CHAPTER 4
How Common Is IBS?
Irritable bowel syndrome is one of the most common medical conditions encountered by health care providers of all types—nurse practitioners, physician assistants, internists, and physician specialists like gastroenterologists, obstetricians, gynecologists, surgeons, and psychiatrists. It affects more than forty million adult Americans. Each week, 12 percent of all patient visits to a family practitioner or internal medicine physician are for symptoms related to IBS. During the course of one week, more people see physicians for IBS than for other common medical conditions such as asthma, diabetes, or heart disease. In addition, at least 40 percent of the people referred to gastroenterology specialists have IBS. Thus, contrary to popular perception, IBS is a common disorder that occurs not only in Western societies, such as North America and Europe, but throughout the world (see Figure 4.1).
Hank’s story illustrates some of the misconceptions about IBS and the people who have it.
Hank is a 47-year-old truck driver from Oklahoma. He spends most of his time on the road hauling cattle and farm supplies. During the last several years he’s had frequent problems with constipation and abdominal pain. On many days, he notices a persistent ache or discomfort in his lower abdomen. His abdominal pain is relieved temporarily after he has a bowel movement, but his trips to the bathroom are few and far between. If he’s lucky, he has a bowel movement twice a week. When he does go to the bathroom, he has to strain a lot, and he passes rocky, hard stools. Hank’s friends told him that he is “too uptight” and that he should drink some prune juice and eat more fruits and vegetables. Unfortunately, Hank has difficulty eating fruits and vegetables because his meals are mostly at fast food restaurants and truck stops while he is on the road.
Hank went to his local pharmacy and the pharmacist told him to take some fiber pills, which didn’t seem to help (although he only tried them for a week). His wife told him that he probably had irritable bowel syndrome (she had read about it in a magazine), but he laughed and said that irritable bowel was very uncommon and never happened in men. His symptoms persisted for several more months, and despite trying a variety of over-the-counter products (milk of magnesia, magnesium citrate) and herbal remedies (senna and cascara), he didn’t feel any better.
Hank went to see his doctor for his yearly check-up. After a physical examination (which was completely normal) and some simple laboratory tests (blood count), Dr. Liu asked if there was anything else he wanted to discuss. Although he was somewhat embarrassed to discuss his symptoms, Hank told Dr. Liu about his abdominal pain and constipation. Dr. Liu listened carefully, double-checked a few important points in Hank’s medical and family history (see Chapters 3 and 7), and then told him that because of his symptoms and his normal examination, it was quite likely that he did have IBS. Hank was skeptical and told Dr. Liu that he didn’t think that he could have IBS if he was constipated. Dr. Liu reviewed some of the facts and figures about IBS (discussed below) and reassured Hank that IBS is a common problem that occurs in both men and women. Hank seemed reassured and they spent the remaining time discussing different treatment options (see Chapter 15).
Of all the people in the United States who have IBS, only about 30 percent visit a health care provider for treatment of this disease. There are several reasons why people who have IBS may avoid the doctor’s office. One, they have mild, intermittent symptoms that they either ignore or treat at home with over-the-counter medications. Two, they are too embarrassed to discuss their symptoms with a doctor or other health care provider. These people may feel uncomfortable describing their symptoms because they think the symptoms are uncommon or because they do not know the proper terms to describe them. Three, many people who have IBS avoid going to a doctor because they worry that their symptoms indicate a serious illness, and they don’t wish to hear bad news. Ordinarily, you would think that if you had a medical problem that might be serious, you would want to see a doctor and have a thorough evaluation. However, some patients are so fearful of hearing bad news that they put things off as long as they can. A recent study found that nearly 17 percent of people who have IBS incorrectly believe that they will develop cancer. Finally, some people who have IBS don’t see a doctor because of financial reasons. This may be due to a lack of insurance, the inability to pay the copayment for an office visit, or concerns that they won’t be able to pay for medications, laboratory studies, or diagnostic tests.
Figure 4.1. The Worldwide Prevalence of IBS
Although many people think of IBS as a problem only in the United States, people the world over have this condition. Research studies have demonstrated that IBS is quite common worldwide, including in Europe, Japan, China, Australia, and New Zealand. This map provides an estimate of the percentage of the population that has IBS in each country for which data are available. The variation in percentages may be caused by the use of different questionnaires and different definitions of IBS in the studies, but true differences in the prevalence of IBS may also exist, based on ethnicity, social customs, and geography.
How Do We Know How Many People Have IBS?
People who have IBS usually have a lot of specific questions: How common is it for someone to develop IBS? What are my risks for developing IBS? How many other people in the community have IBS? What is the natural history of IBS? In answering these questions, it is best to start by defining the two most common ways to measure the extent of disease: incidence and prevalence. Although these terms are frequently used in the media and in the medical literature, they can still be confusing.
Incidence refers to the number of new cases of a specific disease that develop within a certain time period (such as a given year). For example, if members of a small community (10,000 adults) answered a health questionnaire and 100 said that they had been newly diagnosed with IBS during the past year, the incidence would be 1 percent per year (100/10,000). A study performed in the United States over 10 years ago found that the incidence of IBS was 9 percent (that is, 9 new cases of IBS diagnosed for every 100 people surveyed; see Figure 4.2). A European study performed using a different method found the incidence of IBS to be lower, approximately 2 percent (2 of 100 people had been newly diagnosed with IBS during the previous year). Most likely, the incidence of IBS differed in these two studies because the types of questions physicians used to diagnose IBS were different, and the study populations themselves were also somewhat different (Europeans vs. Americans).
Incidence only describes the number of new cases of a disease during a certain time period. Researchers use incidence to determine the frequency of IBS in a given population, not to study why the disease develops, its natural history, or the nature of its symptoms. Because people with typical symptoms of IBS are not always formally diagnosed and therefore cannot be accurately counted in a study, incidence may underestimate the frequency of the disease. As noted above, only 30 percent of people who have symptoms of IBS ever see a doctor for their problems; thus up to 70 percent of people who have IBS may not be included in research studies or surveys.
Another common way of measuring the extent of disease, which may be more familiar to people and is often more useful, is prevalence. The prevalence of a disease is a measure of how many people have the disease at any given time. The prevalence of IBS is commonly derived from survey studies involving large groups of people. In these surveys, various populations (young, old, European, American, Asian, etc.) are asked a series of questions to determine whether they have symptoms of IBS at the present time. The prevalence of IBS is the percentage of people who, at the time of the survey, have all of the symptoms consistent with IBS. So, if the same community mentioned earlier (population of 10,000) fills out a questionnaire asking if they currently have IBS (or have symptoms that fit the definition of IBS), and 1,500 people say yes, the prevalence would be 15 percent (1,500/10,000; see Figure 4.3). Health care providers often use the concept of prevalence because it lets them know how many people with the disease are in the community.
Figure 4.2. Incidence
This diagram illustrates the statistical term incidence. Incidence is defined as the number of people who develop a disease during a specific time period. Typically, incidence is defined as the number of new cases that develop over the course of a year. In this example, at the start of the year, 100 people were surveyed and none had symptoms. At the end of the year, the same 100 people were surveyed and 9 had developed symptoms. This means that the incidence of the disease in this particular population is 9 percent (9/100).
People often wonder about the relationship between incidence and prevalence. It seems logical to think that if the incidence of a disorder was 5 percent, then at the end of 10 years, the prevalence should be 50 percent because 5 percent of the population develops the disease each year (incidence of 5 percent per year × 10 years = prevalence rate of 50 percent). However, this equation assumes that the population never changes. As we all know, our local population is changing all the time. People move into our communities and people leave. People who have IBS may have their symptoms disappear over time, and other people may develop new symptoms consistent with IBS. So, this seemingly logical line of thinking is flawed, because it is based on the assumption that once a person is diagnosed with the disorder, that person will always have it. Fortunately, that is not the case for people who have IBS.
If we look at the example of incidence and prevalence in Figure 4.4, we can see that there are 17 people who currently have symptoms of IBS and have been formally diagnosed with this problem (prevalence). At the same time, three people have been newly diagnosed with IBS. This should raise the prevalence to 20 percent (20 of 100 people), but we also need to take into account 3 people who were previously diagnosed with IBS but whose symptoms have since changed. These three people no longer fit the definition of IBS: one person had all of her symptoms resolve, one person died of a medical condition not related to IBS, and one person had the diagnosis of IBS changed to celiac disease. (Please note that IBS is never a lethal disease and it does not shorten someone’s lifespan. In addition, physicians rarely diagnose IBS as another disorder [see Chapter 7]).
Figure 4.3. Prevalence
This diagram illustrates the statistical term prevalence. Prevalence is defined as the number of people who have a disease at a specific point in time. In this diagram, 100 people were asked if they had the symptoms consistent with a specific disease. Fifteen people said yes, and thus the prevalence is 15 percent (15/100). Note that this is different from incidence, since prevalence does not signify how many people developed the disease or disorder during a given time but rather how many people have symptoms of the disorder at a specific point in time.
Who Gets IBS?
If you review the map of the world in Figure 4.1, you may notice that the prevalence rates of IBS seem to differ in various parts of the world. The prevalence rates of IBS from different research studies vary for many reasons, including what type of questionnaire was used, how the questionnaire was administered (in person, by phone, by mail), and what definition of IBS was used. Overall, the worldwide prevalence of IBS is approximately 10 to 35 percent. Multiple studies from the United States have consistently found a prevalence rate of 10 to 20 percent, with an average of 15 percent (see Figure 4.1). This prevalence rate means that between one in six to one in seven adult Americans suffers from IBS. Most people who have IBS begin to develop symptoms in their late teenage years or early twenties, although the problem may not be diagnosed for many years (see Chapter 7). The prevalence of IBS peaks in the third and fourth decades of life and decreases in the sixth and seventh decades of life. The prevalence of IBS in people over age 60 is approximately 11 percent (11 of 100 people over 60 years old have IBS, based on symptoms). Physicians may diagnose IBS in some people who are well into their seventies and even in their eighties, although this is not common (see Figure 4.5), and they are quite cautious about doing so because other diseases (colon cancer, diverticulitis) may have similar symptoms.
Figure 4.4. How Prevalence Changes over Time
This diagram illustrates how the prevalence of a disorder can change with time and why the incidence of a disorder does not mean that eventually everyone will develop that disease. On the left side of the diagram, 3 people develop IBS during the course of one year, so the incidence is 3 percent. If more and more people develop the disease, then the prevalence will increase. In the middle portion of the drawing, a survey of the population (20 people) shows that 3 people are afflicted, and thus the prevalence is 15 percent (3/20). However, as time progresses, the prevalence may change as the population changes. Some people’s symptoms may resolve, some patients may have been incorrectly diagnosed, and some people may move away. In addition, some people will be newly diagnosed while other people have their diagnosis changed, move away, or get better. Thus, the prevalence of a specific disease in a given population may change over time or it may remain the same.
In regard to other factors that may influence who gets IBS, race/ ethnicity does not appear to play a major role in the development of IBS. Several studies have reported a similar prevalence among European Americans and African Americans, and two studies have reported that the prevalence of IBS is somewhat lower in Asians and in Hispanics when compared to European Americans or African Americans.
Socioeconomic status may play a role in the development of IBS, although data from research studies are not very clear on this issue. One study showed that people in a lower socioeconomic group were more likely to develop IBS symptoms than people of higher socioeconomic status, but this study was not designed to determine why finances appear to be related to the development of IBS. It is quite possible that finances, as a single, specific issue, do not play any role in the development of IBS. Someone who is better off financially can afford to see a physician or take medications that relieve symptoms of IBS; thus, people in a higher socioeconomic group might be more likely to have their symptoms improve or resolve. Conversely, a person who is financially more stressed might not be able to see a physician or afford medications that improve their symptoms of IBS; thus, their symptoms might persist for a longer period of time. In addition, we already know that stress and emotions can affect the GI tract through the brain-gut axis (see Chapter 2), so it seems logical that financial stress could worsen IBS symptoms.
In large population studies, women are at least twice as likely as men to be diagnosed with IBS. In studies conducted at referral medical centers (usually large, university-associated or university-owned medical centers), the ratio of women to men who have IBS and are enrolled in research studies is usually three to one and may be as high as four to one. Researchers have found that throughout the world, women consistently outnumber men when it comes to having IBS—except in India and Sri Lanka. Studies performed in these two countries have shown that IBS is actually more common in men than in women, most likely because men generally have much greater access to health care than do women in this area of the world and are thus more frequently diagnosed.
Figure 4.5. The Relationship of IBS to Age
This diagram illustrates the prevalence of IBS in relation to age. IBS is uncommon in children. The peak prevalence of IBS is in people in their late twenties to late thirties. After that, the prevalence slowly decreases. Older individuals can have IBS, although it is much less common than among younger adults.
The disparity in the diagnosis of IBS in men and women may occur for a number of different reasons. One, in most countries women are more likely than men to make routine health care visits, usually to see a gynecologist or an obstetrician. Many young women use their gynecologist as their primary health care provider. During these visits, women may be asked about the presence of other symptoms, including a change in bowel habits, bloating, or abdominal pain. Two, women who see a physician more regularly (such as a gynecologist) may be more willing to discuss IBS-related problems, as opposed to men, who are less likely to see a physician on a regular basis and who may wish to focus on other health issues during these less-frequent visits. Three, some studies have showed increased health care–seeking behavior in women who have IBS. However, the results of these studies probably represent only a small fraction of all women who have symptoms of IBS and describe only people with the most severe symptoms, rather than all women (or men) who have IBS.
Differences in hormone levels might also explain the gender disparity in diagnosis of IBS. Elevated hormone levels (estrogen and progesterone) during pregnancy can cause smooth muscles in the GI tract to relax, which partially explains the increase of acid reflux disease and constipation in pregnant women. Emerging data show many women experiencing changes in their IBS symptoms that fluctuate with their menstrual cycle. One recent study found that 50 percent of women note a worsening of IBS symptoms at the onset of menses. In addition, there is now a small amount of data showing that some women who have IBS note an improvement in their symptoms after menopause, a time when levels of hormones drop significantly. In summary, hormones are likely to play a key role in the presentation and severity of IBS symptoms, and they should be factored into current and future treatment programs.
Summary
• IBS is very common. Approximately 15 percent of adult Americans (1 in 7) suffer from this disorder.
• Although IBS can develop and be diagnosed at virtually any age, the most common time of diagnosis is in the third or fourth decade of life.
• Women are more likely to be diagnosed with IBS than men. The reasons for this are not clear, and this issue is an active area of research.
• Because many people who have IBS never see a physician or other health care provider for their problems, IBS is underreported.
• IBS does not increase a person’s risk of developing cancer and does not shorten a person’s lifespan.