IBS is the most common gastrointestinal disorder seen in general practice, representing 30% to 50% of all referrals to gastroenterologists.^2,³ Determining incidence or prevalence figures is virtually impossible, because many sufferers never seek medical attention. It has been estimated, however, that approximately 15% of the population has complaints of IBS, with women predominating in a ratio of 2:1 (it is likely that an equal number of men have IBS but that they do not report symptoms as often). The etiology of the greater colonic motility seen in IBS has been attributed to physiologic, psychological, and dietary factors.

Although IBS is often a diagnosis of exclusion, clinical judgment must be used to determine the extent of the diagnostic process required. A detailed history and physical examination have been shown to eliminate much of the vagueness involved in diagnosing IBS.⁴ Distention, relief of pain with bowel movements, and the onset of loose or more frequent bowel movements with pain seem to correlate best with the diagnosis of IBS (any three, P = 0.84; just one, P = 0.25).²

In most cases, a comprehensive stool and digestive analysis (see Chapter 27) with special attention to dysbiosis, complete blood count, and measurements of erythrocyte sedimentation rate, free thyroid T₃ hormone level, and celiac testing (antiendomysial antibody test; see Chapter 155) should be performed to establish the diagnosis and exclude other diseases. If diarrhea-predominant IBS symptoms are more pervasive, panendoscopy with duodena, colonic, and terminal ileal biopsies should be obtained to rule out autoimmune conditions such as celiac disease, inflammatory bowel disease, lymphocytic colitis, and collagenous colitis. If food allergy is suspected as a contributor, stool testing for eosinophilic cationic protein should be considered. If no discernible cause can be identified, screening for occult fecal blood and flexible sigmoidoscopy are indicated in patients younger than 50 years and colonoscopy in older patients. Conditions that may mimic IBS are listed in Box 183-1.

BOX 183-1 Conditions That May Mimic Irritable Bowel Syndrome

• Gastrogenic dietary factors such as excessive consumption of tea, coffee, carbonated beverages, and simple sugars

• Infectious enteritis such as amebiasis and giardiasis

• Inflammatory bowel disease

• Lactose intolerance

• Laxative abuse (an easy test to eliminate this possibility is to add a few drops of sodium hydroxide solution to a stool specimen; since most laxatives contain phenolphthalein, the stool will turn red)

• Intestinal candidiasis

• Disturbed bacterial microflora as a result of antibiotic or antacid use

• Malabsorption diseases such as pancreatic insufficiency and celiac disease

• Metabolic disorders such as adrenal insufficiency, diabetes mellitus, and hyperthyroidism

• Mechanical causes such as fecal impaction

• Diverticular disease

• Neoplasm

Therapeutic Considerations

Once other conditions have been ruled out, there appear to be three major treatments to consider in the formulation of a therapeutic regimen for IBS:

• Increasing dietary fiber.

• Eliminating allergic/intolerant foods.

• Controlling psychological components.

• Peppermint oil for spastic colon.

Additional considerations include:

• Acupuncture for pain modulation and motility regulation.

• Melatonin as a stress adaptogen, mood stabilizer, and to promote restful sleep.

• Small intestinal bacterial overgrowth, intestinal dysbiosis and food allergies may be silent triggers of the disease.

• Dietary FODMAPS (Fermentable Oligo-, Di-, and Monosaccharides, And Polyols) appear to play a role in promoting disease activity and should be limited in IBS.

Diet

Dietary Fiber

The treatment of IBS through an increase in dietary fiber has a long though irregular history.² Patients with constipation are much more likely to show a response to dietary fiber than are those with diarrhea. One problem that has not been addressed in studies on the therapeutic use of dietary fiber is the role of food allergy. The type of fiber often used in both research and clinical practice is wheat bran.⁵ Wheat and other grains are among the most commonly implicated foods in malabsorptive and allergic conditions, and food allergy is a significant etiologic factor in IBS, so the use of wheat bran is usually contraindicated.

Increasing dietary fiber from fruit and vegetable sources rather than cereal sources may offer more benefit to some individuals, although in one uncontrolled clinical study there was no significant difference in improvement when a diet composed of 30 g of fruit and vegetable fiber and 10 g of cereal fiber was compared with a diet consisting of the opposite ratio.⁶ Although the two diets resulted in similar significant improvements in abdominal pain, bowel habits, and state of well-being, the presence of large quantities of potentially allergenic cereal fiber in both diets would probably have obscured any differences.

One type of fiber that may be useful and that is without the allergenic component of a wheat-based fiber is partially hydrolyzed guar gum (PHGG). The guar plant, Cyamopsis tetragonolobus, has been grown in India and Pakistan since ancient times. PHGG is a natural water-soluble dietary fiber derived from the guar plant.⁷ One study of 134 patients found that consumption of 5 g/day of PHGG decreased the frequency of IBS symptoms such as abdominal spasms, flatulence, and abdominal tension.⁸ The researchers concluded that PHGG works well in cases of altered intestinal motility and is easy to use because of its nongelling properties, unlike unhydrolyzed gum, which is much higher in viscosity and more difficult to incorporate into the diet. A recent study has shown that the guar gums when combined with the antibiotic rifaximin (Rifaxam) provide an additional benefit to eradicating dysbiosis in small intestinal bacterial overgrowth (SIBO) associated with IBS.⁹ Put simply, for most cases of IBS, nonwheat sources of fiber, such as vegetables and fruits, may be the best choicees to help reduce symptoms. For some cases of IBS, especially those with a strong diarrheal component, cooked vegetables in small quantities at first may be most helpful. Because each case is unique, clinical judgment and close patient monitoring are needed. In some cases, fiber may aggravate diarrhea and is therefore contraindicated. For a more detailed explanation of dietary fiber, see Chapter 52.

Food Allergy

The importance of food allergies in the etiology of IBS has been recognized since the early 1900s.^10,¹¹ Later studies have further documented the association between food allergy and the irritable bowel.¹²^–¹⁵ The type of food allergy most significant in IBS is believed to be nonimmunologic, so food intolerance rather than food allergy may be a more appropriate diagnosis. According to double-blind challenge methodologies, the majority of patients with IBS (approximately two thirds) have at least one food intolerance, and some have multiple intolerances.¹² Foods rich in carbohydrates as well as fatty food, coffee, alcohol, and hot spices are most frequently reported to cause symptoms.¹ The most common allergens are dairy products (40% to 44%) and grains (40% to 60%).¹⁴ Because in most cases the reaction appears to be related to prostaglandin synthesis or immunoglobulin G (IgG)–mediaed rather than IgE-mediated reactions, skin tests and the IgE-radioallergosorbent test are poor indicators of food intolerance in these patients. The enzyme-linked immunosorbent assay (ELISA) allergen challenge test or ELISA IgE/IgG₄ may be a better indicator (see Chapter 15), although many sensitivities may still be undetectable by currently available laboratory procedures.¹⁶ Many patients have noted marked clinical improvement with the use of elimination diets.¹²^–¹⁵ ¹⁷

Interestingly, many patients with IBS have associated symptoms suggestive of vasomotor instability (such as palpitations, hyperventilation, fatigue, excessive sweating, and headaches), which is consistent with food allergy/intolerance reactions.

Sugar

Meals high in refined sugar can contribute to IBS as well as small intestinal bacterial overgrowth by decreasing intestinal motility.¹⁸ When blood glucose levels rise too rapidly, gastrointestinal tract peristalsis slows down. Because glucose is absorbed primarily in the duodenum and jejunum, the message affects this portion of the gastrointestinal tract most strongly. The result is that the duodenum and jejunum become atonic. A diet high in refined sugar may be the most important reason that IBS is such a common condition in the United States.