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Level 1 - Empirical data gathered by one or more individual researchers
Research Levels Explained

SIBO stands for Small Intestinal Bacterial Overgrowth. SIBO may be associated with digestive symptoms similar to those found in Irritable Bowel Syndrome (IBS). Some SIBO experts recommended diets with low–Fermentable Oligo-, Di-, and Monosaccharide And Polyol (Fodmaps), Specific Carbohydrate Diet (SCD), and Gut and Psychology Syndrome (GAPS.)


1.     Too many bacteria in the small intestine cause intestinal symptoms

2.     Bacteria in the small intestine give off hydrogen and methane

3.     Breath tests that measure these gases can show who has SIBO

4.     Antibiotics that reduce bacteria improve SIBO symptoms

5.     Carbohydrates have unique effects on SIBO producing bacteria

6.     Diets that restrict foods that feed bacteria improve SIBO symptoms

SIBO is defined by the number of bacteria in the second part of the small intestine. These bacteria can be directly measured by removing fluid from the middle of the small intestine (jejunum) with a tube.

How many bacteria are too many is a point of some debate. Most experts say that excess of 106 bacteria per milliliter of fluid taken from the jejunum is considered SIBO. Others say that 105 bacteria/mL is adequate for the diagnosis. Some argue that 104 bacteria/mL may be enough to cause it if they are dangerous types of bacteria.

If these bacteria cause symptoms, there should be a relationship between how many bacteria are in the small intestine and how severe are the person’s symptoms.

There is no such relationship.

When the number of bacteria is compared to the presence or the severity of symptoms, it is clear that they do not relate. Researchers found no correlation with the number of bacteria and how bad the patients’ symptoms were.[1]

Bacteria in the small intestine give off hydrogen and methane

The SIBO phenomenon is based on the premise that hydrogen in the breath comes from the activity of bacteria in the small intestines. A few years ago, leading experts thought this was true.

On closer scrutiny, it turns out that it never was true.

The test used to diagnose SIBO does not measure SIBO. The gasses measured in the breath do not come from the activity or the number of bacteria in the small intestines.

A recent study showed that the hydrogen present in breath tests is not related to the activity of small intestinal bacteria. Instead, it is a function of the transit time of the small intestine - how quickly food moves through it. Those with SIBO have faster transit times. Food moves too quickly through their small intestine and is dumped into the large intestine. Normal amounts of typical types of bacteria then generate hydrogen from this food. They create more hydrogen then would normally be found because the food reached them so quickly.

In another study, scientists took a group of 126 people with IBS symptoms and did jejunal aspirates on all of them and hydrogen breath tests on 80 of them. They also concluded that abnormal hydrogen breath tests related to small intestinal transit time. Their work went on to show that frequency of diarrhea could predict small intestinal transit time. It does not relate to the number of bacteria in the small intestines.

They never had too many bacteria in the small intestine.

When the number of bacteria in the small intestine were measured directly by researchers, they also found that under 3% of those with IBS had true SIBO. There was also no relationship between the number of bacteria present in the small intestines and the severity of their symptoms.[2]

The idea that IBS symptoms relate to abnormal transit times is not new. This mechanism is the reason why antispasmodic treatments like peppermint are often helpful.[3] [4]

Breath tests that measure these gases can show who has SIBO

The only solid diagnosis for SIBO at present, is culture and analysis of fluid aspirated (drained) from the jejunum - the second part of the small intestine.[5] [6]

Note that this is not a procedure that is available to patients, only research centers offer it. Biopsy of the jejunal lining is a procedure that is available during an endoscopy, however, it is not as accurate. [7]

The most popular testing in use, measures the amount of hydrogen or methane in the breath. The availability of breath testing has driven the popularity of the diagnosis of SIBO and the confusion about its treatment.

Before the test, the administrator of the test gives the person tested an oral measured dose of lactulose, glucose, xylose, or sucrose to provoke the bacteria.

SIBO Tests are not Consistent


When people with suspicious symptoms are tested for SIBO, the range can be as large as from 4% to 78%.

In healthy people displaying no digestive symptoms, the number that ‘have SIBO’ is anywhere from 1% to 40%.[8]

The ranges vary so widely due to the choice of challenge agent used, how large of a dose is given, how frequently the tests are done, how soon the tests are started, and what is defined as abnormal levels.

There are many different ways the tests are done and none are validated as being superior to others.

There is also a high rate of random variability within the same person. One study saw that 67% of people tested got different results when retested 6 weeks later even though they were given no treatment or dietary change.[9]

Other studies have shown that as many as 29% of people who fail a breath test have a normal test on a repeat without any treatment.[10]

SIBO Tests Don’t Predict Symptoms

The other assumption in the SIBO hypothesis is that SIBO tests show results that are not present in people without IBS symptoms.

The rate of abnormal breath tests is roughly the same in those with severe IBS symptoms as it is in control patients with no IBS symptoms.[11]

Just as many healthy people with no digestive problems test positive for SIBO as do people with ongoing IBS.

This variance is also a problem when it comes time to see if the treatment worked or not when there are no clear guidelines.

The conclusion of a 2017 review article entitled The Clinical Value of Breath Hydrogen Testing was:

“breath hydrogen tests contribute little value to the clinical management of patients with FBD (functional bowel disease), whether for guidance on instituting a low FODMAP diet or prescription of antibiotics for eradication of SIBO”

The next assumption in the SIBO hypothesis is that higher numbers of bacteria should cause more severe symptoms.

Antibiotics that reduce bacteria improve SIBO symptoms

Many with IBS have taken antibiotics and had a temporary reduction in symptoms. However, the rates of improvement have not been higher than placebo.

The following table shows how well certain therapies worked compared to placebo. For example, if a placebo worked 30% of the time and a diet worked 40% of the time, that would yield a 10% success rate over placebo.

Therapy                                                            Success Rate Over Placebo

Sustained-release mentha oil                          40%

*Placebo without deception                            25%

Antidepressants                                               25%

Antispasmodics                                                20%

Rifaximin                                                           9%

*A placebo without deception is a placebo given openly. Researchers told patients that the pills they were taking were placebos.

The exact wording used is:

“placebo pills made of an inert substance, like sugar pills, which have been shown in clinical studies to produce significant improvement in IBS symptoms through mind-body self-healing processes.”

In a randomized trial, they were shown to be helpful for a high percent of IBS sufferers.[12]

Just because placebos work, IBS is still real, and people who suffer from it are not deluding themselves. It means that the mind has a dramatic ability to improve the underlying, likely neurologic, issues behind IBS.

Carbohydrates have unique effects on SIBO producing bacteria

The assumption that carbs cause SIBO is behind many of the SIBO diets. These diets specifically restrict various types of prebiotics from carbohydrate that are thought to stimulate bacterial growth. In truth, bacteria in the small intestines do feed off of carbs. However, they also can feed off of dietary fats, bile acids, and short chain fatty acids.[13]

Diets that restrict foods that feed bacteria improve SIBO symptoms

Technically diets can help SIBO, but they do not work better than hypnotherapy. If there was some special effect these diets had on bacteria, hypnotherapy should not be as effective.

The most studied diet to date on SIBO symptoms is the Fodmap diet. Results have shown improvement in the 40-70% range. This sounds good until the diet is compared to placebo or hypnotherapy.

In one study, seventy-four patients were treated with hypnotherapy, Fodmap diet, or both. Improvement rate for hypnotherapy was 72%, diet was 71%, and both combined helped 73%.[14]

How do we know if an infection causes a disease? Researchers follow a set of principles called Koch’s postulates named after Robert Koch.

They are:

1.     The pathogen must be present in all cases of the disease

2.     The pathogen can be isolated from the diseased host and grown in pure culture

3.     The pathogen taken from pure culture must cause the disease when inoculated into a healthy susceptible laboratory animal

4.     The pathogen must be re-isolated from the new host and shown to be the same as the originally inoculated pathogen

In the case of an infection like strep throat, all these postulates apply. Everyone who has it has group a streptococcus. A swab can be taken and grown in culture. The bacteria from the culture can infect a new host. The new host can be shown to carry the same bacteria.

In the case of SIBO, none of these apply.

Further arguments against the SIBO hypothesis:

  • We now know that the test that popularized SIBO never measured SIBO.
  • SIBO symptoms respond to other treatments that have nothing to do with bacteria such as muscle relaxants, sedatives, and placebos
  • There is no dose dependent relationship between SIBO bacteria counts and severity of symptoms
  • The connection between any level of SIBO and digestive symptoms is not consistent. Some studies even show that IBS symptoms make the presence of SIBO less likely

  1. Grover M, Kanazawa M, Palsson OS et al. Small Intestinal bacterial overgrowth in irritable bowel syndrome: association with colon motility, bowel symptions, and psychological distress. Neurogastroenterol Motil 2008;20:998-1008.
  2. Lupascu A, Gabrielli M, Lauritano EC, et al. Hydrogen glucose breath test to detect small intestinal bacterial overgrowth: a prevalence case-control study in irritable bowel syndrome. Aliment Pharmacol Ther 2005;22:1157-1160.
  3. Ford AC, Spiegel BM, Talley NJ, et al. Small intestinal bacterial overgrowth in irritable bowel syndrome: systematic review and meta-analysis. Clin Gastroenterol Hepatol 2009;7:1279-1286
  4. Yu D, Cheeseman F, Vanner S. Combined oro-caecal scintigraphy and lactulose hydrogen breath testing demonstrate that breath testing detects oro-caecal transit, not small intestinal bacterial overgrowth in patients with IBS. Gut 2011;60:334-340
  9. Barrett J, NG PSm Muir J, Gibson P. Letter: oral fructose-breath hydrogen response, symptionsm both or neither? Aliment. Pharmacol. Ther. 2013;38:442-3.
  10. Komericki P, Akkilic-Materna M, Strimitzer T, Weyermair K, Hammer H, Aberer W. Oral xylose isomerase decreases breath hydrogen excretion and improves gastrointestinal symptoms in fructose malabsorption-a double-blind, placebo-controlled study, Aliment. Pharmacol. Ther. 2012;36:980-7
  11. Walters B, Vanner SJ. Detection of bacterial overgrowth in IBS using the lactulose H2 breath test: comparison with 14C-Dxylose and healthy controls. Am J Gastroenterol 2005;100:1566-1570
  12. Kaptchuk TJ, Friedlander E, Kelley JM, et al. Placebos without deception: a randomized controlled trail in irritable bowel syndrome. PLoS One 2010;5:e15591