The exact cause of irritable bowel syndrome (IBS) is unknown, but most experts think that it's related to problems with digestion and increased sensitivity of the gut.
Many causes have been suggested – including inflammation, infections and certain diets – but none have been proven to directly lead to IBS.
Problems with digestion
Your body usually moves food through your digestive system by squeezing and relaxing the muscles of the intestines in a rhythmic way.
However, in IBS it's thought that this process is altered, resulting in food moving through your digestive system either too quickly or too slowly.
If food moves through your digestive system too quickly it causes diarrhoea, because your digestive system does not have enough time to absorb water from the food.
If food moves through your digestive system too slowly it causes constipation, as too much water is absorbed, making your stools hard and difficult to pass.
It may be that food does not pass through the digestive systems of people with IBS properly because the signals that travel back and forth from the brain to the gut are disrupted in some way.
It has also been suggested that problems such as bile acid malabsorption (where bile produced by the liver builds up in the digestive system) may be responsible for some cases of IBS.
Increased gut sensitivity
Many sensations in the body come from your digestive system. For example, nerves in your digestive system relay signals to your brain to let you know if you are hungry or full, or if you need to go to the toilet.
Some experts think that people with IBS may be oversensitive to the digestive nerve signals. This means mild indigestion that is barely noticeable in most people becomes distressing abdominal (stomach) pain in those with IBS.
Psychological factors
There is also some evidence to suggest that psychological factors play an important role in IBS.
However, this does not mean that IBS is "all in the mind", because symptoms are very real. Intense emotional states such as stress and anxiety can trigger chemical changes that interfere with the normal workings of the digestive system.
This does not just happen in people with IBS. Many people who have never had IBS before can have a sudden change in bowel habits when faced with a stressful situation, such as an important exam or job interview.
Some people with IBS have experienced a traumatic event, usually during their childhood, such as abuse, neglect, a serious childhood illness or bereavement.
It is possible that these types of difficult experiences in your past may make you more sensitive to stress and the symptoms of pain and discomfort.
IBS triggers
Certain foods and drinks can trigger the symptoms of IBS. Triggers vary from person to person, but common ones include:
alcohol
fizzy drinks
chocolate
drinks that contain caffeine – such as tea, coffee or cola
processed snacks – such as crisps and biscuits
fatty or fried food
Keeping a food diary may be a useful way of identifying possible triggers in your diet.
Stress is another common trigger of IBS symptoms. Therefore, finding ways to manage stressful situations is an important part of treating the condition.