Epidemiology is the study of how often medical conditions occur in different groups of people. It characterises the condition in terms of age, sex, lifestyle habits, social and economic behaviours, ethnic origin and association with other diseases, conditions or treatments. This information is intended to give us clues to the cause of the condition and to suggest treatments or preventive strategies.
Can you define the irritable bowel syndrome in one sentence?
In irritable bowel syndrome (IBS), the function of the bowel is disturbed, causing symptoms, but its structure is normal.
How common is irritable bowel syndrome?
The individual symptoms that constitute irritable bowel syndrome – constipation, diarrhoea, bloating and abdominal pain – are so common that most of us can expect to experience them at some time. It’s when they come together, persist and interfere with daily life that we begin to feel that something is wrong. This constellation of bowel symptoms has come to be known as irritable bowel syndrome, having previously been known in different guises as an irritable or spastic colon.
Depending on the criteria used to define irritable bowel syndrome, about 9-23% of the world population are affected. In the UK, about 4-6% of the population have been given a formal diagnosis of irritable bowel syndrome, but surveys suggest that a further 8% have undiagnosed irritable bowel syndrome. In other words, at least 1 in 10 people have irritable bowel syndrome to some extent. It is the most common gastro-enterological problem seen by family doctors even though many people never go to the doctor with their irritable bowel syndrome symptoms, so a formal diagnosis is never made.
If you have had symptoms for over a year without a recent change, without unintended weight loss or blood in the stool, there is no need to see your doctor for a ‘formal’ diagnosis of irritable bowel syndrome. Many people come to terms with their symptoms of irritable bowel syndrome, learn to live with them and lead perfectly normal lives.
LIFE FACTORS
We’re always being told our Western lifestyle is unhealthy. Is irritable bowel syndrome predominantly a disease of Western developed countries?
Irritable bowel syndrome is not just related to modern living or the lifestyle of developed countries. It exists in every society that has been examined. The number of people said to be affected varies from 3.5% in Iran to 30% in Nigeria. These differences probably, at least partly, reflect the criteria used to diagnose irritable bowel syndrome, how the questions about it were posed and cultural differences in answering them. Interestingly, the balance between constipation-predominant and diarrhoea-predominant irritable bowel syndrome also varies. Constipation is said to dominate in Bangladesh and Singapore, whereas diarrhoea is more common in parts of China and India.
Is irritable bowel syndrome more common in women?
Irritable bowel syndrome appears to be at least twice as common in women than in men. In one survey, 86% of people with irritable bowel syndrome attending their family doctor in the UK were women. The reason for this is unknown. In a few countries (India, Sri Lanka and Japan, for example), irritable bowel syndrome isn’t more common in women. This may be because Asian men are more likely than Asian women to tell their doctor about symptoms that could be caused by irritable bowel syndrome. Since the diagnosis of irritable bowel syndrome depends entirely on people describing their symptoms, cultural issues may contribute to the number of men and women being diagnosed. British men are notoriously reluctant to talk about bodily functions, and this may partly explain why more women than men in the UK appear to have irritable bowel syndrome.
I’m sure my irritable bowel syndrome is worse during my period. Is irritable bowel syndrome affected by menstruation?
Many women report that their irritable bowel syndrome symptoms are worse during their periods. Occasionally, they feel their symptoms are worse in the middle of the cycle. It is possible that these changes in severity relate to changes in the levels of the hormones oestrogen and progesterone.
Oestrogen levels are highest near the mid-point of the cycle, and in women without irritable bowel syndrome, the stools tend to be firmer at this time. When oestrogen and progesterone levels fall towards the end of the cycle, the lining of the uterus breaks down and menstruation starts. In women without irritable bowel syndrome, we know that the stools tend to be somewhat looser and more frequent during menstruation. These effects may be exacerbated in women with irritable bowel syndrome, so that diarrhoea is worse during menstruation and constipation is worse between periods.
Interestingly, a recent study has suggested that the rectum becomes more sensitive to distension in women with irritable bowel syndrome during menstruation, which would explain an increase in pain. Alternatively, some women may find menstruation stressful anyway, so that other symptoms can become less tolerable.
I’ve just been put on hormone replacement therapy (HRT). Does HRT affect irritable bowel syndrome?
There is just one study suggesting that women with irritable bowel syndrome have less bloating when they take HRT, but it doesn’t seem to affect other symptoms. irritable bowel syndrome is not a reason to try HRT.
I’ve got irritable bowel syndrome and I’m worried that my son will get it too. Is there a genetic influence?
One way of assessing the heritability of a condition is to look at how often the disease occurs in bothmembers of a pair of twins. This is called the concordance rate. Both twins are assumed to have had the same environmental influences, but identical twins have all their genes in common, whereas non-identical twins have about half their genes in common. Therefore, if genes are important in the aetiology of a disease, that is, in what causes the disease, the concordance rate will be higher in identical twins.
One study of twins has shown a concordance rate of 33% for irritable bowel syndrome in identical twins compared with just 13% for non-identical twins. This suggests that genetic influences are important. Although no specific gene has been identified, a recent study has demonstrated an association between certain forms of the gene coding for a protein that transports a substance called serotonin (which affects the movement of the gut) and diarrhoea-predominant irritable bowel syndrome. Hopefully, as we discover more information on the human genome, we will better understand how different genes interact with the environment to produce illnesses.
How old are people when they develop irritable bowel syndrome?
In a European survey of over 40 000 people published in 2003, symptoms that could be explained by irritable bowel syndrome were found in 12.2% of 18–34-year-olds, 9.9% of 35–54-year-olds and 7% of over 55-year-olds. Similar results were found in an American study. irritable bowel syndrome seems to be most common in young women but exists in all age groups.
How often will the average family doctor see a new case of irritable bowel syndrome?
Large surveys from the UK and USA have estimated that about 2–4 new cases of irritable bowel syndrome occur for each 1000 people and are diagnosed by family doctors every year. As the average family doctor in the UK has about 2000 patients, he or she will see 4–8 new cases of irritable bowel syndrome each year. As there is no cure for irritable bowel syndrome, many patients will see their doctor again and again. It has been estimated that 1 in 12 of family doctor consultations in the UK are to do with gastroenterological problems. About a third of these patients will be asking about symptoms related to irritable bowel syndrome, so that irritable bowel syndrome is probably the most common gastroenterological problem seen by family doctors.
Although family doctors probably refer only about a third of the patients they see with irritable bowel syndrome for a specialist opinion, irritable bowel syndrome may form up to 40% of a hospital gastroenterologist’s outpatient practice.
Is smoking linked to irritable bowel syndrome?
Smoking has not been linked to irritable bowel syndrome. However, it is known to exacerbate Crohn’s disease, even though it may actually help ulcerative colitis.
Is alcohol linked to irritable bowel syndrome?
Excess alcohol consumption tends to make the stools looser and is in some people a cause of diarrhoea. Nausea, vomiting and pain in the upper abdomen are also frequently associated with excess alcohol, or with alcohol withdrawal. Alcohol consumption is not, however, associated with irritable bowel syndrome.
Irritable bowel syndrome and other conditions
What about obesity? It’s always on the TV and in the papers about how obesity is rising. Is irritable bowel syndrome associated with this?
There is no association between irritable bowel syndrome and obesity; obesity does not cause irritable bowel syndrome. Losing weight can cure some cases of heartburn and indigestion, such as from acid reflux from the stomach, but it will not improve irritable bowel syndrome.
Is irritable bowel syndrome more common after a hysterectomy or any other operation?
In a study of over 40 000 European women, hysterectomy (removal of the womb) was not associated with irritable bowel syndrome. There was a slightly higher rate of previous appendectomy (removal of the appendix) in irritable bowel syndrome sufferers. This might be because both conditions cause pain in the right lower corner of the abdomen, so the irritable bowel syndrome in these cases may initially have been mistaken for, for example, appendicitis. The rates of gall bladder, ovarian or bladder surgery were no higher in people with irritable bowel syndrome.
I’d had bad gastroenteritis not long before I got irritable bowel syndrome. Is irritable bowel syndrome more common after a bout of gastroenteritis?
irritable bowel syndrome occurring after gastroenteritis probably accounts for at least 10% of all cases of irritable bowel syndrome.The risk of getting irritable bowel syndrome following gastroen¬teritis has been estimated to be 4–7% (4–7 people in every 100) in the year after the infection. This compares with an average risk in the general population of 0.2% (2 in a 1000) each year.
Is irritable bowel syndrome more common after taking antibiotics?
This is something that some patients notice, but surveys haven’t supported the idea. Antibiotic treatment frequently causes a loosening of the stool or actual diarrhoea because many of the bacteria that normally live in the large bowel are killed. This is not irritable bowel syndrome, and the bowel habit usually returns to normal once the antibiotic treatment has been completed and the bowel’s population of bacteria returns to normal.
Is irritable bowel syndrome associated with indigestion?
In a study of over 40 000 Europeans, indigestion (dyspepsia), including gastro-oesophageal reflux (reflux of acid back from the stomach into the oesophagus) and peptic ulcer disease, was more common in those with irritable bowel syndrome. In clinical practice, between a third and a half of people coming to the clinic with irritable bowel syndrome-type symptoms will also describe symptoms of indigestion. Most of these people will not have ulcers or severe inflammation of their oesophagus (oesophagitis). Rather, they tend to have dysmotility (a lack of co-ordination of the stomach muscles) or mild acid reflux from the stomach into the oesophagus.
The relationship between indigestion and irritable bowel syndrome is not surprising as stomach function can affect bowel function and vice versa. Filling of the stomach promotes muscular activity and emptying of the bowel. This is called the gastrocolic reflex, and it explains why people some¬times need to empty their bowel immediately after or even during a meal. Conversely, constipation slows down the emptying of partly digested food from the stomach, and it is easy to understand how severe constipation can produce symptoms of indigestion, nausea and vomiting.
Will having irritable bowel syndrome mean I’ll get cancer later?
No. irritable bowel syndrome does not predispose to cancer or to any other serious disorder.
THE FUTURE
What is the prognosis for irritable bowel syndrome? And how long does it last?
The symptoms of irritable bowel syndrome usually come and go, so that people can have long-lasting periods when they hardly notice they have it. In one study, fewer than a third of people were free of symptoms after 2 years, and in another, 1 in 20 people were free of symptoms after 5 years. irritable bowel syndrome occurring after a bout of gastroenteritis (post-infectious irritable bowel syndrome) may have a better prognosis – about 40% of people with this appear to recover after 5–6 years.
There is no cure for irritable bowel syndrome, but you can rest assured that nothing more serious will develop from it. The symptoms of irritable bowel syndrome do, however, tend to persist over the years. There is a lot you can do to make sure that they interfere as little as possible with your everyday life.
SUMMARY
irritable bowel syndrome is very common, affecting about 10% of people of both sexes in all age groups and in all ethnicities and cultures.
the usual suspects – smoking, alcohol, obesity and a Western lifestyle – are not to blame.
English 
Spanish