Your gut flora helps you get the most from your food, trains your immune system and is in constant contact with your brain. Here we look at what research has found so far about the role of gut flora in IBS, how it works with the immune system and how you can influence it through what you eat.

Around 38 trillion bacteria from hundreds of different species live in the gut, mainly in the large intestine. Together they weigh about 200 grams and are known as the gut flora or microbiota. Counted by genes, the bacteria have far more than we do.

Most of these bacteria are helpful, but some can make us ill. These are called pathogens. In a balanced gut, the bacteria keep each other in check. If that balance is disturbed, harmful bacteria can grow, and this is called dysbiosis.

What gut bacteria do

  • Produce vitamin K and some B vitamins.
  • Produce butyrate from fibre. Butyrate is an important fuel for the cells lining the gut.
  • Stimulate cells in the gut to make serotonin. About 90 per cent of the body’s serotonin is made in the gut, where it helps control how the bowel moves, among other things.
  • Protect against infection by competing with disease-causing bacteria.
  • Train the immune system and help it protect the body against invaders.

Gut flora trains the immune system

A large share of the body’s immune cells sit in the gut wall, separated from the bacteria by only a thin layer of mucosa. Here the immune system has to act against anything dangerous while leaving food and helpful bacteria alone. It develops this ability together with the gut flora. Animals raised entirely without gut bacteria have fewer immune cells in the gut and a less developed immune system.

Bacteria influence the immune system partly through the substances they make. When they break down fibre, they produce short-chain fatty acids such as butyrate. These strengthen the gut barrier and promote regulatory T cells, immune cells that dampen inflammation. The relationship works in both directions. The immune system also affects which bacteria thrive in the gut, partly through the antibody IgA.

In some people with IBS, this relationship seems to be disturbed. Studies have found more activated immune cells in the gut wall, especially mast cells, and some have linked this to a more sensitive gut and pain. Results vary between studies and do not apply to everyone with IBS. It is also different from IBD, where the immune system causes damage to the gut. This kind of damage is not seen in IBS.

The gut and brain communicate

There is a lot of research into the link between the gut and the brain, known as the gut-brain axis. IBS is not a psychological illness, but the brain plays an important role because it is in constant communication with the gut and its bacteria.

With today’s technology, researchers can extract DNA from a stool sample and see which bacteria are present and in what amounts. This has brought a lot of new knowledge, but so far these tests do not help choose treatment for an individual person.

How gut flora is linked to IBS

Disturbances in the gut flora can contribute to IBS, at least in some people. This is what points to a link:

  • Antibiotics disrupt the gut flora and can increase the risk of IBS.
  • About one in seven people who have had gastroenteritis or another gut infection go on to develop IBS. This is called post-infectious IBS, or PI-IBS.
  • Low-grade inflammation can occur in the gut wall of people with IBS, which could be due to a disturbed relationship between the bacteria and the immune system.
  • SIBO, bacterial overgrowth in the small intestine, can cause IBS-like symptoms. Its link to IBS has not been established.
  • Changing the gut bacteria with antibiotics or probiotics can improve or worsen IBS symptoms.
  • People with IBS have been found to have lower levels of bifidobacteria than healthy people.

In one study, researchers found a gut flora signature of 90 bacteria that separated people with severe IBS from those with milder symptoms and from healthy people. It was marked by low diversity, low methane production and an absence of methane-producing bacteria, among other things. The degree of dysbiosis also seems to affect how people respond to the low FODMAP diet. In another study, those who did not respond to the diet had more dysbiosis from the start.

Bifidobacteria are a good first step

The low FODMAP diet can reduce levels of bifidobacteria, which may already be low in IBS. That is why we usually start there. The probiotic Alflorex contains Bifidobacterium longum 35624, is well studied in IBS and is included in the guidelines from the WGO (World Gastroenterology Organisation). You can add it for 4 to 8 weeks before assessing the effect.

Food changes gut flora quickly

Gut flora responds quickly to diet. In one study, the mix of bacteria changed within about a day when participants switched diets. For a lasting change, the new way of eating needs to continue, so think long term. Increase fibre gradually and choose types your gut tolerates. Fibre gives the bacteria what they need to make short-chain fatty acids, which also matter for the immune system. To increase butyrate production, resistant starch is a good choice, so keep some cooked potatoes in the fridge.

Belly Balance app subscribers can now take our in-depth course on gut flora!

Sofia Antonsson
Registered Dietitian, Belly Balance