Celiac disease weakens gut microbes that digest fiber

By Dr. Liji Thomas, MDReviewed by Lauren HardakerApr 2 2026

The problem in celiac disease may not be what patients eat, but what their microbes can process; microbial fiber metabolism could be the missing link. 

Study: Small intestinal microbial fiber metabolism dysfunction in celiac disease. Image credit: Marian Weyo/Shutterstock.com

Celiac disease is a gluten-triggered, immune-mediated condition that causes intestinal inflammation and damage. The gluten-free diet (GFD) often relieves its symptoms but may cause fiber deficiency. However, a study published in Nature Communications found that, irrespective of a gluten-free diet (GFD), these patients had reduced fiber-degrading taxa in their small-intestinal microbiota and correspondingly lower microbial fiber metabolism in the small intestine.

What is CeD?

Celiac disease (CeD) is triggered by gluten in wheat, rye, and barley. This causes small intestinal inflammation, with lymphocytes migrating into the epithelium and crypt hyperplasia. CeD shares features of autoimmune disease. For instance, it is associated with HLA-DQ2 or DQ8, autoantibodies to tissue transglutaminase 2 (TG2), and immune-mediated destruction of intestinal epithelial cells.

Of the 30 % of the population with a genetic predisposition to CeD, the disease develops in only 2–3 % of those at risk, but the incidence has shot up over the last few decades. This suggests that contributing environmental factors exacerbate the genetic risk, such as gut infections coupled with high gluten intake, or that altered gut microbiota composition and function in the small intestine.

CeD and dietary fiber

The existing literature suggests that eating more fiber-rich foods, such as vegetables, reduces the odds of developing celiac autoantibodies. Microbes act on fiber to produce short-chain fatty acids (SCFAs) like acetate and propionate, which have been linked to improved gastrointestinal function, including motility and immune regulation, while supporting beneficial gut microbial growth.

The GFD is the only accepted treatment for CeD, but it results in slow, irregular mucosal healing. The GFD is typically low in fiber, and little is known about how gut microbes process this fiber in CeD. Many practitioners recommend that these patients increase their fiber intake, despite limited clarity about its metabolic fate. To clarify this situation, the authors conducted this study.

Comparing untreated, treated, and healthy gut microbiota

The study included 16 newly diagnosed patients with CeD (11 on a GFD for 2 or more years [T-CeD]) and 26 healthy controls. Using gene sequencing, the researchers examined the small intestinal microbiota of each participant, using only duodenal aspirates. This showed distinct clustering of microbial profiles between groups.

Reduced microbial fiber metabolism

Fiber intake was typically suboptimal according to Health Canada guidelines. No obvious correlations were observed between symptoms and dietary fiber.

The authors found reduced predicted abundance and, for some targets, expression of genes encoding fiber-degrading enzymes in CeD, starch-degrading α-amylase, and fructan β-fructosidase. This was independent of GFD status and of fiber intake. This reduction was primarily due to the depletion of Prevotella species that degrade fiber.

Reduced microbial metabolism was indicated by decreased SCFA production in CeD patients compared with controls, as measured in fecal samples as a proxy for microbial fermentation. Interestingly, T-CeD patients had higher SCFA levels than untreated CeD. This suggests a modest recovery of fiber-degrading function with treatment despite inadequate fiber consumption.

Fiber supplementation in a mouse model

Secondly, the authors tested the effect of additional fiber in a mouse model. These mice were genetically predisposed to the condition and had been induced to develop gluten sensitivity. They were put on GFD, with added fiber either in the form of inulin or HylonVII (a resistant starch prebiotic substrate). Neither inulin nor HylonVII improved alpha-diversity, but beta-diversity in the HylonVII group changed distinctively compared to inulin or no-added fiber groups.

Inulin supplementation increased microbial saccharolytic activity (fiber metabolism) in microbiota-containing mice compared with germ-free controls, supporting the role of the microbiota in fiber metabolism.

Both the HylonVII and inulin groups became seronegative at 12 weeks in the mouse model, although antibodies were not normalized at earlier time points. Histological findings suggest that inulin enhanced GFD response by accelerated healing.

Microbial manipulation of a mouse model

The authors compared germ-free mice (free of all organisms) with mice colonized with complex microbiota (SPF mice). The SPF mice had higher SCFA production (mostly acetic acid). The increase was 10 times higher in feces than in the small intestine.

Inulin supplementation was associated with increased SCFA receptor expression along the small intestine in SPF mice but not germ-free mice. Based on this, germ-free DQ8 mice were colonized with ten Prevotellaceae strains. With inulin supplementation, this resulted in higher small intestinal SCFA levels.

Study limitations and future directions

This was a small exploratory study, and too few small intestinal samples were available for microbial metabolic analysis. Small intestinal SCFAs were not directly measured; instead, they were inferred or predicted in human subjects. However, the results were tested in mouse models.

Microbial deficits, not just diet, impair fiber metabolism 

“The results support clinical investigations of dietary fiber supplementation and microbial fiber degradation to enhance responses to the GFD in CeD.” Many CeD individuals eat too little fiber, especially on the GFD. Even though these individuals have similar proportions of plant-based foods, specific whole grains (wheat, rye, oats, and barley) and the flours derived from them were missing from the GFD.

This may contribute to lower total SCFA in CeD compared to controls. The highest fiber deficiency was in T-CeD, but they had higher SCFAs than active CeD patients. This suggests a partial restoration of microbial fiber metabolism with treatment, though this hypothesis requires confirmation by direct evidence.

Findings from earlier studies have also shown increased abundance of taxa associated with SCFA production and indirectly with recovered gut health. Taken together, these data suggest that impaired microbial metabolism of fiber in CeD patients is not solely due to low intake but also to a deficit in key fiber-degrading microbes.

This supports further investigation into supplementing CeD patients with fiber while also adding in appropriate microbes to colonize the gut, although current evidence for therapeutic benefit remains preclinical. Importantly, Prevotellaceae contains species that hold promise in immunomodulatory therapy, and are being investigated for systemic autoimmune disease in ongoing trials.

Future prospective trials should further examine the relationship of dietary fiber with gut symptoms in celiac disease.

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