Up to 84% of people with IBS state that eating foods containing carbohydrates, fried and fatty foods, and foods rich in histamine trigger digestive symptoms. Foods containing gluten, a protein found in wheat, barley, and rye, are also considered a common culprit that worsens bloating, diarrhea, and abdominal pain. This leads many patients with IBS to avoid gluten or wheat, which contains gluten, “just in case”, often without a formal medical diagnosis.
A new study from McMaster University researchers has revealed that gluten and wheat are safe for many people living with IBS, despite self-perceived sensitivity to these food components.
To find out whether gluten was the cause of IBS symptoms, the researchers gave cereal bars containing either gluten but no other wheat components, whole wheat, or neither of these ingredients (sham challenge) to adult participants with a diagnosis of IBS who previously reported improvement on a gluten-free diet. The participants were informed that the bars could worsen symptoms, but were not told which ones contained specific ingredients. At the end of the study, each participant ate all three varieties of the bars.
After eating one of three types of cereal bars every day for seven days, the number of people who experienced worse IBS symptoms was similar across all three groups: eight of the participants reported worse symptoms after eating bars without gluten and wheat, ten of the participants after the gluten-only bars, and eleven of them after consuming the wheat ones. These findings suggest that individual expectations and beliefs surrounding 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.
Regarding what else beyond gluten and wheat could 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 decrease bacterial fermentation, with less gas production in the colon. Wheat, apart from gluten, which is a main antigen in celiac disease, also contains other antigenic proteins that may stimulate innate immune responses in the gut. And finally, beliefs in detrimental effect of wheat and gluten may play a significant role in many patients. Implementing a specific diet, which removes a perceived symptom trigger food, provides at least some form of control over their disease.”
The central role of personal beliefs in symptom onset was supported by the fact that when participants were told at the end of the study which bars led to digestive symptoms, most did not change their beliefs or their diets.
While the findings suggest gluten and wheat are not the most relevant causes of symptoms for many cases, people’s expected symptoms are likely the culprits for many people with IBS. This is the psychological phenomenon known as the nocebo effect, in which a person develops side effects or symptoms that can occur with a treatment (including foods and whole diets), just because expecting to have symptoms leads to real symptoms.
Bercik says: “We have not expected the findings we obtained, but this is how the science works. We were well aware of the nocebo effect in patients with IBS, as it can affect up to 4 out of 10 patients, but we were very surprised by how strong the effect was in our study. After challenges with cereal bars, the patients attributed worse symptoms to consumption of gluten or wheat, although they might have been given a sham challenge. Even more surprising finding was that when we shared with them our results and explained that only few reacted to wheat or gluten, this did not change their beliefs in the detrimental effect of gluten and wheat, and they continued adhering to a gluten-free diet. This suggests that these individuals would benefit from ongoing support from dieticians and psychologists.”
“We have assessed the patients’ commitment to follow the study (adherence) by several ways. First, our dietician verified adherence to a gluten-free diet by a validated questionnaire. We also asked the patient whether they ate all challenge bars. And in both of these 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 is worth noting is that although most patients with IBS in the study reported eating all challenge bars, lab tests showed that many of them did not consume them and only about a third actually followed the diet as instructed by the dietitian. “We found that many patients consumed gluten when they were supposed to be gluten-free, and on contrary, we could not detect any gluten residue in some patients after having gluten or wheat challenge. And this may be due to the fact that the patients were afraid of having more symptoms and that’s why they skipped some of the challenge bars”, explained Bercik.
A recent review of more than 58 studies explored whether gluten is the cause of the problem. While 1 in 10 adults reported being sensitive to gluten or wheat, true gluten-specific reactions were uncommon and typically mild. Fermentable carbohydrates (known as FODMAPs, in particular fructans found in wheat, onion, and garlic), and expectation effects explain most cases.
The bottom line is that gut or other symptoms appearing after eating foods containing gluten in people who tested negative for celiac disease or wheat allergy are real, but not gluten-driven. These people could be sensitive to fructans or other wheat proteins. Another explanation could be that symptoms are explained by an alteration in how the gut communicates with the brain. Rather than following a lifelong restriction of gluten, the authors suggest that ruling out celiac disease, improving diet quality, and considering short-term, dietitian-supervised trials of gluten reintroduction 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 behaviours in adults with irritable bowel syndrome: a single-centre, randomised, double-blind, sham-controlled crossover trial. Lancet Gastroenterol Hepatol. 2025; 10(9):794-805. doi: 10.1016/S2468-1253(25)00090-1.
Biesiekierski JR, Jonkers D, Ciacci C, et al. Non-coeliac gluten sensitivity. Lancet. 2025. doi: 10.1016/S0140-6736(25)01533-8. Online first.
