Small intestinal bacterial overgrowth (SIBO) is defined as an increase in the quantity or an alteration in the type of bacteria present in the small bowel. SIBO was first described in 1939, in association with intestinal strictures.1 Indeed, SIBO is more likely to occur as a complication of gut motility disorders, including strictures, scleroderma, or diabetic enteropathy.2 Recently, however, the association between SIBO and irritable bowel (IB) has peaked clinicians’ interest. In 2000, Mark Pimental, MD, and his colleagues at Cedars-Sinai Medical Center reported a 78% prevalence of SIBO in patients with irritable bowel.3 Since that time, SIBO is consistently identified more frequently in patients with irritable bowel than in healthy controls.4
There is a lack of consensus, however, regarding the natural history of SIBO in irritable bowel or other functional gastrointestinal disorders. Symptoms such as bloating, abdominal pain, or diarrhea are nonspecific and unreliable for diagnosis.5 Controversies now surround the best practices for the diagnosis and management of SIBO, particularly in patients with gastrointestinal dysfunction.
Symptoms of SIBO vary for individuals, from mild to severe, from annoying to debilitating, and from chronic to sporadic.
The symptoms of SIBO most commonly include an increase in gas and bloating. Other symptoms may include abdominal pain and cramping, diarrhea, and/or constipation. As with any gastrointestinal/digestive impairment, symptoms can also present outside of the GI tract.
The upper portions of the small intestine normally contain minimal numbers of bacteria. Gastric acid, intestinal motility, biliary secretions, and immunoglobulins keep bacterial growth in check. With the loss of any of these protective mechanisms, bacteria can thrive.6 The gold standard for SIBO assessment is a duodenal culture, but a less costly and less invasive test that has gained favor is the hydrogen breath test.
The basis for the breath test is that the ingestion of sugar leads to fermentation by small intestinal bacteria, producing hydrogen gas that is exhaled. Glucose is arguably the most reliable substrate for breath testing (as it is entirely absorbed in the proximal small intestine), but it will not identify all cases of SIBO: a recent study found glucose hydrogen breath testing to have a specificity of 84% but a sensitivity of only 42%.7 Some forms of bacteria produce methane rather than hydrogen, so the combination of hydrogen and methane testing will identify more true cases of SIBO.8 Diagnostic tests remain so controversial, however, that some experts argue for initiating SIBO interventions even in the absence of confirmatory tests.9
It is common for the symptoms of SIBO to reoccur. The plan of care is focused on management, namely achieving and maintaining remission.
The mainstay of SIBO management is antibiotic therapy. Rifaximin is the most thoroughly studied antibiotic for SIBO, with a reported success rate of 64%.10 Interestingly, the FDA approved Rifaximin (Xifaxan) for the treatment of diarrhea-predominant irritable bowel, even in the absence of testing for SIBO.11 The empiric use of Rifaximin for irritable bowel further muddles the distinction between these two diagnostic entities. The choice of antibiotic is also an important consideration: Rifaximin is effective against hydrogen-producing bacteria, but additional antibiotics are indicated for methane-producing bacteria.12 In a 2014 study, researchers compared Rifaximin to herbal therapy.13
Of concern, relapse of SIBO occurs in as many as 44% of cases that initially resolve with therapy.14 Relapse prevention relies on addressing the underlying cause or causes. Supporting GI health may include appropriate use of therapeutic diets, promotility (prokinetic) agents, probiotics, and other gastrointestinal support protocols.*15
The food we consume feeds bacteria in our body, making nutrition and diet implications for patients with SIBO particularly important. Nutritional intervention is common, particularly intervention that provides adequate nutrition, but starves the bacteria population in the intestine. Popular diets include the Specific Carbohydrate Diet which removes grain, starchy vegetables, lactose, some beans, and sweeteners. The Low FODMAP diet is also common; however, because this diet allows for grains, sugars, and other foods it may need to tailored to those with SIBO.
The elemental diet is a medically supervised, sole nutrition dietary management given to individuals with moderate to severe impaired gastrointestinal function for 14-21 days.
The diet consists of macronutrients broken down into their elemental form requiring little to no digestive functionality allowing time for the gut to rest. Elemental formulations are believed to be entirely absorbed within the first few feet of small intestine.16
Products designed for the Elemental Diet contain anywhere from 14-18% of calories from protein in the form of amino acids, 42-76% calories from carbohydrate in the form of monosaccharaides and 6-43% of calories from fat in the form of fatty acids. The micronutrient composition of an elemental diet is tricky because it must be sufficient for up to 3 weeks, but not exceed safe levels of ingestion even for impaired individuals.
A medically supervised elemental diet is often used by patients with SIBO because the nutrients in an elemental diet are easily assimilated into the body the bacteria residing in the gastrointestinal tract are thought to have reduced activity and proliferation, making elemental diets an effective tool. When an elemental diet is determined to be the next step for an individual, the patient’s caloric need is first determined. After the elemental diet is complete, a reintroduction and assessment of foods is conducted.
- Barker WH, Hummel LE. Macrocytic anemia in association with intestinal strictures and anastomosis. Bull Johns Hopkins Hospital. 1939;46:215. Cited by: Reynolds KH. Small intestinal bacterial overgrowth: a case-based review. J Patient Cent Res Rev. 2015;2:165-173.
- Bures J, Cyrany J, Kohoutova D, et al. Small intestinal bacterial overgrowth syndrome. World J Gastroenterol. 2010;16(24):2978-2990.
- Pimentel M, Chow EJ, Lin HC. Eradication of small intestinal bacterial overgrowth reduces symptoms of irritable bowel syndrome. Am J Gastroenterol. 2000;95(12):3503-3506.
- Ghoshal UC, Srivastava D. Irritable bowel syndrome and small intestinal bacterial overgrowth: meaningful association or unnecessary hype. World J Gastroenterol. 2014;20(10):2482-2491.
- Baker JC, Saad WJ. Common gastrointestinal symptoms do not predict the results of glucose breath testing in the evaluation of suspected small intestinal bacterial overgrowth. Am J Gastroenterol. 2015. Cited by: Rezaie A, Pimentel M, Rao SS. How to Test and Treat Small Intestinal Bacterial Overgrowth: an Evidence-Based Approach. Curr Gastroenterol Rep. 2016;18(2):8.
- Bures J, Cyrany J, Kohoutova D, et al. Small intestinal bacterial overgrowth syndrome. World J Gastroenterol. 2010;16(24):2978-2990. [REPEAT]
- Erdogan A, Rao SS, Gulley D, Jacobs C, Lee YY, Badger C. Small intestinal bacterial overgrowth: duodenal aspiration vs glucose breath test. Neurogastroenterol Motil. 2015;27(4):481-489.
- Saad RJ, Chey WD. Breath testing for small intestinal bacterial overgrowth: maximizing test accuracy. Clin Gastroenterol Hepatol. 2014;121964-72; quiz e119.
- Pimentel M. Breath Testing for Small Intestinal Bacterial Overgrowth: Should We Bother. Am J Gastroenterol. 2016;111(3):307-308.
- Lauritano EC, Gabrielli M, Scarpellini E, et al. Antibiotic therapy in small intestinal bacterial overgrowth: rifaximin versus metronidazole. Eur Rev Med Pharmacol Sci. 2009;13(2):111-116.