A study tested the opposite of what is typically recommended for irritable bowel syndrome

There’s plenty of official advice available for people with irritable bowel syndrome (IBS). One of them is to eat regularly and avoid long periods without food.

But in one Norwegian professional environment, the opposite idea emerged: Maybe the symptoms can be relieved by eating less frequently?

“We were a small group of professionals who came together as a result of both a growing interest in intermittent fasting in the field and many anecdotes about how people who fasted experienced fewer digestive problems,” says Marit Kolby.

She is a university lecturer and researcher at Oslo New University College.

“But there was no research on this,” she says.

Showed improvement in most participants

In 2021, Kolby and her colleagues Asgeir Brevik and Marianne Molin began planning a study of time-restricted eating for IBS.

This is a type of intermittent fasting where you only eat for a limited period of the day – typically 8 hours – while you fast for the rest of the day, i.e. 16 hours.

However, it turned out to be difficult to obtain funding for such a study.

“One of the reasons for rejection was that this had not been researched before,” says Asgeir Brevik, an associate professor at OsloMet.

The researchers, however, believed that the lack of knowledge was actually a very good reason to investigate the idea. So they decided to carry out what they could without funding.

That ended up being a pilot study – a small study to pave the way for more research.

So master's students Maria Thompson Clausen and Henrik Sverdrup from the University of Oslo conducted a trial with 97 patients, under the supervision of the three researchers. The Norwegian Gastrointestinal Association helped recruit patients.

The results showed that most participants experienced significant improvements in symptoms.

"Could be relevant"

Gülen Arslan Lied is a professor and head of the University of Bergen's Centre for Nutrition, as well as a senior consultant and gastroenterologist at Haukeland University Hospital. She also researches IBS but was not involved in this study.

“You can't say much from a pilot study. Nevertheless, this may suggest that meal timing itself – regardless of specific food restrictions – could be relevant for the symptom burden in IBS,” she says.

“The findings may challenge the one-sided focus on what you eat and open up for the idea that when you eat may also matter,” she adds.

Lied emphasises, however, that the results must be interpreted with great caution.

"Do not eat full meals"

Professor Bodil Ohlsson at Lund University in Sweden also studies IBS and has published studies showing that IBS patients improved on a low-carb diet.

She believes eating patterns may play a role.

“In dietary studies on IBS, we have seen that many patients eat small meals all the time,” she writes in en email. “They eat irregularly and do not eat full meals; they just snack. So I can imagine that when the stomach gets some rest at times, they feel better.” 

However, Ohlsson points out that more research is needed before anything can be said for certain.

“The weakness of this study is that it only followed patients with time-restricted eating,” she writes. “There will always be a placebo effect when you follow patients and give them attention. A new study with a control group that does not practice time-restricted eating is needed.”

No food restrictions

The participants in the study were individuals who reported having been diagnosed with IBS by a doctor. They practiced time-restricted eating for eight weeks.

Participants could choose whether their eight-hour eating window should start early or late in the day. It was also possible to adjust the timing somewhat – for example, if they had a late dinner – as long as the fasting period lasted at least 16 hours.

Otherwise, there were no restrictions on what or how much the participants ate. They were encouraged to eat until full at meals and maintain the same weight.

The participants kept in contact with the researchers and with each other in a closed Facebook group.

Both before and after the eight weeks, the participants answered an IBS-SSS questionnaire, which is designed to assess IBS symptoms.

The results showed substantial improvement in many participants.

Made a difference

“We were very excited before conducting the first analyses,” says Clausen.

It then became clear that the IBS-SSS score decreased by an average of 100 points from the beginning to the end of the experiment. This is double what is considered clinically relevant – meaning an effect large enough to make a difference for the patient.

But the results were not the same for everyone.

“We saw the best results in those who had IBS with constipation,” says Clausen.

There were also significant differences between participants.

Many got better – some got worse

“We also looked at what happened to each individual participant. That was very interesting,” Clausen says.

This overview showed that most people improved after fasting. By the end of the study, there were clearly fewer people with severe IBS. Some participants had improved a great deal. 12 participants no longer had IBS symptoms.

On the other hand, a few patients became worse. A couple became much worse. Based on the data in the study, it's not possible to say why. For example, the researchers do not know whether some of the participants followed the regimen more closely than others.

Kolby believes that the variation suggests a need for individualised treatment.

Large variations among patients

Lied from the University of Bergen also believes that patients need individually tailored IBS treatment.

“We know from IBS in general that there are large variations among patients,” she says.

IBS can, for example, have different underlying causes. This and many other factors can have an impact on the effect of time-restricted eating, Lied believes.

“Differences in circadian rhythm, stress response, timing of the eating window, food intake, and psychosocial factors may play a role. And, not least, for some people fasting itself can be a stress factor that worsens symptoms," she says.

Several possible mechanisms

The pilot study cannot tell us anything about why time-restricted eating may alleviate symptoms in people with IBS.

However, Kolby and her colleagues still have some ideas about about which biological mechanisms may be involved.

It is, for example, known that fasting stimulates the so-called migrating motor complex – muscle movements that help clear the small intestine of bacteria and food residues. Fasting is also associated with less inflammation in the  body.

Many hours without food may also change the composition of intestinal bacteria. It can also cause the body to switch to using fat as an energy source and become more efficient at removing cells that are not functioning properly.

"Larger studies are needed"

“Several of these mechanisms overlap with known pathophysiological features of IBS, but the study did not measure any biological markers or mechanisms,” says Lied.

“We still lack scientific evidence. Time-restricted eating should therefore not be seen as a replacement for existing treatment. Larger randomised, placebo-controlled studies are needed to determine whether it was really the fasting that led to the improvement,” she adds.

Ohlsson from Lund University believes we should work towards helping patients develop regular eating habits with full meals.

“You can start there. Then fasting can be evaluated further through more studies. Since there is a connection between eating disorders and IBS, it's important not to introduce measures that could trigger eating disorders,” she writes.

Significant symptoms

Kolby hopes the results from the pilot will make it easier to get funding for larger studies.

“A recent Norwegian study showed that many IBS sufferers have significant symptoms and do not feel that they are receiving enough help from the healthcare system,” she says.

Time-restricted eating is free, quite easy to implement, and places no restrictions on what food you can eat. If it proves effective, this eating pattern could become a new tool in the IBS treatment toolbox.

Too little research funding

Previous studies have also shown that both low-carb diets and the Mediterranean diet can relieve IBS symptoms. There is also some evidence that the condition is linked to ultra-processed foods.

In 2022, Kolby published a book about fasting and whole food-based diets.

However, she says that there are no financial interests behind her desire to study how eating patterns can affect IBS.

Kolby explains that the research group has been expanded, with Jørgen Valeur and Hanna Fjeldheim Dale from Lovisenberg Diaconal Hospital. They are currently applying for funding to test the effect of a combination of a whole food-based diet and time-restricted eating on IBS.

Perhaps the results from the pilot study could help in the competition for funding.

Marianne Molin, professor at Kristiania University College, believes there is far too little research funding available in this field.

“It’s a problem that there’s not more public funding to investigate a health issue that affects so many people,” she says.

References:

Clausen et el. Time-Restricted Eating and Symptom Severity in Irritable Bowel Syndrome: Results from a Pilot Study, Nutrients, 2026. DOI: 10.3390/nu18050765

Dale et al. Ultra-Processed Food Consumption and Irritable Bowel Syndrome: Current Evidence and Clinical Implications, Nutrients, 2025. DOI: 10.3390/nu17223567

El-Salhy et al. Quality of life, functional impairment and healthcare experiences of patients with irritable bowel syndrome in Norway: an online survey, BMC Gastroenterology, 2025. DOI: 10.1186/s12876-025-03685-6

Nilholm et al. Assessment of a 4-Week Starch- and Sucrose-Reduced Diet and Its Effects on Gastrointestinal Symptoms and Inflammatory Parameters among Patients with Irritable Bowel Syndrome, Nutrients, 2021. DOI: 10.3390/nu13020416

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Translated by Nancy Bazilchuk

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Read the Norwegian version of this article on forskning.no