Small Intestinal Bacterial Overgrowth (cont.)
How does small intestinal bacterial overgrowth cause symptoms?
When bacteria digest food in the intestine, they produce
gas. The gas can accumulate in the abdomen giving rise to abdominal bloating or
distension. Distension can cause abdominal pain. The increased amounts of gas
are passed as flatus (flatulence or farts). The bacteria also probably convert
food into substances that are irritating or toxic to the cells of the inner
lining of the small intestine and colon. These irritating substances produce
diarrhea (by causing secretion of water into the intestine). There is some evidence that the
production of one gas by the bacteria-methane-causes constipation.
Bacteria in the small intestine, when present in large
numbers, can compete with the human host for the food that is eaten. This can
lead to malnutrition with vitamin and mineral deficiencies. In advanced cases of SIBO, the bacteria use up enough food that there are insufficient calories for
the host, thereby leading to weight loss.
How is small intestinal bacterial overgrowth diagnosed?
Culturing bacteria from the small intestine
One method of diagnosing bacterial overgrowth is
culturing (growing) the bacteria from a sample of fluid taken from the small
intestine. The culturing must be quantitative, meaning that the actual number of
bacteria must be determined. Essentially, the bacteria in a known quantity of
fluid are counted. Culturing requires a long flexible tube to be passed through
the nose, down the throat and
esophagus, and through the stomach under x-ray guidance so that fluid can be
obtained from the small intestine.
There are several problems with diagnosing SIBO
by culturing. Passage of the tube is uncomfortable and expensive, and the skill
necessary to pass the tube is not commonly available. The quantitative culturing
of intestinal fluid is not a routine procedure for most laboratories, and,
therefore, the accuracy of the cultures is questionable. Finally, with the tube
only one, or at most a few, locations of the small intestine can be sampled. Usually
it is the duodenum. It is possible that the overgrowth involves just the jejunum
or ileum, and overgrowth may be missed if only the duodenum is sampled. Because
of all these potential problems, quantitative culturing for intestinal bacteria
usually is utilized only for research purposes.
Hydrogen breath test (HBT)
Bacteria that live in the colon are capable of digesting and using sugars and
carbohydrates as food. When the bacteria normally present in the colon digest
sugars and carbohydrates, they produce gas, most commonly carbon dioxide, but
also smaller amounts of hydrogen and methane. (The types of bacteria normally
found in the esophagus, stomach, and small intestine produce little gas.) Most of
the sugars and carbohydrates that we eat are digestible and are digested and
absorbed in the small intestine, never reaching the colonic bacteria. Moreover, more
than 80% of the gas that is produced by bacteria in the colon is used up by
other bacteria within the colon. As a result, relatively little of the gas that
is produced remains in the colon, and it is eliminated as flatus (farts). Although the
overwhelming majority of the hydrogen and methane produced by colonic bacteria
is used up by other bacteria, small amounts of these gases are absorbed through
the lining of the colon and into the blood. The gases circulate in the blood and
go to the lungs, where they are eliminated in the breath. The gases can be
measured in the breath with special analyzers (usually a gas chromatograph).
Hydrogen breath test
procedure
For the hydrogen breath test, individuals fast for at least 12 hours. At the start of
the test, the individual fills a small balloon with a single breath of air and
then ingests a small amount of the test sugar (usually lactulose or glucose).
All samples of breath are analyzed for hydrogen and methane every 15 minutes for
three or more hours.
Lactulose is a sugar that is digested only by colonic
bacteria and not by the human host. The ingested lactulose travels through the
small intestine undigested and reaches the colon where the bacteria produce gas. In
the normal individual, there is a single peak of gas in the breath following the
ingestion of lactulose when the lactulose enters the colon. Individuals with SIBO have two peaks of gas in the breath. The first abnormal peak occurs as the
lactulose passes the gas-producing bacteria in the small intestine, and the second
normal peak occurs as the lactulose enters the colon.
The principle difference
is when glucose is used for the hydrogen breath test. Glucose is a sugar that is digested and
absorbed by everyone. None of it reaches the colon. However, if large amounts of
glucose are ingested (50-100 grams), the glucose is steadily absorbed in the
small intestine. As a result, the concentration of glucose in the small
intestine
decreases steadily as the glucose travels down the intestine until eventually there
is no more glucose in the small intestine. If the glucose passes through a segment
of the small intestine that contains overgrowing bacteria (i.e., SIBO is present),
the bacteria produce gas from the glucose, and the gas is excreted in the
breath. Normal individuals secrete no gas in their breath after ingesting
glucose because the glucose never reaches the gas-producing bacteria that are
normally present only in the colon.
Limitations of hydrogen breath test
There are several
limitations to the hydrogen breath test for the diagnosis of SIBO.
- Hydrogen breath test with lactulose may be able to diagnose only 60%
of patients with SIBO, and glucose may be only slightly better. Since glucose
is absorbed completely before it completes passage through the small intestine, it
may not be able to diagnose SIBO of the distal small intestine (ileum). A major
problem is that there is no "gold standard" for the diagnosis of SIBO since
culture of the bacteria has its own limitations, as discussed previously.
Without a testing gold standard, it is difficult to
know just how good hydrogen breath test is for the diagnosis of SIBO.
- Any condition that impairs the digestion or
absorption of sugars and carbohydrates in the small intestine can produce an
abnormal hydrogen breath test when dietary sugars, i.e., glucose, are used for testing.
Therefore, conditions other than SIBO, such as pancreatic insufficiency and
celiac sprue, can result in abnormal breath tests. In the
former instance, the pancreatic enzymes that are necessary for the digestion of
carbohydrates are missing, and in the latter condition, the lining of the small
intestine is destroyed and digested food cannot be absorbed. Hydrogen breath
test using lactulose is
not affected by impaired digestion or absorption.
- There may be similarities in
the pattern of gas production with SIBO and rapid intestinal transit, thus
making distinctions difficult.
- Some normal individuals may have slow transit
through the small intestine making prolonged testing--up to five
hours--necessary and many individuals are unwilling to undergo such prolonged
testing.
- A small number of individuals with SIBO have bacteria that do not
produce hydrogen or methane, and, therefore, their SIBO cannot be detected with
the hydrogen breath test.
- Some individuals produce only methane or a combination of hydrogen
and methane. There is much less experience with methane as compared with
hydrogen for the diagnosis of SIBO, however, and the production of methane is
more complex than the production of hydrogen. Therefore, it is not clear if the
pattern of methane production after ingestion of sugars can be interpreted in
the same way as hydrogen production.
- A positive hydrogen breath test does not
always mean that a patient's symptoms are caused by SIBO. For example, Crohn's
disease of the small intestine, small intestine strictures (narrowing due to scarring),
or other anatomical abnormalities of the small intestine can cause symptoms of
bloating, distension, pain, and diarrhea from the obstruction to the intestine
they cause. These other conditions also can cause bacterial overgrowth, which
can produce similar symptoms. How can it be determined whether the underlying
condition or the bacteria is causing the symptoms? The only way to establish
whether the symptoms are caused by the intestinal disease or by the SIBO is to
treat and eradicate the bacteria. If the symptoms disappear, then it is likely
that SIBO rather than the underlying disease is responsible for the symptoms. If
symptoms don't improve, however, it is possible either that the symptoms are
those of the underlying disease or that the treatment of the bacteria was
ineffective.
For more information on the hydrogen breath test, please read the
Hydrogen Breath Test article.
Next: Is there a relationship between small intestinal
bacterial overgrowth and irritable bowel syndrome? »
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