If you’re searching for the causes of IBS symptoms, you’ve probably Googled a few hundred pages!  This blog post suggests five key reasons for someone developing IBS. 

What is IBS?

Irritable Bowel Syndrome (IBS) is a disorder of the gut-brain connection. It’s a condition made up of a collection of digestive symptoms such as bloating, abdominal pain, gas, constipation or diarrhoea.

Each person with IBS will have a different experience and triggers, partly down to different actual root causes.

This means it can be hard to define a treatment approach based on a diagnosis of IBS. What works for some people won’t have any benefit for others. 

Searching for answers online you might have found a range of suggestions from ‘the cause is unknown’ to ‘high FODMAP foods’ with a few suggestions of stress along the way.

Most of my clients are well researched in all the possibilities, but it’s hard to objectively filter information from so many sources, particularly if you are unsure whether to trust the internet ‘experts’.

Here are my top five causes of IBS.


1. Post infectious IBS

Did your IBS start after an episode of food poisoning? That one dodgy seafood meal, or slightly off salad on holiday might be your cause of IBS.

After a bout of gastroenteritis or any stomach infection you are up to six times more likely to get IBS. This risk may still persevere even after 2-3 years.

Post infectious IBS is one of the common causes of IBS, thought to cause of 5-32% of IBS

How does an infection cause IBS?

When we eat that dodgy kebab or dirty water we take pathogenic organisms such as Campylobacter, Shigella, Salmonella and Escherichia coli (E. coli), into our body. These organisms affects the whole digestion by:

  • disrupting intestinal barrier function
  • altering neuromuscular function
  • triggering chronic inflammation

These can cause IBS symptoms, and drive ongoing issues after the infection is cleared.

When we have an infection our body draws water into the bowel to create diarrhoea, and we also produce an antibody called Cytolethal Distending Toxin (Cdt-B).  

Our motility (the way food passes through the gut) can be affected by this toxin, and our immune response to the toxin. People with IBS-D have been shown to have higher levels of anti-cdt-B antibodies, showing a potential test for IBS in the future. 




2. Motility issues can cause IBS

Motility is the speed with which food travels through the intestines. The problems in IBS can be down to your smooth muscles moving the gut either too fast or too slow.

Slow motility

Slow movement of food through the digestive tract can cause an overgrowth of bacteria, which can increase constipation. These microbes often produce methane gas, which slows down the gut transit even more. 

A sluggish digestion, common in constipation predominant IBS, can be caused by things like:

  • not enough food (are you eating enough)
  • insufficient fibre (fibre provides bulk which triggers an urge for a bowel movement)
  • structural issues (e.g. strictures, growths, adhesions)
  • smooth muscle or nerve impairment that weakens digestion
  • slowing down digestion before your period (see Periods and IBS).

Slow transit time results in more water being absorbed from the stool, which leaves dry, hard to pass poos. This can also create more gas due to additional fermentation by large intestine gut microbes. 

Fast motility

Food passing through your gut too quickly could be caused by:

  • High anxiety or chronic stress – your body is evacuating food in order to deal with a ‘fight or flight’ situation
  • Food intolerance – when some sugars aren’t well digested it can cause water to be drawn into the bowel creating diarrhoea (see 4 below)
  • Gastric dumping – when food is passed out of the stomach too quickly. This causes bloating, feelings of fullness, pain and excessive gas. 
  • High caffeine or alcohol intake
  • Hormone changes as your period starts – see Periods and IBS

Addressing the speed at which food travels through your digestion is really key to overall gut health. This may be done with diet change, supplements, or changing when and how you eat. 

3. Small Intestinal Bacterial Overgrowth – common cause of IBS

We can get increased growth of microbes in the small intestine which causes symptoms of IBS. Although SIBO is a specific condition, and separate from IBS, it might be the cause of your irritable bowel. 

The microbes can overgrow in the small intestine due to poor diet, blockages in the gut, slow transit time (see above), medication (e.g. antibiotics), or impaired digestion (e.g. lack of stomach acid or digestive enzymes). Read more about SIBO here.

 The cross over of SIBO in people with IBS is thought to be between 16-70%, so this is worth considering as a cause of your symptoms. 

4. Maldigestion of food causing IBS

If you struggle to eat fruit, artificial sweeteners, or bread for example you may have an issue absorbing certain carbohydrates.

When these foods aren’t broken down properly in the small intestine they travel to the large intestine and can cause bloating, gas and cramps. 

These foods may be an issue for you:

  • Fructose – high in fruits, honey and high fructose corn syrup
  • Lactose – dairy foods
  • Galacto-oligosaccharides (including fructans) – wheat, onions, garlic, beans & pulses
  • Polyols – mushrooms, cauliflower, peaches, blackberries, avocado, xylitol, sorbitol

A low FODMAP diet can improve symptoms for some people who react to these foods, or identifying triggers through diet tracking can also help.

Eliminating certain foods for a set period can help you identify if these kinds of foods are causes of your IBS. You can also read about the difference between carbohydrate malabsorption and SIBO in this blog post. 

5. Gut – Brain connection

The way we think and how we feel can hugely impact on our digestion. There is a large connection between our brain, our mood and our ability to break down food. In high stress situations, when the body is in ‘fight or flight mode’ we do not prioritise digestive processes.

This can lead to less blood flow to the gut, resulting in constipation, bloating, fermentation, diarrhoea or gas. 

You might consider whether these affect you:

  • Stress (e.g. parenting, work stress, relationships) and the knock on impact on your sleep
  • Adverse childhood experiences may also affect IBS. People who experienced trauma (e.g. abuse, accidents, domestic violence) were twice as likely to have IBS as those who didn’t. 

Many of my clients find managing their stress levels makes a huge difference on symptoms. Learning techniques such as deep, diaphragmatic breathing can help. 


The causes of IBS are very varied, and may overlap. The best thing to do is work with an IBS specialist who can guide you through identifying what might help you get better digestion. 

If you need help identifying your IBS triggers and what to eat please get in touch with me.

Read about my Gut Reset programme, and you can see how I work, then make an appointment for a free call if you’d like to meet me and talk about what you need help with. 

Hi I'm Anna Mapson, registered Nutritional Therapist.

I help people with IBS and SIBO get control of unpredictable gut symptoms to find long term relief from painful and embarrassing IBS without restrictive dieting.

I can help you to:

  • understand your digestion better, so you recognise your triggers
  • eat a well balanced diet, with tasty meals that are simple to prepare
  • develop better digestion and more energy

Find more about my 3 month 1:1 Gut Reset programme

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