Fermentable carbohydrates in functional bowel disorders: New insights

There has been a dramatic increase in the use of diet therapies for the management of functional bowel disorders and, in particular, irritable bowel syndrome (IBS).1 These include diets such as the NICE (National Institutes for Clinical Excellence) guidelines, gluten-free diets, and diets restricting food constituents such as capsaicin1; however, diets low in fermentable carbohydrates have received the greatest attention over recent years. FODMAPs refer to fermentable oligosaccharides (fructans, galacto-oligosaccharides), disaccharides (lactose), monosaccharides (excess fructose), and polyols (eg, sorbitol, mannitol). FODMAPs are poorly absorbed in some people (and in the case of oligosaccharides in all people) and upon reaching the colon are fermented by the microbiota, producing gases, short-chain fatty acids, and other potentially biologically active metabolites. While the low FODMAP diet is increasingly embraced by physicians, dietitians, and patients alike, there are ongoing questions regarding the magnitude of the benefit, the inability to identify patients most likely to respond, a lack of understanding about the pathophysiology and mechanisms, and the safety implications.1 To assist in addressing these concerns, the editors of two leading journals in the area, Neurogastroenterology and Motility and the Journal of Human Nutrition and Dietetics, have assembled key papers in a single virtual issue, and all are available through open access. This issue contains three comprehensive reviews that carefully examine the impact on symptoms of the numerous low FODMAP diet intervention studies1-3 and safety issues. While almost all studies report significant symptom improvement, these reviews also discuss the limitations of these studies with particular focus on the challenges of conducting well-designed dietary clinical trials. Consequently, the reported 50%-80% improvement of symptoms is not solely a result of the diet and reflects a number of other factors, including the impact of intensive counseling and support, patient unblinding, and expectation bias. As a result, the true benefit of the dietary intervention remains unclear. In view of this knowledge gap, two of these reviews provide clinical guidelines for the implementation of dietary therapies including a low FODMAP diet.2, 3 Despite these concerns, in a proof-of-concept study, an algorithmic approach to diagnosis and management prior to the assessment by a specialist was evaluated and patient introduction of a low FODMAP was reported to be effective.4 One of the ongoing issues regarding the low FODMAP diet is its complexity and patient compliance. The low FODMAP diet is designed in three stages, starting with short-term (4-8 weeks) FODMAP restriction, followed by increasing doses of select foods during FODMAP reintroduction and finally long-term maintenance through FODMAP personalization that allows consumption of a wider range of foods deemed suitable during reintroduction.2 Many investigators report that involvement of dietitians is essential for the diet to be followed successfully and safely. However, there is a significant limitation in the numbers of suitably trained dietitians, and consequently, one study examines the clinical and cost-effectiveness of group vs 1:1 education of patients by dietitians.5 In long-term follow-up studies where dietitians were involved, the low FODMAP diet was superior to a habitual diet and no safety issues were identified.6 No doubt many recommendations will emerge as more information comes forward, for example, with improved food composition data assisting patients who have multiple dietary restrictions, for example, vegetarians trialing a low FODMAP diet, and reductions in FODMAP content of plant-based foods following processing.7 A better understanding of the mechanisms of action of FODMAPs is needed to identify biomarkers for selecting true responders and to explore alternatives to dietary restriction, for example, microbiome-manipulation therapies. In addition to distention resulting from the gas produced by fermentation in patients with underlying visceral hypersensitivity, some studies provide evidence of immune activation and a possible link to symptoms.8 Certain FODMAP subgroups can also have osmotic effects in the small intestine, and one study suggests that fructans can stimulate motility in the upper gastrointestinal tract.9 Of the FODMAP subgroups, the actions of excess fructose have been particularly challenging to elucidate, especially given their complex interaction with glucose co-transport. Unfortunately, combining glucose with either excess fructose or fructans does not mitigate their effects on symptoms.10 Tests that identify susceptible patients that malabsorb FODMAPs, and in particular excess fructose, have remained elusive. Using a complex design, one study confirms the inadequacy of breath testing to detect fructose malabsorption, but raises the potential of the test for detecting intolerance.11 There are also a few studies in the pediatric literature showing efficacy, and one intriguing proof-of-concept study suggested that reducing FODMAPs in breastfeeding mothers could reduce infantile colic.12 In summary, this virtual issue highlights key publications that provide new insights into our understanding of FODMAPs and their influence on symptom induction and the role of the low FODMAP diet in managing functional bowel disorders, and gaps for future research.

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