The Relationship of Irritable Bowel Syndrome to the Gut Microbiome

Irritable bowel syndrome (IBS) is a disorder that affects the intestines and involves problems with the movement of food that is being digested through the gastrointestinal tract.  Canada has one of the highest rates of IBS in the world with an estimated 18% of citizens being affected, while the global IBS rate is 11%. People with IBS experience belly pain, cramping, bloating, gas and diarrhea or constipation or both.  Sufferers cannot predict when symptoms will flare up.  They report being depressed and embarrassed and face significant burdens on their working life, their participation in social activities, their personal relationships and their ability to sleep. IBS is not life-threatening and does not lead to cancer or other serious illnesses. There are no medical tests for IBS but the symptoms are real. Its underlying cause has been unclear but things are changing in that department.  (1)

Scientists and doctors have been puzzling about how to approach IBS for years.  However, as the study of gut microbes advanced over the past two decades, it became evident that the tiny residents living in our gut may have an important role to play in IBS.  (2)

During the early 2000s it was observed that some people who had experienced a serious gastrointestinal infection had at least doubled their chance of developing IBS.  Their gut microbe community appeared to be modified and researchers began to wonder if the infection had caused a disruption in the life of the microbes in the gut.  Continuing scientific investigations are now revealing that those with IBS indeed have an altered gut microbial community and that this state can also apply to others with IBS who have had no gut infection.  (2)

When the gut microbiome is disrupted, a state known as dysbiosis, the result is a less diverse community of microbes in the gut microbiome.  In other words, there are fewer species of microbes (bacteria, archaea, fungi, viruses, and protozoa) living in the gut. Why is this a problem?  It turns out that a balanced microbiome made up of certain particular species of microbes is crucial for maintaining gut health.  A healthy microbiome consists of several microbial groups including approximately 40% to 60% Firmicutes, 30% to 40% Bacteroidetes and 5% to 10% Actinobacteria along with a low proportion of 5% of Proteobacteria.  This mix of microbe communities works together to maintain microbial balance and healthy intestinal function with effects such as inhibiting the growth of harmful pathogens and supporting a robust gut barrier to prevent the leaking of potentially harmful fragments from inside the intestine out into the bloodstream.  (3)

With our increased understanding of the gut microbiome, IBS is now being defined as a disorder of the brain-gut axis, a complex two-way communication network between the brain and the gut that links the central nervous system to the gastrointestinal tract via the large vagus nerve. This connection influences both the function of the gut and the activity of the brain.  In fact, there is a great deal of overlap between mood and digestive health.  The vagus nerve is the physical connection which carries signals in both directions.  The gut is constantly updating the brain about what is happening in the digestive system and the brain can influence the gut.  This communication is essential for maintaining both a healthy gut and a healthy brain.  Research has illustrated the beneficial effect of a healthy gut on mental health. On the other hand, up to 50% of people with IBS also have major depression and/or generalized anxiety disorder.  When IBS was first recognized, it was thought that it started in the brain.  It is now known that gut symptoms come first.   (4,5,6)

How does the brain-gut connection distribute all this information?  The cells lining the gut produce gut hormones that send signals to the brain concerning things like the regulation of bowel function, appetite and food intake.   More importantly, the microbes living in the gut metabolize fiber to generate short-chain fatty acids and bile acids which are key activators of neurons in the brain-gut axis.  These metabolites enable the transmission of chemical signals from the gut to the brain that are crucial for influencing brain function, its response to stressors like inflammation and oxidative stress, maintenance of the blood-brain barrier, and support of the regular movement of material passing through the intestines that encourages digestion and the absorption of nutrients. (4,7,8)

Additionally, a healthy microbiome can stabilize the immune system.  As the millions of nerves in the gut sense conditions within the intestines, they communicate with the cells of the immune system, the bulk of which conveniently sits right next door just across the thin intestinal gut barrier.  This interplay can influence the balance between pro-inflammatory and anti-inflammatory responses, preventing inflammation that can lead to autoimmune diseases and protecting against infections.  (9,10)

 

Sources of the pain and discomfort of IBS

When the microbiome is in a state of dysbiosis, the function of the intestines is jeopardized.  The rhythmic pattern of the muscles that line the intestines and move its contents along their way can become uncoordinated.  If the movement slows down, build up of gas and stool along with stretching of the intestine causes pain.  If the movement speeds up, urgent and painful bowel movements can be the result.  Additionally, changes in the immune system can cause the release of substances like histamine that increase the sensitivity of the gut nerves and change the motility of the gut.  Stress, anxiety and poor sleep can also affect IBS pain through the brain-gut axis by increasing the awareness of gut signals surrounding altered gut motility and gut pain and intensifying the discomfort.  (11)

 

What Can You Do To Help Yourself If You are Suffering from IBS? 

Medical advice for IBS was once to severely cut down on fiber intake. However, research is continually illustrating that when fiber comes into contact with the gut microbes and they make short-chain fatty acids such as butyrate, acetate and propionate, the nerves lining the gut are calmed.

Fiber is a form of carbohydrate that our bodies cannot easily digest. It remains mostly intact as it moves through the digestive tract, but it has the ability to support gut health by reducing inflammation, maintaining healthy microbes while discouraging unhealthy ones and producing compounds that strengthen the gut lining.  Fiber is found naturally only in plant-based foods such as vegetables, legumes, fruit, whole grains, nuts and seeds.   (12)

There are two types of fiber,

• Soluble fiber that dissolves in water and forms a gel-like texture in the digestive tract. Soluble fiber slows down digestion but promotes the regularity of bowel movements.  Most soluble fiber is fermentable, allowing the beneficial gut bacteria to use it to create their healthy short chain fatty acids.
• Insoluble fiber does not dissolve in water but adds bulk to the stools making them easier to pass through the digestive tract.

Eating fiber must be handled carefully when you are experiencing IBS.  Every case is different and it may take some trial and error to discover what works for you.  In most IBS sufferers, soluble fiber is more easily tolerated, and it may even help with symptoms.  Insoluble fiber can make symptoms worse.

The key to adding fiber to your diet is to start off very slowly.  If you have been eating a typical Western diet, you will have been consuming around 15 grams of fiber every day, an amount much too low for optimal gut health.  This amount may be even lower if you have IBS and are staying away from fiber.  Here are some suggested steps to take to increase your fiber intake, hopefully without increasing your pain.  (18)

 

 

Start off by adding about 3 grams of soluble fiber per day to your diet for a week.

Begin with gentle sources of fiber like cooked foods, soups and blended smoothies, not raw foods.

 

If this is uneventful, increase your soluble fiber by 3 more grams per day for the next week.

 

Be sure to also add more fluids to your daily habits.

Not consuming enough water can worsen the symptoms of IBS even when fiber intake is adequate.

 

If the fiber increases are going okay but you’re encountering minor problems,

keep the fiber level the same for the next  week. 

 

Once you are tolerating the last increase,

go week by week, adding another 3 grams of soluble fiber per day.

If you experience increases in intestinal pain, gas, diarrhea, cramping or bloating,

wait another week to increase your fiber uptake.

 

If you started out eating 15 grams of fiber daily, you could double your fiber intake after 5 weeks.

 

As your gut microbiome strengthens, you will be able to handle more complex sources of fiber.

Start to add insoluble fiber at the same 3 grams per day rate.

 

As time goes on, you can continue to  increase the variety of sources of the fiber you are consuming.

 

 

What does 3 grams of fiber look like?  (13,4)

Most food sources of fiber contain both soluble and insoluble fibers.  Here is a list of those higher in each fiber type.

 

Soluble fibers: 

¼ cup cooked lentils

¼ cup cooked split peas

¼ cup cooked black beans

¼ cup cooked other sorts of beans like navy, kidney or cannellini beans

½ medium baked sweet potato

2 medium cooked carrots

¾ cup boiled sweet corn

½ medium apple with skin on

½ medium pear

1 medium orange

1 medium banana

1/3 of an avocado

½ cup cooked oatmeal

2 tsps chia seeds

 

 

Insoluble fibers:

1/2 cup of raspberries

1 cup of strawberries

1 cup of blueberries

1/3 cup boiled green peas

½ cup boiled broccoli

1 small potato cooked with skin on

1 cup raw chopped cauliflower

2 medium raw carrots

½ cup cooked whole-wheat spaghetti

½ cup cooked quinoa

½ oat bran muffin

1 cup cooked brown rice

1 slice whole wheat or rye bread

1 tbsp flax seeds

1 ounce almonds

1 ounce pistachios

¼ cup sunflower seeds

 

 

Current fiber dietary guidelines suggest 25 grams of fiber per day for women and 38 grams of fiber per day for men. That’s a good beginning target. However, going even higher is better.  Our Paleolithic ancestors ate close to 100 grams every day!  You don’t need to aim that high.  A goal of 50 to 60 grams is doable and enough to gain the benefits of fiber. But you don’t have to worry about eating too much fiber once you’ve acclimatized your system to eating it. (4)

If you are having trouble with increasing your fiber intake, your health care provider may suggest trying out a low FODMAP diet. FODMAPS can trigger gastrointestinal symptoms in sensitive individuals. FODMAPs  encompass a group of fermentable carbohydrates comprising oligosaccharides, disaccharides, monosaccharides and polyols.  This includes such foods as asparagus, beetroot, broccoli, brussels sprouts, cabbage, eggplant, fennel, garlic, leeks, onions, green bell peppers, mushrooms and sweet corn; large amounts of wheat and rye; an excess of fruits like apples, mangos, pears, watermelon, cherries, nectarines, peaches, pears, plums and prunes; lactose-containing foods like milk, soft cheeses and milk-based yogurt; and sweeteners like sorbitol, mannitol, isomalt, maltitol and xylitol. (14,15)

Removing FODMAPs reduces fermentable fibers and lowers the amount of gas produced in the intestine, relieving abdominal pain and bloating.  Taking this step can also help to identify food triggers as these foods are reintroduced.  But, a low FODMAP diet should only be temporary.  Aiming for a future of eating an abundance of fiber-rich foods that supports a thriving community of healthy gut microbes is the way to overall health and vitality.   (4)

 

The PREDICT Study (16,17)

There is another way to look at eating a diet rich in fiber.  New research reveals that increasing your fiber intake until you are incorporating more than 30 different varieties of high fiber foods into your weekly diet is a good goal.  This includes consuming lots of vegetables, legumes, fruit, whole grains, tofu, nuts, seeds, herbs and spices.  The more variety in your fiber sources, the more likely it will be that you are consuming all the necessary types of fiber and receiving all their benefits.

Having a flourishing, varied gut microbiome is important for overall health, not just gut health.  Zoe Health, a pioneering personalized nutrition company focusing on improving health through innovative research and technology, is in the midst of the PREDICT trial, recently publishing their first investigation results regarding the links between one person’s microbiome and their personal health.  Your individual gut contains a complex ecosystem of trillions of different microorganisms.  They play an important role in many aspects of health including the production of thousands of key metabolites, enzymes and vitamins, many of which cannot be produced by humans and are not directly available through diet.  Humans share 99% of the same DNA but the human gut is hugely variable.  It appears that genetics have only a limited influence on the composition of the microbiome.  But you can change your microbiome by what you eat.

The PREDICT study uses deep metagenomic sequencing, together with long-term dietary data and hundreds of blood marker measurements from each PREDICT participant to identify a clear set of microbial species that are strongly and consistently linked to dietary patterns, biomarkers of the heart, blood vessels, the metabolic system and responses to food choices.  Their data also categorizes sets of “good” and “bad” gut bugs that correlate consistently with “healthy” and “unhealthy” foods and dietary patterns. And they have identified specific foods that correlate with individual microbes and thus likely increase the population of these particular microbes. These findings strongly support the interactions between the food we eat, the microbes they support, and chronic disease outcomes, and can predict personal responses to different types of food.

 It is not surprising that the PREDICT study identified fiber as the nutrient that supported the growth of “good” microbes.  Without these microbes, humans are unable to make full use of either soluble or insoluble fibers.  That takes specific microbes that are specialized in breaking down the various kinds of fiber.  Accordingly, a wide variety of fiber types in the diet is needed to support the optimal health of each  gut microbiome.  The naturally occurring fiber found in unprocessed whole foods is highly complex and is linked to better health outcomes.  Highly-processed food sources are not associated with health improvements.

 

Final Message

IBS doesn’t have to be a life sentence.  With the right support and attention to diet, many people find their symptoms become far more manageable and, in some cases, dramatically improved.  Take a look at what you are eating and, with patience, start your journey along the path towards an improved digestive state.  The road might turn out to be long and winding but, if you reach the goal of a stable digestive system without pain, it will all be well worth your while.

 

SOURCES:

1  https://cdhf.ca/en/digestive-conditions/irritable-bowel-syndrome-ibs/

2  https://cdhf.ca/en/ibs-whats-dysbiosis-got-to-do-with-it/

3  Li, X., Yuan, Q., Huang, H., Wang, L. Gut microbiota in irritable bowel syndrome: a narrative review of mechanisms and microbiome-based therapies. 2025.  Front. Immunol. 16:1695321. Doi: 10.3389/fimmu.2025.1695321.

4  Dr. Will John Bulsiewicz, an American triple board-certified gastroenterologist, author, and advocate for the exploration of the relationship between the gut microbiome, dietary fiber, and plant-based nutrition. He is also an Adjunct Assistant Professor of Medicine at the Emory University School of Medicine.         https://zoe.com/learn/brain-gut-mystery-ibs-dr-will-bulsiewicz

5 Appleton, J. The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integr Med (Encinitas). 2018 Aug;17(4):28-32. PMID: 31043907; PMCID: PMC6469458.

6    https://www.health.harvard.edu/digestive-health/how-the-gut-brain-connection-influences-mood

7  https://www.healthline.com/nutrition/gut-brain-connection#neurotransmitters

8  Sun, L.J., Li, J.N., Nie, Y.Z. Gut hormones in microbiota-gut-brain cross-talk. Chin Med J (Engl). 2020 Apr 5;133(7):826-833. Doi: 10.1097/CM9.0000000000000706. PMID: 32132364; PMCID: PMC7147657.

9  Réka, F., Zsófia, B., Ádám, B, Péter F. The Gut–Muscle–Immune Axis in Motion: Mechanistic Synergies of SCFA Metabolism, Exercise, and Microbial Cross-Feeding. Nutrients. 2025; 17(23):3786. https://doi.org/10.3390/nu17233786.

10  Wiertsema, S.P., van Bergenhenegouwen, J., Garssen, J., Knippels, L.M.J. The Interplay between the Gut Microbiome and the Immune System in the Context of Infectious Diseases throughout Life and the Role of Nutrition in Optimizing Treatment Strategies. Nutrients. 2021 Mar 9;13(3):886. Doi: 10.3390/nu13030886. PMID: 33803407; PMCID: PMC8001875.

11  https://cdhf.ca/en/understanding-ibs-abdominal-pain-cramping/

12  https://www.eatingwell.com/fiber-and-ibs-11861089

13  https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/high-fiber-foods/art-20050948

14  https://www.gastroconsa.com/wp-content/uploads/2019/09/Low-FODMAP-Diet-FODMAP-Foods-Updated.pdf

15  https://www.niddk.nih.gov/health-information/digestive-diseases/irritable-bowel-syndrome/eating-diet-nutrition

16  https://zoe.com/learn/how-to-improve-gut-health

17  https://zoe.com/whitepapers/gut-microbiome

18  https://www.todaysdietitian.com/fiber-and-irritable-bowel-syndrome-strategies-for-counseling-patients/