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Optimising dietary therapy in patients with irritable bowel syndrome-type symptoms

par Van Ouytsel, Pauline
Président du jury Le Moine, Alain
Promoteur Louis, Hubert
Co-Promoteur Piessevaux, Hubert
Publication Non publié, 2026-09-07

Thèse de doctorat

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Résumé :

Background and aims: Irritable Bowel Syndrome (IBS) is characterised by recurrent abdominal pain associated with changes in stool frequency and/or consistency, affecting 4.1% of the global population. Its pathophysiology is multifactorial, involving Disorders of Gut-Brain Interaction (DGBI) which leads to altered gastrointestinal (GI) motility and visceral hyperalgesia. Intolerance to short-chain fermentable carbohydrates (FODMAP) is increasingly recognised as a contributing factor in IBS, not as a primary cause, but as a trigger for symptoms. A low-FODMAP diet (LFD) has been shown to improve symptoms and quality of life in patients with IBS. However, LFD implementation raises important practical and safety considerations. As a restrictive diet, it may expose patients to nutritional deficiencies, alterations of the gut-microbiota, and the development or exacerbation of disordered eating behaviours, such as Avoidant/Restrictive Food Intake Disorder (ARFID). This thesis aimed to optimise dietary therapy delivered by dietitians and gastroenterologists in the management of IBS-type symptoms by contributing to the development of practical clinical tools for both clinicians and by better characterising the target populations. Ultimately, the goal was to support targeted, effective dietary interventions while minimising the risks associated with restrictive diets, thereby improving the quality of care for patients presenting IBS-type symptoms.Results: A preliminary study highlighted the importance of dietitian support alongside LFD initiation, combined with gastroenterologist guidance. A second study identified a relationship between IBS-type symptom severity and some specific FODMAP intake following bariatric surgery. This research also emphasised the need for a simplified Food Frequency Questionnaire (FFQ) to reliably assess FODMAP intake. Building on these findings, we developed and validated the first European FFQ specifically designed to assess FODMAP consumption. This FFQ-FODMAP-BE demonstrated good reproducibility and allowed the identification of moderate- and high-FODMAP consumers with 80% sensitivity. In a sub-study, lactose was found to account for the highest proportion of FODMAP intake, while Galacto-Oligo-Saccharides (GOS) were the least consumed in the Belgian adult population. In parallel, a French-translated and validated version of the Nine Item ARFID Screen (NIAS-Fr) was developed through a multicentre collaborative work involving French and Belgian centres. Finally, our last cross-sectional study showed that 26% of patients with quiescent Inflammatory Bowel Disease (IBD) screened positive for ARFID risk, which was associated with lower FODMAP consumption and increased anxiety.Conclusions: Overall, our findings provide meaningful contributions for both researchers and clinicians by providing practical tools and improving the characterisation of the clinical context in which restrictive diets such as the LFD are prescribed. The NIAS-Fr is now available to screen for ARFID risk in French-speaking populations. The FFQ-FODMAP-BE provides a practical, rapid, and reliable alternative to reference dietetic methods for assessing FODMAP intake. A better understanding of FODMAP consumption in the Belgian population, alongside improved characterisation of ARFID risk in vulnerable groups, represents a contribution to the current literature. Taken together, these findings represent a step toward the development of personalised and targeted dietary therapies for patients with IBS-type symptoms. Future research should focus on developing predictive models of dietary response to guide dietary interventions more precisely. The development of a multidisciplinary clinical decision tree integrating dietary, clinical, and psychological factors represents the next key step toward delivering personalised dietary care and improving patient outcomes.