Small Intestinal Bacterial Overgrowth (cont.)
Bhupinder Anand, MD
In this Article
How does small intestinal bacterial overgrowth cause symptoms?
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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 including sugar and carbohydrate 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 type of 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 the 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 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, greater 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 to be eliminated, 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. These 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). Samples of breath are analyzed for hydrogen and methane every 15 minutes for the next 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 situation is slightly different 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 small 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 excrete no gas in their breath after ingesting glucose because the glucose never reaches the gas-producing bacteria that normally present only in the colon.
Limitations of hydrogen breath test
There are several limitations of hydrogen breath test for the diagnosis of SIBO.
Reviewed by Bhupinder Anand, MD on 3/26/2012
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