IBS diet

Which foods may be worth avoiding?

Qualified Nutritionist (BSc, MSc, RNutr)
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An introduction to IBS and common dietary triggers

Dietary factors are thought to play an important part in IBS. People often experience a ‘flare up’ after eating or drinking certain types of food.

Although ‘trigger foods’ can vary from one person to another, some will lead to IBS symptoms more consistently. It may be worth eliminating or incorporating these categories of food, one at a time, to see how your gut responds. This means you can potentially take your first big step in managing your symptoms.


Gluten is a protein found in grains such as wheat, rye and barley. Gluten intolerance or sensitivity is becoming increasingly more common in Western societies.

Sensitivity to gluten should be differentiated from gluten allergies such as coeliac disease. These are more serious conditions, which involve the activation of your immune system and long-term damage to your gut as a consequence of its interaction with gluten. In addition, a host of physical symptoms may also be experienced.

In gluten intolerance, physical symptoms are present but the immune system is unaffected. These symptoms may include: bloating, flatulence, stomach pain, nausea, diarrhoea or constipation.

Gluten or wheat sensitivity (less common) occurs when your body lacks an enzyme required to break down these components. If this doesn’t happen efficiently, undigested wheat or gluten is forced to travel down into your large intestine where it is then attacked by bacteria in a process called fermentation. Excess or abnormal fermentation is more likely to give rise to the undesirable symptoms associated with IBS.

Gluten is found in all products containing wheat, rye and barley and is often hidden in many goods, some of which may surprise you including: soy sauce, processed meats such as sausages and beer! Always read food labels where the allergens can be found in bold.

Gluten intolerance may occur alone or in combination with additional sensitivities. Grains such as wheat, barley and rye, which contain gluten, also fall into the FODMAP group.  Wheat products also contain components called fructans which are thought potentially to exacerbate IBS. Fructans are present in wheat but not in gluten.

Therefore, being sensitive to wheat does not necessarily mean that you are also sensitive to gluten but perhaps the fructans present. Fructans are found in many gluten-free fruit and vegetable sources too which makes it easier to determine which sensitivities you have.  Read more about high FODMAP foods by following the above link.


Lactose intolerance is arguably the most common food intolerance, with prevalence ranging from 5%-90% of people across different countries and communities throughout the world.

Lactose is the natural sugar found in milk and dairy products. In your body you have an enzyme called lactase, which is responsible for breaking lactose down into its simple units: glucose and galactose, which can be readily absorbed in the small intestine.

In lactose intolerance, however, this process doesn’t work so well. People with lactose intolerance produce insufficient lactase, which means lactose isn’t properly broken down. Unless this break down happens, lactose can’t be absorbed and carries on down the digestive system, from the small intestine to the large where it undergoes a process called fermentation. Here bowel bacteria existing naturally in the gut digests lactose, converting it into gas and short-chain fatty acids.

Lactose intolerance may occur alone or in combination with other sensitivities. Lactose is a disaccharide included in the FODMAP framework that, along with other categories of food, it may be useful to cut out in the treatment of IBS. Refer to our FODMAP diet section to learn more.

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Dietary fibre is the part of the plants you eat that can’t be broken down and absorbed into your body. It is normal for some of the food you eat to pass through the digestive system partially or completely undigested, ending up in the large intestine ready to undergo fermentation.

Generally, fibre is recommended as part of a healthy, balanced diet. This is because the natural bacteria living in your gut, called microbiota, feed on dietary fibre to produce beneficial compounds such as short-chain fatty acids, which are then absorbed.

There are two types of fibre we can include in our diets: soluble and insoluble.

  • Soluble fibre – sources include oatmeal, nuts, beans, lentils, apples and blueberries and may be of benefit if diarrhoea is present
  • Insoluble fibre – this can be found in wholegrains and the seeds and skin of many fruit and vegetables. This type of fibre is often helpful for constipation.

However, despite its supposed beneficial properties, some people simply don’t react well to fibre. Responses vary, with some people reacting badly only to fibre that is part of the FODMAP framework (with a specific molecular structural). In rare cases, even FODMAP free sources of fibre act as an irritant to the gut and cause grief in people with IBS.

So, it is important to consider the different types of fibre and to see if the introduction of more of this type of food, or reducing your intake, can have a positive impact on your symptoms.


FODMAPs are classes of specific, simple, short-chain carbohydrates, which are not well broken down in the gut and, as a consequence, not well absorbed. The term is an abbreviation derived from: Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols.

Fermentation is the process whereby bacteria, residing in our large intestine, feed on undigested food that has not been properly absorbed in the small intestine. Many types of food, mainly classes of dietary fibre, undergo fermentation in the body, but specific groups of oligosaccharides, disaccharides, monosaccharides and polyols have been specifically identified as the biggest culprits for producing adverse effects. See our FODMAP page to gain a better understanding of what foods fall into these categories.

Other triggers

As well as the common trigger foods that are most likely to cause chaos in your gut, it is thought that people suffering from IBS have a more sensitive gut, reacting adversely to foods that wouldn’t necessarily affect the average person.

Examples of these foods include:

  • Sugar. Sugar is likely to feed the ‘bad’ strains of bacteria or yeast living in your gut, allowing them to thrive and contribute to the unpleasant symptoms associated with an IBS flare up
  • Alcoholic drinks have a similar effect as gut bacteria are able to feed on the sugars present, so try to avoid as much as possible
  • Acidic foods and drinks such as soft drinks, tomatoes and animal protein and fat, can disrupt the gut environment and adversely affect our digestive enzymes
  • Caffeine. Caffeine triggers your ‘fight or flight’ nervous system, which takes the focus off your digestive system. This means you are more likely to experience symptoms such as diarrhoea as your gut contractions become uncoordinated.

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