study commencement
* Are currently undertaking behavioral therapy for IBS (including cognitive behavioral therapy, CBT), or have undertaken clinician-delivered behavioral therapy for the treatment of IBS
* Are currently undertaking or have undertaken a dietitian-prescribed and delivered restrictive diet for IBS (e.g., FODMAP diet).