Gluten may not actually trigger many irritable bowel syndrome cases

Some people who think gluten worsens their irritable bowel syndrome (IBS) symptoms actually experience no more discomfort when they eat the protein, which is found in wheat, barley and rye.

IBS commonly causes abdominal pain, bloating, diarrhoea and constipation. While the mechanism is unknown, many people with the condition believe eating gluten or wheat, which contains gluten, worsens their symptoms.

To understand whether these really are common triggers, Premysl Bercik at McMaster University in Canada and his colleagues recruited 28 people with IBS who said they had experienced improvements on a gluten-free diet.

The researchers asked the participants to eat a gluten-free diet for three weeks, before ranking the severity of their symptoms on a scale of 0 to 500, with an average score of 183.

They then randomly assigned the participants to eat one of three types of cereal bars, which looked and tasted the same, every day. One of the bars contained wheat, the second contained gluten but no other components of wheat and the third was a free of both ingredients. The first two bars contained doses of gluten equivalent to four slices of bread, says Bercik.

The participants were told the bars could worsen their symptoms, but weren’t told which ones contained what ingredients.

One week later, the participants ranked the severity of their symptoms, then switched back to a gluten-free diet for two weeks, to reverse any of the bars’ effects. They then repeated the experiment twice, so that each of the participants ate all three varieties of the bars.

After the sham bars, the participants reported experiencing a 50-point worsening in their symptoms – a level of change doctors consider to be significant, says Bercik. Meanwhile, this occurred in 10 of the participants when eating the gluten-only bars and in 11 of them after consuming the wheat ones.

“All three challenges induced symptoms in a similar proportion of patients,” says Bercik.

While gluten and wheat are probably genuine triggers for some people with IBS, the results suggest others are impacted by the nocebo effect – where the expectation symptoms will worsen causes this to actually occur, says Bercik.

In a comment piece accompanying the paper, Sigrid Elsenbruch at the University of Duisburg-Essen in Germany noted the participants were told any of the bars could worsen their symptoms, which may have enhanced the nocebo effect compared to real-world settings.

Stool samples from the participants also revealed a handful of them didn’t eat the bars as instructed. This could mean they were not consuming enough gluten or wheat for these ingredients to particularly affect their IBS.

Bercik says the researchers are exploring pathways through which gluten and wheat may cause IBS symptoms in some people, for instance by altering the gut microbiome.