Irritable Bowel Syndrome (cont.)
Is there a relationship between IBS and small
intestinal bacterial overgrowth?
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 than 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 the bacteria of 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 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 for diagnosing SIBO. In addition, some
patients with IBS have improvement of 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 in which probiotics may be having 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.
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 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 among patients with IBS are oral antibiotics and
probiotics. Probiotics are live bacteria that when ingested by an individual,
result in a health benefit 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 inhibit 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 40-70%. When one antibiotic fails, doctors may add
another antibiotic or change to a different antibiotic, but the doses 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 effects 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) 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 important side effects. Higher-than-normal doses of rifaximin (1200 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; however, it is not yet known whether the larger
dose is any better at suppressing symptoms.
- 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 to be effective for treating patients with
IBS.
Treatment with antibiotic versus probiotic.
There are no trials of treatment
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.
Next: What is a reasonable approach to IBS? »
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