A new study from researchers at McMaster University revealed that gluten and wheat are safe for many people with IBSdespite self-perceived sensitivity to these food components.
To find out whether gluten was the cause of IBS symptoms, the researchers gave cereal bars containing gluten but no other wheat ingredients, whole wheat, or none of these ingredients (mock challenge) to adult participants diagnosed with IBS who previously reported improvement on a gluten-free diet. Participants were told that the bars could worsen symptoms, but were not told which specific ingredients contained them. At the end of the study, each participant ate all three variations of the bars.
After eating one of three types of granola bars every day for seven days, the number of people who experienced worse IBS symptoms was similar in all three groups: Eight of the participants reported worse symptoms after eating bars without gluten and wheat, ten of them after consuming bars with only gluten, and eleven of them after consuming bars with wheat. These findings suggest that individual expectations and beliefs around gluten and wheat, rather than gluten and wheat themselves, may be the cause of symptoms in many people.
To better understand these findings, we asked Premysl Bercik, MD, PhD, senior author of the study and professor in the Department of Medicine at McMaster University, to share his insights on the study.
As for what else beyond gluten and wheat might explain why some people feel better after going gluten-free, Bercik notes: “There are several possible explanations. First, a gluten-free diet is generally low in fiber, and this may reduce bacterial fermentation, with less gas production in the large intestine. Wheat, apart from gluten, an important antigen in celiac disease, also contains other antigenic proteins that can stimulate innate immune responses in the intestines. And finally, beliefs in the harmful effect of wheat and gluten may play an important role in many patients. Implementing a specific diet, which eliminates a perceived symptom caused by food, offers at least some form of control over their illness.”
The central role of personal beliefs in the onset of symptoms was supported by the fact that when participants were told which bars led to digestive symptoms at the end of the study, most did not change their beliefs or their diet.
Although the findings suggest that gluten and wheat are not the most relevant causes of symptoms in many cases, people’s expected symptoms are likely the culprits for many people with IBS. This is the psychological phenomenon known as the nocebo effectwhere a person develops side effects or symptoms that may occur with treatment (including food and whole diets) just because expecting symptoms leads to real symptoms.
Bercik says: “We didn’t expect the findings we obtained, but this is how science works. We were well aware of the nocebo effect in patients with IBS, as it can affect up to 4 in 10 patients, but we were very surprised by how strong the effect was in our study. After cereal bar challenges, patients attributed worse symptoms to gluten or wheat consumption, even though they may have been given a sham challenge. An even more surprising finding was that when we shared our results with them and explained that few reacted to wheat or gluten, this did not change their belief in the harmful effects of gluten and wheat, and they continued to adhere to a gluten-free diet. This suggests that these individuals would benefit from continued support from dietitians and psychologists.”
“We assessed the patients’ commitment to follow the study (adherence) in several ways. First, our dietitian verified compliance with a gluten-free diet using a validated questionnaire. We also asked the patient whether he ate all the challenge bars. And in both tests, the patients did very well,” says Bercik.
After eating each bar, participants provided stool samples to quantify gluten intake before and after each challenge. What’s worth noting is that although most patients with IBS in the study reported eating all the challenge bars, laboratory tests showed that many of them did not consume them and only about a third actually followed the diet as prescribed by the dietitian. “We found that many patients were consuming gluten when they should be gluten-free, and on the contrary, we could not detect any gluten residue in some patients after a gluten or wheat challenge. And this may be due to the fact that the patients were afraid of developing more symptoms and therefore they skipped some of the challenge bars,” explains Bercik.
A recent one review of more than 58 studies investigated whether gluten is the cause of the problem. Although 1 in 10 adults reported being sensitive to gluten or wheat, true gluten-specific reactions were uncommon and usually mild. Fermentable carbohydrates (known as FODMAPs, especially fructans found in wheat, onion and garlic) and expectancy effects explain most cases.
The bottom line is that intestinal or other symptoms that occur after eating foods containing gluten in people who tested negative for celiac disease or wheat allergy are real, but are not caused by gluten.. These people may be sensitive to fructans or other wheat proteins. Another explanation could be that the symptoms are explained by a change in the way the intestines communicate with the brain. Instead of following a lifelong restriction of gluten, the authors suggest that ruling out celiac disease, improving diet quality and considering short-term studies of gluten reintroduction under the supervision of dietitians may be the most effective approach.
References:
Larissa Seiler C, Horacio Rueda G, Miguel Miranda P, et al. Effect of gluten and wheat on symptoms and behavior in adults with irritable bowel syndrome: a single-center, randomized, double-blind, sham-controlled crossover study. Lancet Gastroenterol Hepatol. 2025; 10(9):794-805. two: 10.1016/S2468-1253(25)00090-1.
Beesyirerski JR, Jonkers D, Cycic C, et al. Non-celiac gluten sensitivity. Lancet. 2025. doi: 10.1016/S0140-6736(25)01533-8. Online first.
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