Eat more fiber — patients with irritable bowel disease (IBD) hear it all the time from well-meaning experts. But what if that advice is wrong, or even harmful?

According to a new study in Gastroenterology, certain types of fiber can cause inflammation in people with IBD, actually making symptoms worse.

IBD, which includes ulcerative colitis and Crohn’s disease, affects an estimated 3 million people in the U.S., says the CDC.

Abdominal pain, diarrhea, bloody stools, weight loss, late puberty, and long-term risk of colorectal cancer are all associated with IBD cases. The exact cause is unknown, but some risk factors include genetics, diet, environmental factors, and changes in gut microbes.

“We know there are health benefits to consuming dietary fibers and they promote good gut health in healthy individuals, but IBD patients quite frequently complain about a sensitivity when they consume dietary fibers,” says Heather Armstrong, who started the research as a post-doctoral researcher at the University of Alberta, Canada and is now an assistant professor of medicine at the University of Manitoba and Canada Research Chair in Integrative Bioscience. Armstrong has IBD herself. “We really wanted to understand the mechanisms behind this.”

The research team is developing a stool test to examine the microbes found in patients’ guts to predict who will have a negative response. Ideally, the stool test will help tell patients exactly how to adjust their diets to avoid flare-ups.

Not All Fibers Are the Same

Unlike most of the food we eat, fiber is not digested in the small intestine. Tiny bacteria and fungi or “microbiota” in the large intestine or colon produce enzymes to ferment fiber. Chemically, fiber can be a short string of sugars like pectin, which is found in citrus fruit, or a very long and branched structure that is harder to ferment.

The researchers identified that β-fructan fibers found in foods such as artichoke, chicory roots, garlic, asparagus, and bananas are especially hard to ferment if certain microbes are missing or malfunctioning, as is often the case for IBD patients.

Fiber has a beneficial anti-inflammatory effect in most healthy people and aids with digestion, but the researchers found that select unfermented fibers actually increase inflammation and worsen symptoms in some IBD patients.

Conclusion

While fibers are typically beneficial in individuals with normal microbial fermentative potential, some dietary fibers have detrimental effects in select patients with active IBD who lack fermentative microbe activities.

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