What causes irritable bowel syndrome?

We don't know the exact cause of IBS, but there are lots of theories about it. Many think that IBS is caused by problems in the way the nerves in the gut and the brain are talking to each other - gut-brain connection.

Other theories include the gut working too quickly or too slowly, that nerves in the gut are over-sensitive, and an imbalance of normal bacteria in the gut. It's likely too that psychological distress - such as stress, anxiety, and depression - is closely linked to IBS, but the relationship between this and IBS is complicated.

If you think you may have IBS find out what to do here. This will tell you if you need to see a doctor and how it is treated.

In this series of articles centred around irritable bowel syndrome you can read about the symptoms of IBS, IBS treatment and IBS causes- all written by one of our expert GPs.

The rest of this feature will take an in-depth look at the causes of IBS as, at Patient, we know our readers sometimes want to have a deep dive into certain topics.

What causes IBS?

IBS is a complex condition. The term IBS probably includes several different conditions which cause different symptoms - with some causes in common and others that are different.

We don't fully understand what causes IBS, but there has been lots of research into it. There are several different theories. It's likely that more than one cause is involved, and that the exact causes differ from person to person.

The brain-gut connection

The digestive system has to do a lot of things, and these are controlled by a big network of nerves. There are over 100 million nerve cells in the gut, making up the 'enteric nervous system' - sometimes called the 'second brain'. This second brain is constantly talking back and forth with the brain.

The brain can make the gut behave in ways that cause strange or uncomfortable sensations. An example of this is the feeling of butterflies in your stomach. Almost everyone has felt this feeling of discomfort when feeling anxious or nervous. When this happens, the brain - which is experiencing anxiety - is subconsciously sending messages to the second brain in the gut, causing the gut to squeeze harder and work more, which leads to a feeling of discomfort.

We know, too, that the gut's nervous system can affect the brain. For example, studies show that stretching the walls of the rectum - the last part of the bowel, which joins up to the anus - activates parts of the brain that process emotions, such as stress and anxiety, and pain. Studies show that this response is greater in some people with IBS.

IBS is sometimes called a type of 'functional bowel disorder'. In functional conditions, there are no signs of damage to the organs, but there's a problem in the way they are working, leading to distressing symptoms.

If you imagine the gut nervous system as a computer, in IBS, the hardware - our stomach and bowels - look normal when tests are done. However the software or the programme the gut nervous system is running - isn't working properly. This is similar to what happens in the brain and spine in functional neurological disorders.

So, it's likely that the way the gut and the brain are working, and interacting with each other, is an important cause of IBS symptoms.

Most of the following causes are linked to this theory of the brain-gut connection - they might be a cause, or a consequence, of this.

Sensitive nerves in the gut

Some people with IBS seem to have extra-sensitive nerves in their digestive system. These might be more sensitive to the gut stretching - for example, leading to the feeling of bloating or pain. Whereas people without IBS, and without sensitive gut nerves, might not feel anything at all. The medical term for this is visceral hyperalgesia - meaning organs that are extra-sensitive to pain.

Problems with gut movement

The digestive system moves food through it by a complicated process where muscles in the gut squeeze and relax. In some people with IBS, this appears to occur too quickly - causing spasms, cramps and diarrhoea - or too slowly causing constipation.

Imbalances in the gut microbiome

All of us have bacteria, viruses, and fungi living in our digestive tract - the microbiome of the gut. These are important for the function of the digestive system - they protect against infection, help with digestion, and can help control the gut's immune system. Imbalances in the microbiome - such as having too few good bacteria - might lead to symptoms of IBS in some people. The microbiome can be affected by antibiotics, gut infections, and diet. Some people find that probiotics, which are meant to help restore a healthy microbiome, help their symptoms.

Food intolerance and sensitivities

People with IBS often find that certain foods trigger their symptoms. It's rare for people to have a food allergy in IBS - when the immune syndrome reacts against something in the food - and tests for food allergies are usually negative. It is more common for people to have an intolerance, where some foods cause symptoms to flare. Some types of foods become fermented by bacteria in the gut, producing gas, which can lead to bloating, pain, and discomfort. A special diet, called the low FODMAP diet, aims to manage IBS by cutting out these foods.

Psychological trauma, stress, and anxiety

There are close links between IBS and psychological stress, which relates to the brain-gut connection. When the brain is feeling stressed or anxious, it can affect the nervous system in the gut and trigger IBS symptoms. The opposite is also true where signals from the nervous system in the gut can worsen feelings of anxiety, low mood, and stress.

There is evidence that previous psychological trauma may be linked to IBS. For example, a history of sexual abuse is more common in people with IBS. We don't know exactly why this happens - but it probably comes back again to the complicated way in which the brain and gut interact with each other.

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Irritable bowel syndrome

IBS stands for irritable bowel syndrome, and it is a long-term chronic condition of the gut (bowel) that causes episodes of tummy (abdominal) cramps, bloating and either constipation or diarrhoea. IBS is a problem with how the bowel works.

by Dr Rachel Hudson, MRCGP

How common is IBS?

IBS is common and it is estimated that up to one in five people have symptoms of IBS at some point in their lives. Although, as not everyone seeks medical help for it, this number might be even higher.

What causes IBS flare-ups?

People with IBS often find that their symptoms flare up from time to time. The reasons for flares can be different for each person, and identifying these is important, so as to try and avoid them.

Common triggers for flares include:

• Stress and anxiety.

• Some types of foods - such as artificial sugars, spicy or fatty foods, and processed foods.

• Alcohol.

• Caffeine-containing drinks - such as coffee, tea, soft drinks, and energy drinks.

• Infections causing diarrhoea (gastroenteritis).

Risk factors for IBS

IBS is more common in some people. Likely risk factors include:

• A family history of IBS - tends to run in families, probably due to a combination of genetics, and environmental factors (such as diet) that the whole family is exposed to.

• Gender - more common in women.

• Younger age - more common in people under the age of 50.

• Psychological distress and trauma - such as having depression and anxiety, or having been a victim of sexual abuse.

• Gastroenteritis - a common gut infection causing diarrhoea and sometimes vomiting. Some people get 'post-infectious IBS' following this infection.

• Some people are particularly likely to experience body symptoms (such as pain) as a result of psychological distress - called somatisation. People with somatisation are more likely to experience IBS.

How to prevent IBS

The causes of IBS are complex and not fully understood, so we don't know any reliable ways to prevent IBS entirely. For people with IBS, though, identifying triggers for their symptoms, and avoiding them if possible, is an important part of controlling the condition.

Further reading

• Irritable bowel syndrome; NICE CKS, September 2022 (UK access only).

• Camilleri M; Diagnosis and Treatment of Irritable Bowel Syndrome: A Review. JAMA. 2021 Mar 2;325(9):865-877. doi: 10.1001/jama.2020.22532.

• Vasant DH, et al; British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut. 2021 Jul;70(7):1214-1240. doi: 10.1136/gutjnl-2021-324598. Epub 2021 Apr 26.