Gluten may not be the culprit for many people with irritable bowel syndrome

A new study from McMaster University researchers has found that many people with irritable bowel syndrome (IBS) who believe they are sensitive to gluten or wheat may not actually react to these ingredients.

IBS is an intestinal disorder that affects an estimated 10 per cent of Canadians – one of the highest prevalence rates globally, according to the Canadian Digestive Health Foundation. It can be disruptive and debilitating, and its underlying cause is unclear.

The study, published in The Lancet Gastroenetrology and Hepatology on July 21, 2025, included participants with clinically diagnosed IBS who reported feeling better on a gluten-free diet. These individuals were given cereal bars to eat containing either gluten, whole wheat, or neither -- without knowing, which was which, in a random order.

Researchers found the number of people who experienced worse symptoms was similar across all three groups, including the gluten-free placebo, suggesting that expectations and beliefs, rather than the ingredients themselves, may be driving symptoms in many cases.

Not every patient who believes they are reacting to gluten actually does. Some truly have a sensitivity to this food protein, but for many others, it's the belief itself that's driving their symptoms and subsequent choices to avoid gluten-containing foods."

Premysl Bercik, senior author, professor with McMaster's Department of Medicine

When participants were later told which bars caused gastrointestinal symptoms, most did not change their beliefs or their diets. Bercik says the findings suggest that some patients with IBS may benefit, apart from dietary guidance also from psychological support and personalized care.

"What we need to improve in our clinical management of these patients is to work with them further, not just tell them that gluten is not the trigger and move on. Many of them may benefit from psychological support and guidance to help destigmatize gluten and wheat and reintroduce them safely in their diet."

The study thus sheds light on why many people with IBS continue to avoid gluten, even when it may not be the true cause of their symptoms. According to researchers, the psychological phenomenon known as the nocebo effect, where negative expectations alone can trigger real symptoms, could partly be the reason. Bercik says the powerful role of social media and online communities can also fuel ideas that gluten is harmful.

"There is strong influence from the internet. Many patients post how bad they feel about gluten. Of course, it influences others," says Bercik, adding that for some people living with IBS, avoiding gluten is a way of taking control of their situation.

"Continuing the gluten-free diet might provided patients with an actionable method to try to control their symptoms, although it meant to follow unnecessary diet restrictions."

To conduct the study, researchers designed a randomized, double-blind, sham-controlled crossover trial. This means neither the 29 participants nor the researchers knew which bar was being consumed at any time. After each challenge, participants reported their symptoms, and stool samples were analyzed to objectively measure gluten intake.

Even though most participants said they ate the gluten or wheat bars, lab tests showed many didn't. According to the study, only about a third actually followed the diet as instructed, suggesting some may have skipped the bars to avoid symptoms.

Funding for the study was provided by Canadian Digestive Health Foundation and the Society for the Study of Celiac Disease.