Irritable Bowel Syndrome (cont.)

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Is there a relationship between IBS and small intestinal bacterial overgrowth?

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IBS and small intestinal bacterial overgrowth (SIBO)

There is a striking similarity between the symptoms of IBS and a condition known as small intestinal bacterial overgrowth (SIBO).

The entire gastrointestinal tract, including the small intestine, normally contains bacteria. The number of bacteria is greatest in the colon (at least 1,000,000,000 bacteria per ml of fluid) and much lower in the small intestine (less than 10,000 bacteria per ml of fluid). Moreover, the types of bacteria within the small intestine are different from the types of bacteria within the colon. SIBO refers to a condition in which abnormally large numbers of bacteria (at least 100,000 bacteria per ml of fluid) are present in the small intestine, and the types of bacteria in the small intestine resemble more like the bacteria in the colon than the small intestine.

The symptoms of SIBO include excess gas, abdominal bloating and distension, diarrhea, and abdominal pain. A small number of patients with SIBO have chronic constipation rather than diarrhea. When the bacterial overgrowth is severe and prolonged, the bacteria may interfere with the digestion and/or absorption of food, and deficiencies of vitamins and minerals may develop. Loss of weight also may occur. The symptoms of SIBO tend to be chronic; a typical patient with SIBO can have symptoms that fluctuate in intensity over months, years, or even decades before the diagnosis is made.

It has been theorized that SIBO may be responsible for the symptoms in at least some patients with IBS. The estimates run as high as 50% of patients with IBS. Support for the SIBO theory of IBS comes from the observation that many patients with IBS are found to have an abnormal hydrogen breath test, a test used to diagnose SIBO. In addition, some patients with IBS have improvement in their symptoms after treatment with antibiotics, the primary treatment for SIBO. Moreover, small, scientifically sound studies have shown that treatment with probiotics ("good" bacteria) improves the symptoms of IBS. Although there are several ways by which probiotics may exert their beneficial effect, one way is by affecting the existing bacteria in the small intestine. If this is indeed the mechanism of action, it would support the theory that SIBO is a cause of IBS. Nevertheless, it has not been determined if this is the mechanism of action of probiotics in IBS. Finally, it has been shown that treatment with antibiotics that kill or suppress intestinal bacteria improves the symptoms of IBS.

Although the theory that SIBO causes IBS is tantalizing and there is much anecdotal information that supports it, the rigorous scientific studies that are necessary to prove or disprove the theory have just begun. Nevertheless, many physicians have already begun to treat patients with IBS for SIBO. In addition, a lack of adequate rigorous scientific studies demonstrating benefit from antibiotics and probiotics has not stopped physicians from using them for treating patients.

Treatment of IBS based on the theory of small intestinal bacterial overgrowth

The two most common treatments for SIBO in patients with IBS are oral antibiotics and probiotics. Probiotics are live bacteria that when ingested, result in a beneficial response to the individual. The most common probiotic bacteria are lactobacilli (also used in the production of yoghurt) and bifidobacteria, both of which are found in the intestine of normal individuals. There are numerous explanations for how probiotic bacteria might benefit individuals; however, the beneficial mechanism of action has not been identified clearly. It may be that the probiotic bacteria suppress the other bacteria in the intestine that may be causing symptoms, or it may be that the probiotic bacteria act on the host's intestinal immune system to suppress inflammation.

Several antibiotics either alone or in combination are reported to be successful in treating SIBO in patients with IBS. Treatment success, when measured by either improvements in symptoms or by normalization of the hydrogen breath test, ranges from 40% to 70%. When one antibiotic fails, doctors may add another antibiotic or change to a different antibiotic, but the dose of antibiotic, the duration of treatment, and the need for maintenance treatment to prevent recurrence of SIBO have not been adequately studied. Most physicians use standard doses of antibiotics for one to two weeks. Probiotics may be used alone, in combination with antibiotics, or for prolonged maintenance. When probiotics are used, it probably is best to use one of the several probiotics that have been studied in medical trials and shown to have an effect on the small intestine, (though not necessarily in SIBO). The commonly-sold probiotics in health-food stores may not be effective. Moreover, they often do not contain the stated bacteria or the bacteria are dead. Following are some options for treatment:

  • neomycin orally for 10 days (One observation that has been made is that neomycin eradicates methane-producing bacteria and alleviates constipation.)
  • levofloxacin (Levaquin) or ciprofloxacin (Cipro) for 7 days
  • metronidazole (Flagyl) for 7 days
  • levofloxacin (Levaquin) combined with metronidazole (Flagyl) for 7 days
  • rifaximin (Xifaxan) for 7 days. Rifaximin is a unique antibiotic that is not absorbed from the intestine, and, therefore, acts only within the intestine. Because very little rifaximin is absorbed into the body, it has few side effects. Higher-than-normal doses of rifaximin (1200 or 1600 mg/day for 7 days) were superior to standard lower doses (400 or 800 mg/day) in normalizing the hydrogen breath test in patients with SIBO and IBS. Rifaximin at a dose of 1650 mg/day for two weeks is the only antibiotic that has been demonstrated in a vigorous scientific trial to be effective in treating patients with IBS.
  • Commercially available probiotics such as VSL#3 or Flora-Q which are mixtures of several different bacterial species have been used for treating small intestinal bacterial overgrowth and IBS, but their effectiveness is not known. Bifidobacterium infantis 35624 is the only probiotic that has been demonstrated in a rigorous scientific trial to be effective in treating patients with IBS.

Treatment with antibiotic versus probiotic

There are no trials comparing antibiotics and probiotics; however, antibiotics have certain disadvantages. Specifically, symptoms tend to recur after treatment is discontinued, and prolonged or repeated courses of treatment may be necessary in some patients. Physicians are reluctant to prescribe prolonged or repeated courses of antibiotics because of concern over long-term side effects of the antibiotics and emergence of bacteria that are resistant to the antibiotics. Physicians have less concern over long-term side effects or emergence of resistant bacteria with probiotics and, therefore, are more willing to prescribe probiotics repeatedly and for prolonged periods. One option is to treat initially with a short course of antibiotics and then long-term with probiotics. Long-term studies comparing antibiotics, probiotics, and combinations of antibiotics and probiotics are badly needed.

Medically Reviewed by a Doctor on 10/15/2014

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