Causes of Irritable Bowel Syndrome


What triggers irritable bowel syndrome?

The gastrointestinal tract is designed to digest food and propel the unabsorbed waste products to the end of the intestines for excretion. It does this by coordinated contraction and relaxation of the muscles in the bowel wall.

Although we do not completely understand the cause of irritable bowel syndrome, one factor is disordered contractions of these bowel muscles. It is because the abnormalities involve bowel function, rather than any structural damage or abnormality, that irritable bowel syndrome is often described as a functional disorder.

Why should some people develop irritable bowel syndrome, whereas others do not? We do not know all the answers to that question, although it some factors have been identified that are associated with an increased likelihood that an individual will have IBS. The main factors are:

  • Psychological factors
  • Abnormal activity of the bowel muscles and nerves
  • Increased sensitivity of the gut
  • Gastrointestinal infections
  • Diet, food intolerance and food allergy.

Who consults a doctor?

By no means everyone with symptoms consistent with irritable bowel syndrome consults their doctor; the proportion ranges from 10 to 50%, and is influenced by age and sex. Some people find that their symptoms are troublesome whereas others pay them little attention.

Studies that have looked at the reasons why people go to the doctor have found both physical and psychological differences between those who complain of the symptoms and those who do not. As you might expect, people with more symptoms and more severe pain are more likely to complain, as are those with psychological symptoms such as anxiety and depression.

How the bowel muscles work 

When the muscles in the bowel wall contract, they move the contents along. When short sections contract and then relax, the contents move back and forth. If the contractions follow each other in a wave along the length of the bowel, the contents are moved towards the rectum. The difference is not in the strength of the contractions but whether it keeps moving in the same direction, towards the rectum. 

Psychological factors

People with irritable bowel syndrome is who do not consult a doctor are no more likely or less likely to experience psychological symptoms than those who don’t have the condition. Around 18 to 15% of people who consult their GP or family doctor about their IBS symptoms have psychological symptoms, which is only a slightly higher percentage than among people without irritable bowel syndrome.

However, psychological symptoms are much more common in people who are referred to a gastroenterology clinic. They also seem to be more common in people with irritable bowel syndrome attending hospital than in comparable group of people with an inflammatory bowel disease (IBD) such as Crohn’s disease or ulcerative colitis.

Several studies have also found a link between the onset of irritable bowel symptoms and a preceding stressful event such as employment difficulties, bereavement, marital stress or an operation. Some studies have also found links between the development of IBS symptoms and social problems relating to work, housing, finances or personal relationships.

These findings suggest that an individual’s mood and emotions influence the way that they respond to their symptoms (for example, whether they consult a doctor), as well as having a direct effect on the intestines. Stress has also been shown to play an important role in causing intestine pain.

Nevertheless, despite these findings, many people with irritable bowel syndrome do not have any obvious psychological or personality problems.

Depression and irritable bowel syndrome

Around 10 to 15% of people with irritable bowel syndrome who are referred to gastroenterology clinics are found to have a serious depressive illness and a minority of these may even be suicidal. This is why your doctor will want to ask you about symptoms that might suggest that you are depressed – such as sleep disturbance, low mood and changes in energy. However, the detection of depression is sometimes more difficult and, if your doctor is concerned, you may be referred for a full psychiatric assessment. If depression is present, recognised and treated, pain will often disappear, even if severe.

Influence of mood on the gut

When you are depressed, the passage of waste matter throughout your whole gut is likely to be delayed, possibly causing constipation. In contrast, anxiety is associated with accelerated passage of the digestive contents through the small bowel, leading to diarrhoea.

Most people have, at some time or other, experienced cramps and diarrhoea caused by major anxiety. Acute stress also accelerates the passage of bowel contents through the small intestines and speeds up the working of the whole colon, so you have to open your bowels more often, whether or not you have IBS.

Research has shown that pain may be made less distressing by technique risks such as relaxation and hypnosis. On the other hand, hyperventilation (rapid breathing that occurs during anxiety and panic attacks) has been shown to lower the pain threshold, so that any pain becomes more troublesome.

Abnormal activity of the bowel muscles and the nerves

The workings of the bowels are controlled by several different parts of the nervous system. Changes in the activity of the nerves that supply the gut have been found in people with irritable bowel syndrome. Abnormal action in one part of the nerve supply (the vagus nerve) is linked to constipation, whereas in another part (known as the sympathetic nervous system) it is linked to diarrhoea. Psychological factors can affect these nerves and so can alter the speed at which intestinal contents pass through the bowels.

People who are constipated, including those with irritable bowel syndrome for whom this symptom predominates, seem to have weaker and fewer contraction waves in their colonic muscle. However, there is another group who have increase contractions in the last part of the colon.

Studies have generally shown fast passage of bowel contents in diarrhoea predominant IBS and slow transit time in constipation predominant IBS. Some people with irritable bowel syndrome may also experience abnormalities in small-bowel contraction.

Abnormal bursts of nerve and muscle activity in the colon have been linked to episodes of pain in some individuals. In people with functional abdominal pain, the normal stresses of everyday life may provoke unusual muscle and nerve reactions in the stomach and small intestines.

Despite these observations, the relationship between nerve muscle disturbances in the gut and abdominal pain is not clear cut. Nor is it known whether such disturbances are the result of an abnormality in gut muscle or nerve activity or whether they are triggered by some other abnormal stimulus.

Gastrocolonic response

When we eat, our food stimulates an increase in colonic nerve and muscle activity, which is called the gastrocolonic response or reflex. This effect is one of the main reasons why a baby tends to fill his or her nappy after a meal. This reflex is mainly stimulated by the fat content of food which explains why people with irritable bowel syndrome can experience pain after eating, especially after fatty meals.

Increased sensitivity of the gut

Studies have been made using balloons inflated in parts of patients’ guts to study the pain responses of people with IBS. These have shown that such people are more sensitive than others to distension (or stretching).

In people with irritable bowel syndrome, this abnormal sensitivity has been found in all parts of the gastrointestinal tract (oesophagus and small and large intestines). It was also found that trigger areas for the production of pain may occur in the upper, middle and lower gut in the same person.

Pain may be experienced anywhere in the abdomen. It may also be referred to various parts of the body away from the abdomen, such as the back, thigh and arms.

People with functional abdominal pain resulting from IBS have increased sensitivity to the pain caused by the gut being distended with gas, yet their reactions to pain stimuli in other parts of the body are unaffected. They may describe gut stimuli as unpleasantly painful at lower levels of intensity than people who do not have irritable bowel syndrome. However their pain thresholds when subjected to extreme cold or electrical stimulation of the skin may be normal or even increased.

No one knows why this should be the case, but the explanation probably originates in the brain and the way that different types of painful stimuli are perceived.

Gastrointestinal infections

Sometimes, symptoms of IBS can come on after an acute episode of vomiting and diarrhoea. Persistent problems with bowel function (bowel dysfunction) affect around one in four people after food poisoning caused by bacteria such as Campylobacter, Shigella and Salmonella species.

Factors that make the system symptoms more likely include a more severe acute illness. Examples are:

  • Diarrhoea lasting more than seven days
  • Vomiting leading to weight loss
  • Severe abdominal pain and mucus in the stools.

Other factors are higher anxiety levels and a higher number of stressful events in the six months before the illness.

These types of infection are responsible for long-term symptoms in up to 25% of people with IBS. Such people have a good outlook (prognosis) in that their symptoms often improve and disappear within a year or so.

Diet, food intolerance and food allergy

Eating, especially fatty food, triggers functional abdominal pain in around three out of four people with irritable bowel syndrome. It is important to distinguish this generalised intolerance to food from intolerance to specific foods, which may produce symptoms in certain individuals.

The role of true (specific) intolerance as a cause of the irritable bowel syndrome is debatable. True food intolerance is an adverse reaction in the intestines to a particular food and will occur each and every time a person eats that particular food. One example of this is the excess gas and diarrhoea produced as a result of lactose intolerance (inability to digest a particular type of sugar - lactose - in milk).

Food allergy by contrast, brings on immediate symptoms whenever the individual eats a trigger food, such as prawns, strawberries or oysters. These allergic symptoms may involve the digestive system (such as vomiting), but they often affect other parts of the body, causing a rash, an attack of asthma or a running nose.

Food intolerance

Studies were carried out that tested people’s response to individual foods by excluding them from the people’s diets and then reintroducing them one at a time. These studies found specific food intolerance in between a third and two thirds of people with IBS. The most common intolerance reported in the United Kingdom is to wheat, followed by dairy products (especially cheese, yoghurt and milk), coffee, potato, corn, onions, beef, oats and white wine.

Some people develop typical irritable bowel symptoms such as bloating, cramps and diarrhoea after eating carbohydrates that they are unable to absorb. Examples are lactose (milk sugar) and fructose (fruit sugar). If these are not absorbed, they may ferment in the gut and produce gas. Excluding these from the diet can reduce symptoms and also reduce colonic gas production.

This suggests that changing what you eat can affect the fermentation resulting from the action of the bacteria in the colon.

Reduced production of lactase – an enzyme that breaks down lactose – in the lining of the small intestines can develop in adults and is relatively common in the UK. It is estimated to affect 10% of those of northern European descent, rising to 60% in people of Asian origin and 90% of people of Chinese descent.

People taking a substantial amount of lactose (equivalent to more than half a pint of milk per day) can expect to benefit from lactose restriction. On the other hand, those with low lactose intakes may not, because a low intake does not usually cause symptoms of lactose intolerance.

Elimination diets

An initial study using elimination diets (that is, diets that exclude all but a single type of fruit, a single type of meat, a single vegetable, and so on) improved symptoms in two thirds of those who completed the study.

More practical elimination diets, which impose less drastic restrictions on what you can eat, have been developed. These exclude only foods that are commonly implicated in food tolerance. These diets have a lower success rate (around 50%) but are easier to follow.

Whether diets for food intolerance are really worthwhile is hard to assess. This is because of the placebo response (in which you feel better just because you are expecting to). Such an effect cannot be ruled out unless foods are given in such a way that neither you nor the researcher knows what you have just eaten when your response is assessed.

Even if you would be given nothing but blended foods through a tube passing from your nose into your stomach, it is still impossible to assess any particular influences. Examples of such influences are the role played by important social, psychological and physical aspects of eating. These are likely to be at least significant as the direct effects of individual foods on the gut. Interestingly, however, a study in which people were fed in this way with suspect food reported that 6 of 25 people with irritable bowel syndrome correctly recognised that they have been given one of the foods that seemed to trigger the intolerance.

Specific food intolerance does appear to be the cause of symptoms in a small number of people with IBS. If your doctor suspects that this may be so in your case you should ideally be referred to a specialist centre for objective, scientifically controlled tests. An enthusiastic, determined approach is required by everyone involved – you, the doctors and dieticians – because these studies need to be conducted for a number of weeks or months.

Food allergy

True food allergy is much less common than food intolerance and is usually not difficult to recognise. This is especially so when eating a particular food (or foods) is associated with a rash, asthma or a running nose.

Such allergies often give a high incidence (70%) of positive results to allergy tests such as skin prick and blood tests. If you have this type of allergy, you are more likely to see a specialist in immunology rather than a gastroenterologist because your doctor is unlikely to think that you have IBS.

One study tested people with purely intestinal symptoms. It found that only 15 out of 88 people who believed that they had a food allergy had this confirmed in a double-masked trial (a trial in which neither they nor the test you what food they were eating). Skin prick tests are more likely to be positive if your symptoms come on immediately after eating the suspect food than if they develop only some hours later.

Women and irritable bowel syndrome

Although men and women in general are equally likely to develop IBS, studies have shown that women tend to consult the doctor more often than men. Anxiety, depression and stress are known to occur more often in women, and this possibly may play a part in triggering symptoms. It is also possible that hormonal differences may contribute to the differences between the sexes. During menstruation, for examples, IBS symptoms of abdominal pain, diarrhoea and gas tend to become worse in 50% of women.

Women with IBS are also more likely than men to show increased sensitivity of the gut and three times more likely than men to develop irritable bowel syndrome after a gastrointestinal infection.

In 60% of women with irritable bowel syndrome, pain may also sometimes be felt deep in the pelvis following intercourse. It can come on several hours after intercourse, particularly when the woman has constipation.

Key points

  • There are a number of factors, both physiological and psychological that may make an individual more susceptible to IBS.
  • Many people who develop irritable bowel syndrome symptoms have had stressful experiences, such as work difficulties, bereavement, marital problems and operations, in the preceding months
  • People who develop IBS after a gastrointestinal infection tend to have a better prognosis
  • Three quarters of people with IBS experienced abdominal pain after eating, and a small proportion of them are tolerant to specific foods
  • Women are more prone to irritable bowel syndrome than men.