Small intestinal bacterial overgrowth


Small intestinal bacterial overgrowth


The definition of small intestinal bacterial overgrowth (SIBO) is undergoing constant evolution with the development of new techniques to characterize the bacterial (and viral and archaic) colonization of the intestine. Generally, the term SIBO refers to a significant increase in bacterial colonization in the small intestine. Classic causes of SIBO are considered to be changed transport through the gut, for example secondary to drugs, neurodegenerative or inflammatory diseases, or surgery, changes in stomach acidity or immune disorders. Recently, many more possible disease associations have been postulated, including functional gastrointestinal (e.g. IBS), connective tissue, neurological and psychiatric disorders. Much further work needs to be done before these possible associations are clearly proven.

Normally the bulk of bacteria are localized in the large intestine (colonic flora or microbiome) and there is a progressive decrease in the number of the bacteria and also a change in the type of bacteria towards the upper end of the gastrointestinal tract. Each individual has a unique microbiome, but general patterns in health are recognized and depend on the diet, hygienic conditions in infancy amongst other factors. Gut bacteria are involved in several important body functions, including the regulation of immunity, intestinal permeability (abnormalities often described as a “leaky gut”), vitamin production, hormone signaling and food absorption or fermentation. A changed microbiome can potentially affect all these functions, as well as result in symptoms due to fermentation of incompletely absorbed food. This leads to the formation of gases (mainly CO2, hydrogen and methane), as well as other molecules, principally short-chain fatty acids (e.g. butyric, proprionic and acetic acid).

In SIBO large numbers of bacteria are resident in the upper small intestine, where they reach the ingested sugars, proteins or fats before the body has had time to digest and absorb them. This results in the production of gases and short-chain fatty acids.

The effect and symptoms are the same as in diminished absorption or digestion of food for other reasons (e.g. lactose intolerance).

Frequency in population

The reported prevalence of SIBO varies widely, reflecting the lack of a gold standard test for diagnosis. The prevalence in IBS is between 4 and 64% and in 0-20% in healthy individuals.


The main symptoms are increased gas, abdominal pain, cramps, pressure, bloating or fullness, diarrhea or constipation, and sometimes nausea.

There is a wide range of further attributed symptoms outside of the GI tract, including the central nervous system, but these require further validation.

Testing and diagnosis

The optimal diagnostic test for SIBO remains to be determined, but currently the best non-invasive test is the glucose breath test, where the production of hydrogen and methane is repeatedly measured after swallowing a fixed dose of glucose. The lactulose breath test is also used, but is not as accurate due to the lack of distinction between a rapid intestinal passage (as in diarrhea) and bacterial overgrowth. We therefore recommend the glucose breath test for diagnosis of SIBO.


Research is currently clarifying the syndrome of SIBO and its treatment. Antibiotics, such as rifaximin, can relieve the symptoms in a certain percentage of individuals with SIBO, although symptoms may subsequently recur in a significant number. Alternatively, the use of probiotics (bacteria that can be swallowed to achieve health benefits) may be beneficial. A reduction in the amount of fermentable sugars or fat ingested will also lead to symptom improvement. The role of SIBO in IBS remains unclear, but treatment may be worth trying in selected patients with clear signs of malabsorption.