Why IBS diets do not work for everyone - Medical News Today

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Gut bacteria profiles may influence the effectiveness of IBS diets. Guido Mieth/Getty Images

• Irritable bowel syndrome (IBS) is a gastrointestinal condition. It causes symptoms such as abdominal pain and bloating.
• A common treatment for IBS is a special diet called a low FODMAP diet. However, this does not work for everyone.
• New research suggests that people with IBS who have a specific gut bacterial makeup might respond best to a low FODMAP diet.

IBS is a chronic condition that affects an estimated 12% of people in the United States. People with IBS often experience unpleasant symptoms.

Treatments that influence gut bacteria can sometimes help. These treatments include dietary changes, probiotics, and fecal transfers.

Some people with IBS benefit from a diet called a low FODMAP diet. However, researchers are still trying to understand why this helps.

A recent study in the journal Gut found that people with a specific gut bacterial makeup might respond best to a low FODMAP diet. Testing for this specific gut bacterial makeup could help doctors develop an appropriate treatment plan for someone with IBS.

IBS is a functional gastrointestinal disorder. It is related to the interaction between the gut and the brain.

People with IBS tend to have a digestive tract that is more sensitive, and the muscles in the intestinal tract tend to contract too often. However, this varies among individuals.

Some common symptoms of IBS include:

For some people with IBS, food moves too slowly through the digestive tract. For others, food moves too quickly. Doctors diagnose different types of IBS based on the presence of constipation, diarrhea, or mixed bowel habits.

Experts do not yet know the precise causes of IBS. However, people with certain risk factors are more likely to develop the condition.

For instance, IBS tends to affect more women than men and more people under the age of 50 years than over.

People who have experienced severe infections that affected their digestive tract, such as gastroenteritis, and people who have experienced highly stressful life events also have a higher risk of developing IBS.

A person with IBS might want to change their diet to help control the condition. Doctors sometimes suggest a low FODMAP diet. FODMAP is an acronym that refers to specific types of carbohydrates: fermentable oligo-, di- and monosaccharides, and polyols.

The American College of Gastroenterology explains that the low FODMAP diet is an elimination diet.

People following a low FODMAP diet avoid all high FODMAP foods for 2–4 weeks. Then, they slowly start reintroducing certain groups of FODMAPs. Through this process, people with IBS can learn which foods trigger their symptoms and which foods do not.

People who are following a low FODMAP diet typically avoid foods containing high amounts of fructose and lactose. They also tend to avoid certain fruits and vegetables, artificial sweeteners, and certain starches. For example, they usually avoid wheat-based bread, but they are able to consume corn-based bread.

Researchers are still trying to discover why a low FODMAP diet helps some people with IBS and not others.

Evidence does suggest that IBS is connected to and involves the bacteria in the gut. To investigate, the authors of the new study looked at the gut bacterial makeup of people with IBS and then examined how it changed in response to following a low FODMAP diet.

First, they assessed the bacteria in stool samples from people with IBS while those people consumed their regular diet. They also examined the bacterial makeup of stool samples from a member of the same household to act as a control.

They found two main profiles among the participants with IBS. One group had a "pathogenic-like" profile. The other group had a "health-like" profile.

The pathogenic-like profile included more species that are involved in disease, such as Clostridium difficile, C. sordellii, and C. perfringens. Conversely, this profile exhibited low numbers of beneficial species, such as Bacteroidetes. There was also an abundance of bacterial genes for amino acid and carbohydrate metabolism.

The participants with health-like profiles had similar profiles to the study's control participants.

Each participant followed a low FODMAP diet for 4 weeks. During this time, the team monitored the participants for clinical response and changes to the gut bacteria.

They found that participants with pathogenic-like profiles experienced a more significant reduction in IBS symptoms than those with health-like profiles. They also found that their gut bacterial makeup and metabolic genes shifted toward a health-like profile.

In a companion editorial, Prof. Peter R. Gibson and Dr. Emma Halmos — from the Department of Gastroenterology at Monash University and Alfred Health, both in Melbourne, Australia — write:

"Their study has suggested that initial FODMAP restriction might actually correct dysbiosis in a proportion of [people] with IBS with consequent durable symptomatic benefit without the need for major FODMAP restriction."

They also note that "the structure of fecal microbiota might predict the degree of response to restriction of FODMAPs." In other words, looking at the gut bacterial makeup might help doctors predict how well a person with IBS will respond to following a low FODMAP diet.

The study did have limitations.

First, there was a limited sample size. Second, the researchers only collected dietary information from the participants on the final week they were following the low FODMAP diet. As the study authors explain, "participants could have been tempted to follow a more rigorous diet on the week they had to report their dietary intake."

They also acknowledge that other factors besides diet could have affected the improvement in the participants' symptoms. The study authors encourage similar testing and studies in other populations with different dietary habits to help confirm their findings.

The researchers hope that if their findings are validated, they will significantly benefit people with IBS.

Doctors may be able to look for these specific gut bacteria profiles in people with IBS. Based on the results, they may be able to predict the best course of treatment for someone with IBS. For example, it could help doctors predict whether or not following a low FODMAP diet would be helpful.

Dr. Allen Andy Lee, a gastroenterology specialist at the University of Michigan Health in Ann Arbor, was optimistic about the study's results. He told Medical News Today that this study suggests that we can create further subtypes of IBS to help predict how people with the condition will respond to dietary changes.

"Overall, this is an important study that supports prior studies and significantly enriches our understanding of the microbiota in IBS by suggesting that microbial composition and function may allow for subtyping of [people with IBS], which may serve as a biomarker for predicting clinical response to dietary interventions."

– Dr. Allen Andy Lee

However, he also noted that the study requires follow-up. He told MNT that "this was a small, single-center study and [that] these findings require validation in larger multi-center cohorts with diverse geographical and cultural backgrounds."