Prebiotics and Gut Health: Implications for Managing IBS and Functional GI Disorders

The gut microbiota represent the collection of bacteria, archaea and eukarya that colonize the GI tract. Increasingly, alterations to their diversity and abundance are recognized as symptom drivers in IBS, due to a range of mechanisms.

Disruptions to the gut microbiota are observed in the majority of people with IBS and lower levels of bifidobacteria are of particular concern. While there is no evidence yet that can support the definition of a “healthy” microbiome composition, microbial signatures seen in IBS are associated with a greater number of days of abdominal pain in both healthy adults and those with IBS.2 Gastroenteritis, which disrupts the microbiota, also increases risk for post-infectious IBS (PI-IBS) and functional diarrhea, further suggesting gut microbiota play a role in symptom onset.3

There is also evidence to show that the low-grade inflammation sometimes seen in IBS disrupts signaling to the gastrointestinal immune system via gut microbiota pathways. This may increase risk for intestinal permeability and other altered immune responses.4 Furthermore, metabolites of gut microbiota such as serotonin and their influence on enterochromaffin cells may also play a role in pain signaling or visceral hypersensitivity in IBS and other DGBIs.2

These various mechanisms by which gut microbiota influence symptoms have led to an increased interest in ways to support and restore microbiota health in IBS.

Prebiotics are “substrates that are selectively utilized by host microorganisms conferring a health benefit to the host.”5 They may consist of carbohydrates naturally present in food, such as fructans and galactooligosaccharides (GOS), and are also available as dietary supplements, often in the form of inulin or fructooligosaccharides (FOS).

Prebiotics provide a promising path to support gut microbiota health based on their theoretical ability to increase fecal short-chain fatty acids (SCFAs), feed microbiota and reduce gut-associated inflammatory markers.6 Increased production of SCFAs may regulate motility by increasing the release of serotonin and stimulating the gut-brain-axis through the enteric or vagal nerves, which help regulate colonic smooth muscle.7

Byproducts of prebiotic fermentation in the colon may provide several physiologic effects, such as greater calcium absorption, increased fecal weight, regulation of GI transit time and reduced blood lipid levels, though the extent to which they may occur depend on baseline gut microbiota and diet. In particular, benefits related to prebiotics’ ability to increase colonic bifidobacteria include support for production of pathogen-inhibiting compounds, reductions in blood ammonia levels and facilitation of vitamin and digestive enzyme production.8

Still, while the mechanistic benefits of prebiotic are diverse and well-identified, there is less clarity into their clinical role in managing symptoms in DGBIs and research on the benefits of prebiotic supplementation is rather limited. Data on the impact of prebiotic supplementation on symptoms themselves is mixed and the impact of the placebo response cannot be ruled out.2 Among existing research, there is little standardization among prebiotic dose, prebiotic type and study protocols. This heterogeneity makes interpretation of findings difficult to synthesize into clinical recommendations.

Still, evidence illustrates that supplementation with inulin-type fructans supports increased fecal bifidobacteria in healthy adults at doses of > 6 g/day.[9] Increased bifidobacteria levels may provide symptom relief due to the known benefits of bifidobacteria for gut health, but clinical findings remain inconclusive. For those with chronic constipation, prebiotic supplementation may provide benefit. However, it can also lead to increased flatulence, suggesting alternative methods to address constipation may be preferable.7,10

There is a significant dose-respondent relationship and the type of fiber matters when it comes to prebiotics. Lower supplemental doses can improve symptoms but higher doses can exacerbate them. Inulin-type prebiotics are most likely to worsen symptoms like flatulence, while non-inulin-type prebiotics, such as GOS and guar gum, can improve them. In short, the data is highly mixed. A recent meta-analysis concludes that prebiotic supplementation does not provide benefit for symptom management, stool output or QoL in IBS.2

There is more evidence to suggest that inclusion of prebiotic-rich fruits, vegetables, and whole grains, as well as foods enriched with prebiotic fibers, can help with symptom management. Prebiotics exist in various forms, such as polysaccharides, oligosaccharides, resistant starch, conjugated linoleic acid and polyphenols. Foods rich in naturally-occurring prebiotics include buckwheat, kiwi, rhubarb, onions, bananas, honey, garlic and leeks. When used to enrich foods, prebiotics are typically added in the forms of inulin, chicory root, guar gum, FOS, GOS, lactulose and human milk oligosaccharides (HMOs).8

Still, note that the American Gastroenterological Association (AGA) Clinical Practice Guidelines on the role of diet in IBS highlight the lack of scientific data to recommend a diet high in prebiotic fiber. Instead, the guidelines emphasize the importance of increasing overall soluble fiber intake to manage IBS symptoms.11 In other words, prebiotic-rich foods may be part of a diet high in soluble fiber, but should not be the sole focus.

The role of prebiotic supplementation and a diet high in prebiotics for IBS is complex. Many prebiotic types, such as GOS, inulin and FOS, are also Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (FODMAPs). Dietary restriction of FODMAPs is effective for symptom control in IBS so it is logical that addition of high-FODMAP prebiotic sources could exacerbate symptoms. For this reason alone, introduction of prebiotic fiber in either food or supplementation form should be exercised with caution in those who are FODMAP sensitive and low-FODMAP prebiotic fiber sources should be the focus.

The heterogeneous evidence on prebiotics and symptom management highlights the importance of considering prebiotic dose and type in clinical nutrition practice and managing tolerance closely. Due to the significant dose-respondent relationship, patients who do wish to trial prebiotics are advised to introduce them gradually to allow for an adaptation period. This was the approach used in a small, recent proof of concept study in healthy adults, who were given 2.8 g/d of supplementary GOS for an initial 3 weeks adaptation period. While respondents reported initial consumption led to increased flatulence, the symptoms subsided by the third week. This suggests an extended trial period may allow for clearer understanding on response and potential symptom benefit.12

Looking forward, additional research on low-FODMAP prebiotic fiber sources and at varying doses is warranted to clarify whether prebiotic supplementation should play a role in treatment plans for IBS and other FGIDs and, if so, optimal strategies for their use. Until then, encourage patients to include prebiotic rich-foods as part of an overall diet high in soluble fiber diet to support symptom management.