Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is characterized by chronic abdominal pain and altered bowel habits without an identifiable organic cause. It affects 10-15% of the US population and represents up to 50% of all referrals to gastroenterologists.[1],[2],[3] It is more commonly diagnosed in younger patients and women.

The pathophysiology is unclear. To date, no physiologic or psychological etiology has been identified. Investigation has centered on abnormal gastrointestinal motility, hypersensitivity of gastrointestinal nerves, microscopic inflammation, changes in gut flora, bacterial overgrowth and change or homeostatic imbalance in gut metabolites (i.e., short-chain fatty acids, lipids, vitamins, amino acids), carbohydrate or bile acid malabsorption, and emotional stress, but clinical studies thus far are inconclusive.[4],[5],[6],[7],[8],[9],[10]

Chronic abdominal pain and changes in bowel habits are the symptoms that commonly prompt patients to seek medical advice. Altered bowel habits may occur as diarrhea, constipation, or alternating diarrhea and constipation.[11] Other symptoms include bloating, early satiety, incomplete evacuation, nausea, dyspepsia, dysphagia, and reflux. IBS may also be accompanied by non-gastrointestinal symptoms (e.g., dysmenorrhea, urinary frequency and urgency, sexual dysfunction, or fibromyalgia and other pain syndromes) and psychiatric disorders (e.g., somatization, depression, anxiety).[12]

Risk Factors

At least half of cases present in patients less than 35 years of age. In Western societies women are affected twice as often as men.[2]


A careful history and physical examination are essential to avoid unnecessary and costly diagnostic testing. The examining physician should attempt to identify foods, nutrients or additives (e.g., lactose, fructose, sorbitol, saccharin, sucralose), and medications (e.g., antacids, calcium channel blockers, anticholinergics, macrolides, and tetracyclines antibiotics) that are related to symptoms.[13] It is also important to look for factors that suggest organic disease and require further diagnostic testing to rule it out. Examples include hematochezia, rapid unintentional weight loss, family history of colon cancer, recurring fever, anemia, and/or severe diarrhea.

A detailed psychiatric history should also be elicited, given that the treatment of underlying psychological conditions may be an essential part of treatment.[14],[15]

Depending on the predominating symptom, IBS is categorized into four main subtypes, i.e., IBS with constipation; IBS with diarrhea; IBS with mixed symptomology; and unclassified IBS.[16]

Two symptom-based criteria frameworks help with the diagnosis of IBS when there is no evidence of organic factors, but the utility of these criteria has not been fully established.

The Manning criteria consist of a group of symptoms that suggest IBS as a possible diagnosis. These include relief of pain after a bowel movement, pain accompanied by more frequent and loose stools, mucus mixed with the stool, and tenesmus.[17]

The Rome criteria were designed to create a standardized system for diagnosis. The latest revision, Rome criteria IV, defines IBS as having abdominal pain at least once a week for the past 3 months, accompanied by 2 or more of the following:[16]

  • Related to defecation.
  • Changes in stool frequency.
  • Changes in stool consistency, according to Bristol Stool Form Scale (type 1-2 or type 6-7).

Laboratory studies such as complete blood count (CBC), chemistry panel, thyroid function tests, 24-hour stool collection, and stool testing for ova and parasites are all normal in IBS and can be used to rule out organic causes of symptoms.

Colonoscopy with biopsy may be useful to rule out inflammatory bowel disease (IBD) and colorectal cancer, especially in patients over 50. In younger patients with symptoms of IBS, colonoscopy is not usually necessary unless there is a family history of IBD or colorectal cancer.

Small intestinal bacterial overgrowth (SIBO) can present with symptoms of IBS, including bloating and abdominal pain.[18] A lactulose breath test can be diagnostic, as gut bacteria turn lactulose into measurable hydrogen and/or methane.[19]


There is no specific curative treatment. The therapeutic regimen should focus on relieving symptoms and reassuring the patient that a serious illness is not present. The following interventions and several medications have been used with varying success:

  • Lifestyle and dietary modification are initially recommended. Avoiding possible food triggers, including lactose and artificial sweeteners (e.g., sorbitol, saccharin, sucralose). Daily exercise has also been shown to improve pain associated to IBS.[20]
  • Diarrheal symptoms can be treated with loperamide as initial treatment, cholestyramine (a bile acid sequestrant), or other antidiarrheal medications.[2]
  • Constipation can be treated with fiber supplementation or osmotic laxatives (e.g., polyethylene glycol, lubiprostone).
  • Abdominal pain may respond to antispasmodic agents (e.g., mebeverine, dicyclomine, hyoscyamine), but are only used on an as-needed basis.
  • Tricyclic antidepressants (e.g., amitriptyline) may also be used to decrease both pain and intestinal transit time on patient with diarrhea.[21] However, it is important to establish the IBS subtype as tricyclics can worsen constipation-predominant IBS. Serotonin-norepinephrine reuptake inhibitors are also efficacious in IBS with diarrhea, since constipation is a potential side effect.[22]
  • For SIBO, the treatment of choice is antibiotic therapy, typically rifaximin for 14 days.[23],[24] Probiotics may be useful in preventing recurrence. If there is an underlying cause for bacteria overgrowth, it should be addressed as well.

Individuals with IBS may have enhanced autonomic, neuroendocrine, attentional, and pain-modulatory responses to stimuli.[25] Brain imaging studies have demonstrated increased activation of the anterior mid-cingulate cortex that is linked to fear and psychological distress, and repression of descending opiate-mediated inhibitory pathways originating in the anterior cingulated gyrus of the limbic system.[26] Sympathetic activity is increased at rest.[27]

Psychological interventions should also be considered and are often necessary. Psychological distress, major depression, anxiety, panic disorder, agoraphobia, somatization, and illness anxiety disorder are more common in patients with IBS, compared with other patients.[28],[29]

Nutritional Considerations

IBS appears to have both nutrition and stress-related etiologies. As with some other intestinal diseases, it may be more common in individuals consuming Western diets than in persons consuming the high-fiber, low-fat diets that are traditional in unindustrialized societies.[30] Both diet and psychological interventions have resulted in symptomatic improvements, and it is likely that patients will benefit most from a combination of medical, nutritional, and behavioral approaches. The evidence of success with elimination diets for IBS is not strong, as some patients may overly restrict and nutrient shortcomings are a concern.[31],[32] However, dietary changes remain a valuable tool in IBS treatment.[33],[34] The following measures may be helpful:

Increased soluble fiber. The rationale for treatment with increased fiber is the assumption that symptoms are caused by an increase in intraluminal pressure, which is relieved by the bulking action of fiber.[35] A systematic review using 14 randomized controlled trials found soluble fiber to be effective in reducing overall symptoms. More studies are warranted, but a possible mechanism favoring soluble fiber is the fermentation of fiber acting as a prebiotic altering the composition of the gut microbiome.[36]

Controlled insoluble fiber. Increased insoluble fiber (e.g., bran) could help relieve constipation in some IBS patients though may worsen symptoms in others.[37],[38],[39] Some investigations have indicated that fiber types other than wheat bran (e.g., partially hydrolyzed guar gum) are more effective for this purpose.[40] Additional controlled clinical trials are needed to compare the efficacy of different types of fiber. Fiber should be added to the diet slowly and in small amounts.

A low-FODMAP diet. FODMAP is an acronym for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols—short-chain carbohydrates fermented by gut bacteria. Impaired carbohydrate absorption is hypothesized to allow excess undigested carbohydrates to reach the large intestine, thus stimulating the growth of fermenting pathogenic microbes which leads to excess gas, diarrhea, and constipation symptoms that are hallmark for IBS.[41] Theoretically, the restriction of fermentable foodstuffs deprives the dysbiotic gut flora of their energy source and results in decreased symptoms.

Quinoa, oatmeal, many fruits and vegetables, lentils, and chickpeas (tolerable in small amounts) are low-FODMAP foods that are high in soluble fiber. Foods to be avoided on a low FODMAP diet include: fructo-oligosaccharides (e.g., wheat, rye, onions, garlic, artichokes), galacto-oligosaccharides (e.g., legumes), lactose (e.g., milk, cheese), fructose (e.g., honey, apples, watermelon, mango), sorbitol (e.g., apples, pears, stone fruits, sugar-free energy bars, sugar-free mints/gums), and mannitol (e.g., mushrooms, cauliflower, sugar-free mints/gums).

Evidence suggests that a low-FODMAP diet may be an effective treatment of IBS symptoms for some patients.[42],[43] In a randomized control trial comparing a low-FODMAP diet with a modified National Institute for Health and Care Excellence guidelines (mNICE) diet, 52% of the low-FODMAP group reported adequate relief of IBS symptoms and overall had greater reduction in abdominal pain and bloating compared to those in the mNICE group.[44] Other studies have demonstrated that individuals with IBS assigned to a low-FODMAP diet experienced significant improvement in bloating, abdominal pain, and flatulence in comparison with a standard diet group.[45],[46]

Limitations have been documented with the low-FODMAP diet including challenges with teaching the diet, cost, and nutrition adequacy. A low-FODMAP diet can negatively alter the health of the microbiome.[47] Further, not all FODMAPs exacerbate symptoms in the same patient. Low-FODMAP diets are low in fiber, iron, and antioxidants such as flavonoids, carotenoids, vitamin C, phenolic compounds, and anthocyanins.[47] Because of this it is recommended that these diets be used to reduce symptoms for 2 to 6 weeks, followed by reintroducing FODMAP foods one at a time to detect which foods exacerbate symptoms. Following a low-FODMAP diet may be challenging, especially for those without access to a dietitian knowledgeable in the diet.[48]

Avoiding dairy products. In infants, the lactose (sugar) in milk products is digested by the enzyme lactase, which normally diminishes after weaning and eventually disappears. If milk is consumed, undigested lactose is then fermented by gut bacteria, leading to bloating, gassiness, cramping, and diarrhea if milk is consumed. Although “lactose intolerance” was once regarded as a disease, studies conducted since the 1960s demonstrated that it is the biological norm for all mammals, including humans.[49],[50],[51] A genetic mutation, leading to lactase persistence, is carried by many people of northern European or Middle Eastern ancestry. The majority of the world’s population, however, does not digest lactose and can develop similar symptoms to IBS in response to milk ingestion.[52] Removing dairy products may alleviate symptoms in some patients.[53]

Wheat and gluten avoidance. Prevalence of celiac disease may be higher in those with IBS and others may have non-celiac gluten or wheat sensitivity that exacerbates gastrointestinal distress.[54],[55],[56],[57] Testing for celiac disease may be indicated, as a gluten-free diet does not reduce symptoms for most people IBS.[58]

Probiotic therapy. A number of studies have indicated differences in intestinal microbial populations between individuals with IBS and others, suggesting that antibiotic treatments may play a causative role. Repopulating the intestinal tract with “friendly” bacteria may be helpful. Most studies have suggested a benefit from probiotic treatment with Lactobacillus plantarum, Bifidobacterium breve, Streptococcus faecium, and combinations of these with other organisms.[59] Double-blind, placebo-controlled studies indicate significant relief of IBS symptoms with the use of probiotics in adults and in children.[60],[61] However, studies have not yet specifically indicated the strain selection, dose, and viability needed to consistently produce symptom relief in IBS patients.[62],[63]

Peppermint oil. Enteric-coated peppermint oil capsules have been evaluated in controlled clinical trials and found helpful in reducing the symptoms of IBS in more than half of patients overall and in 75% of children.[64],[65],[66] Proposed mechanisms for its effects include local calcium channel blockade causing smooth muscle relaxation, and a direct antimicrobial effect against symptom-inducing bacterial overgrowth in the small intestine.[67],[68]


See Basic Diet Orders chapter.

Consultation with a registered dietitian who specializes in IBS to explore dietary options.[69]

What to Tell the Family

IBS is a frustrating condition that can be exacerbated by stress and, possibly, by poor diet. Patients may benefit from medications, increasing soluble fiber, and eliminating suspected offending foods. There is no one dietary strategy for IBS, and what works for one patient may not work for others. Stress reduction techniques and hypnotherapy may also be helpful.


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Last updated: December 16, 2020