Gut bacteria determine fiber benefits in celiac disease patients

Many people with celiac disease are advised to eat more fiber to support digestion and manage symptoms, either through diet or prescribed fiber supplements. New research from McMaster University shows that the benefits of that fiber may depend on whether the right bacteria are present in the gut to break it down.

The study, published in Nature Communications on March 31, found that people with celiac disease had significantly reduced capacity to metabolize dietary fiber in the small intestine. This is because they lacked a key fiber-degrading bacterial family called Prevotellaceae – organisms that help support gut healing and regulate inflammation.

This discovery was observed in both newly diagnosed people and in those who have been managing celiac disease for years with a gluten-free diet and suggests the lack of small intestinal bacteria such as Prevotellaceae is due to persistent microbiome disruptions linked to the disease itself, rather than diet alone.

"Originally, we thought that the problem was that people aren't getting enough fiber. Then we found out that people might not have the right bacteria to use the fiber that they're already eating. Adding more fiber won't be the solution unless you fix the underlying problems with using it," says Mark Wulczynski, first author of the study and postdoctoral fellow with McMaster's Farncombe Nutrition Initiative.

Celiac disease is a disorder where eating gluten drives inflammation in the small intestine. It impacts approximately one per cent of the Canadian population, with the only current treatment being a strict gluten-free diet. This is particularly noteworthy because the research suggests that removing gluten alone doesn't fully restore the gut's microbiome functional capacity.

Future therapies may need to combine dietary strategies with microbiome‑directed approaches, such as microbial restoration or targeted probiotics.

"While a gluten-free diet remains essential for celiac disease, our findings suggest future therapies may also need to support the gut microbiome. We found reduced fiber-processing activity in the upper gut, the area damaged in celiac disease, which is surprising because this part of the gut has not traditionally been seen as a major site of fiber metabolism. This opens the door to combining dietary strategies like added fiber with probiotics that can metabolize it," says senior author Elena Verdu, professor with McMaster's Department of Medicine and director of the Farncombe Family Digestive Disease Health Research Institute.

Not all fiber is alike

Using preclinical models, the researchers investigated which fiber types are actually helpful for healing the gut. They found inulin – a fiber found in everyday foods like bananas, chicory root, garlic and onions – accelerated healing of gluten-induced intestinal injury by feeding the small intestinal microbiome.

This effect occurred only with the right fibers in the diet. Another source of fiber, Hylon VII, a type of corn-based resistant starch commonly used in food manufacturing, did not affect the microbiome and promote healing in the same way inulin did.

This comparison suggests the type of fiber, not just the quantity, may be important for people with celiac disease

To verify this in patients, researchers collected small intestinal fluid from three groups: people newly diagnosed with celiac disease, people who had followed a gluten-free diet for more than two years, and healthy controls. They analyzed the microbiome in these samples to determine which bacteria were present and whether they carried the genes needed to break down different types of fiber. They also compared these findings with participants estimated dietary fiber intake and with measurements of plant DNA in stool, providing an objective marker of fiber sources in the diet.

The results showed that most individuals, including both healthy controls and celiac patients, consumed less fiber than recommended by Health Canada. Health Canada recommends 25-38g of fiber per day for healthy women and men, respectively, but there are no evidence-based recommendations for different disease states. Diseases may cause the body to be more active and simultaneously less efficient than usual, affecting nutritional needs.

However, celiac patients, whether newly diagnosed or treated with a gluten-free diet, showed a distinct combination of low fiber intake and a lack of bacteria known to break down fiber in the small intestine. This suggests that supporting gut health in celiac disease may require both the right fiber and the right bacteria in the small intestine, known as a synbiotic approach.

Future research is needed to better understand and determine how fiber and microbial restoration could improve symptoms, healing, and treatment responses, before making clinical recommendations.

The study was funded by a Canadian Institutes of Health Research grant and a J.A. Campbell Young Investigator Award to Mark W. from Celiac Canada. Verdu Holds a Canada Research Chair in Microbial Therapeutics and Nutrition in Gastroenterology.