small intestinal bacterial overgrowth (SIBO)

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What is small intestinal bacterial overgrowth (SIBO)?

The small bowel, also known as the small intestine, is part of the digestive system that connects the stomach with the large bowel or colon. The main purpose of the small intestine is to digest and absorb food into the body. The small bowel is divided into three parts: the duodenum (which food from the stomach empties into), the jejunum and the ileum (which empties undigested food into the large intestine or colon).

The entire gastrointestinal tract, including the small intestine, normally contains bacteria. The number of bacteria is greatest in the colon but much lower in the small intestine. Also, the types of bacteria within the small intestine are different to the types of bacteria within the colon. Small intestinal bacterial overgrowth (SIBO) refers to a condition in which abnormally large numbers of bacteria are present in the small intestine, while the types of bacteria found in the small intestine are more like the bacteria found in the colon.

Small intestinal bacterial overgrowth (SIBO) is also known as small bowel bacterial overgrowth syndrome (SBBOS).

How common is small intestinal bacterial overgrowth (SIBO)?

Please discuss with your doctor for further information.


What are the symptoms of small intestinal bacterial overgrowth (SIBO)?

The common symptoms of small intestinal bacterial overgrowth (SIBO) are:

When the overgrowth is severe and prolonged, the bacteria may interfere with the digestion and the absorption of food, so that deficiencies of vitamins and minerals may develop. Patients may also lose weight. Patients with SIBO sometimes also report symptoms that are unrelated to the gastrointestinal tract, such as body aches or fatigue. The symptoms of SIBO tend to be chronic. A typical patient with SIBO can experience symptoms that fluctuate in intensity over months, years or even decades before a diagnosis is made.

There may be some symptoms not listed above. If you have any concerns about a symptom, please consult your doctor.

When should I see my doctor?

If you have any signs or symptoms listed above or have any questions, please consult with your doctor. Everyone’s body acts differently. It is always best to discuss with your doctor what is best for your situation.


What causes small intestinal bacterial overgrowth (SIBO)?

The gastrointestinal tract is a continuous muscular tube through which digesting food is transported on its way to the colon. The coordinated activity of the muscles of the stomach and small intestine propels the food from the stomach, through the small intestine, and into the colon. Even when there is no food in the small intestine, muscular activity sweeps through the small intestine from the stomach to the colon.

The muscular activity that sweeps through the small intestine is important for the digestion of food, but it also is important because it sweeps bacteria out of the small intestine and thereby limits the numbers of bacteria in the small intestine. Anything that interferes with the progression of normal muscular activity through the small intestine can result in SIBO. Any condition that interferes with muscular activity in the small intestine allows bacteria to stay longer and multiply in the small intestine. The lack of muscular activity also allows bacteria to spread backward from the colon and into the small intestine.

Risk factors

What increases my risk for small intestinal bacterial overgrowth (SIBO)?

There are many risk factors for small intestinal bacterial overgrowth (SIBO), such as:

  • Neurologic and muscular diseases can alter the normal activity of the intestinal muscles. Diabetes mellitus damages the nerves that control the intestinal muscles. Scleroderma damages the intestinal muscles directly. In both cases, abnormal muscular activity in the small intestine allows SIBO to develop.
  • Partial or intermittent obstruction of the small intestine interferes with the transport of food and bacteria through the small intestine and can result in SIBO. Causes of obstruction leading to SIBO include adhesions (scarring) from previous surgery and Crohn’s disease.
  • Diverticuli (outpouchings) of the small intestine where bacteria can live and multiply and are not swept away by the intestinal activity. Diverticuli of the colon, an extremely common condition is not associated with SIBO.

Diagnosis & treatment

The information provided is not a substitute for any medical advice. ALWAYS consult with your doctor for more information.

How is small intestinal bacterial overgrowth (SIBO) 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 duodenal fluid 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 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 (for example, 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 are present only in the colon.

Limitations of the Hydrogen Breath Test

There are several limitations of the hydrogen breath test for the diagnosis of SIBO.

Hydrogen breath testing 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 its 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 such a gold standard, it is difficult to know just how effective 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 (for example, glucose) are used for testing. Therefore, conditions other than SIBO, such as pancreatic insufficiency and celiac disease, 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 testing 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, for example, early production of hydrogen or methane.

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 may 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 the production of methane after ingestion of sugars can be interpreted in the same way as the production of hydrogen.

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 of the intestine that they cause. These 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 suppress 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, alternatively, that suppression of the bacteria was ineffective.

How is small intestinal bacterial overgrowth (SIBO) treated?

“Classic” SIBO

SIBO has been recognised for many years as a problem with severe disorders of intestinal muscles and intestinal obstruction. The treatment has been antibiotics, and they are very effective, although the underlying cause is still unclear and symptoms frequently return when antibiotics are stopped.

SIBO associated with IBS

There are very few rigorous, scientific studies on the treatment of IBS with therapies that assume the possibility of underlying SIBO.

The two most common treatments for SIBO among patients with irritable bowel syndrome are oral antibiotics and probiotics. Probiotics are live bacteria that, when ingested by an individual, may result in a health benefit, but it is unclear how they work.

Several antibiotics, either alone or in combination, have been reported to be successful. 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.

Treatment with antibiotics versus probiotics

Some experts believe that for short-term (one to two weeks) treatment, antibiotics are more effective than probiotics. However, antibiotics do 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. Doctors are reluctant to prescribe prolonged or repeated courses of antibiotics because of concern over long-term side effects of the antibiotics. Doctors have less concern over long-term side effects with probiotics and are more willing to recommend probiotics repeatedly and for prolonged periods. One option is to initially treat the patient 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 needed.

Lifestyle changes & home remedies

What are some lifestyle changes or home remedies that can help me manage small intestinal bacterial overgrowth (SIBO)?

The following lifestyles and home remedies might help you prevent small intestinal bacterial overgrowth (SIBO):

Since SIBO usually is a secondary illness that occurs because the intestine has in some way been affected by another disease, it is important to keep chronic diseases properly treated and under control as best as possible.

If you have any questions, please consult with your doctor to better understand the best solution for you.

Hello Health Group does not provide medical advice, diagnosis or treatment.

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Review Date: January 3, 2018 | Last Modified: December 8, 2019

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