Dietary intervention for gut and mental health symptoms in irritable bowel syndrome.

This is a parallel-design, blinded, randomised controlled trial. There will be 2 dietary arms, each administered for 8 weeks. Participants will be randomly allocated to either the traditional Mediterranean Diet (MedDiet; active arm) or a Sham Diet (placebo arm), with the primary objective of measuring differences in gastrointestinal symptom response between groups at week-8 follow-up, defined as a 50 point improvement in the IBS Symptom Severity Scale (IBS-SSS) or greater. The intervention diet (MedDiet) is characterised by a high intake of plant-based foods including olive oil, fruits, vegetables, nuts and seeds, and wholegrains; moderate intake of fish and poultry; and a low intake of dairy products, red meat, processed meats, and sweets. For those who already consume alcohol, optional red wine in moderation (no more than 100mL per day), consumed with meals, is permitted. Participants following the MedDiet will be guided by specific daily and weekly food and food group targets, alongside foods to avoid or limit. The traditional MedDiet guidelines are based off of those previously used in IBS research (see https://doi.org/10.1111/apt.17791). The intervention diet will be administered through dietary counselling (see below for visit schedule). Participants will receive tailored dietary guidance from the research dietitian at each dietitian consultation to support compliance with their allocated diet for the duration of the intervention. Dietitian sessions will also integrate behaviour change techniques such as goal setting and motivational interviewing. The Dietitian sessions will be supported by behaviour change resources including purpose-designed written materials (e.g., recipes and meal planning resources developed for this study) and a food hamper provided once within the first week of the study. The hamper contains core ingredients to support adherence to the allocated diet, including foods from key MedDiet food groups such as olive oil, nuts, and seeds. As hamper contents may vary seasonally, the specific items included may differ between participants but will remain consistent with the principles of the allocated study diet. The study will include 5 scheduled study visits across the 8-week intervention period with a research dietitian. All visits will be conducted remotely via telephone or online consultation. At Baseline, participants will complete a 45-minute dietary consultation with the study dietitian, alongside online questionnaires assessing gastrointestinal symptoms, mental health symptoms, and quality of life, as well as a three-day food record. Subsequent visits at Weeks 2, 4, and 6 will each involve a 30-minute dietary consultation and online questionnaires assessing dietary adherence, with the Week 4 visit also including reassessment of gastrointestinal symptoms, mental health symptoms, and quality of life. The final visit at Week 8 will include a 30-minute dietary consultation and completion of online questionnaires covering gastrointestinal symptoms, mental health symptoms, quality of life, dietary adherence, and an end-of-intervention survey. Dietary adherence to the allocated study diet will be monitored throughout the study via self-reported questionnaires administered at fortnightly intervals corresponding with dietitian visits. Adherence to the Mediterranean Diet will be assessed at baseline and week 8 using the Mediterranean Diet Adherence Screener (MEDAS).