Small Intestinal Bacterial Overgrowth (SIBO) is a condition where an unusually high number of bacteria colonize the small intestine, a region that naturally has a low bacterial concentration. This microbial imbalance interferes with normal digestive processes, leading to uncomfortable symptoms. The misplaced bacterial community often results in excessive gas production, which causes common complaints like burping, abdominal bloating, and chronic digestive distress. SIBO is recognized as a major factor driving symptoms that frequently overlap with Irritable Bowel Syndrome (IBS).
Defining SIBO and the Mechanism of Symptom Generation
Small Intestinal Bacterial Overgrowth occurs when the body’s protective mechanisms fail, allowing bacteria from the large intestine to migrate upward and proliferate in the small intestine. The small intestine is designed primarily for nutrient absorption, not for the extensive fermentation that occurs in the colon. When bacteria inhabit this wrong location, they rapidly consume and ferment ingested carbohydrates and fibers that the human body cannot fully digest.
This microbial consumption produces gaseous byproducts within the small intestine, causing the hallmark symptoms of SIBO. The primary gases generated are hydrogen and methane, though some bacteria also produce hydrogen sulfide gas. The rapid creation of these gases shortly after eating leads to a build-up of pressure inside the small intestine.
The type of gas produced influences the specific symptoms. Hydrogen-dominant SIBO is often associated with diarrhea. Methane gas, produced by single-celled organisms called archaea (a condition termed Intestinal Methanogen Overgrowth or IMO), slows down intestinal transit time, leading to constipation.
Common Digestive and Systemic Manifestations
The most noticeable effects of SIBO relate to the pressure created by excessive gas production. Burping (eructation) is a common attempt by the body to vent this trapped gas from the upper digestive tract. Bloating is a frequent and often severe symptom, resulting from the gases physically distending the walls of the small intestine.
This distension can lead to significant abdominal pain and cramping, as the smooth muscle of the intestinal wall is irritated. SIBO disrupts normal bowel habits, causing persistent diarrhea, chronic constipation, or alternating patterns of both, depending on the dominant gas produced. Patients often report persistent nausea or a feeling of uncomfortable fullness after meals.
The overgrowth also interferes with the body’s ability to absorb nutrients, leading to systemic issues outside of the digestive tract. Bacteria consume essential nutrients, most notably Vitamin B12 and iron, which can cause fatigue, weakness, and anemia. This malabsorption may also contribute to unintended weight loss, brain fog, and chronic fatigue.
Identifying Underlying Causes of SIBO
SIBO is rarely a primary disease, but rather a consequence of an underlying issue that compromises the small intestine’s natural defense mechanisms. A primary protective factor is the Migrating Motor Complex (MMC), a “housekeeping” wave of muscular contractions that sweeps residual food and bacteria out of the small intestine between meals. Impaired MMC function, often due to a prior gut infection, allows bacteria to linger and multiply.
Anatomical abnormalities can also predispose an individual to SIBO by creating areas of stagnation where bacteria can flourish. Examples include strictures, diverticula, or changes to the gastrointestinal tract resulting from prior abdominal surgery. These structural issues prevent the efficient clearance of contents, providing an environment for bacterial colonization.
A reduction in stomach acid is another risk factor, as gastric acid acts as a barrier, sterilizing food and killing bacteria before they reach the small intestine. Conditions causing low stomach acid, or the long-term use of acid-suppressing medications like Proton Pump Inhibitors (PPIs), weaken this barrier. Addressing these root causes is crucial, as treating only the overgrowth often leads to recurrence.
Confirming SIBO Through Testing
Because SIBO symptoms overlap with many other common digestive disorders, diagnostic testing is necessary to confirm the condition. The most widely used non-invasive method is the Lactulose or Glucose Breath Test. This test is based on the principle that the excessive bacteria in the small intestine will produce gas when exposed to a specific sugar substrate that is ingested by the patient.
During the test, the patient consumes a sugar solution (lactulose or glucose) after a period of dietary preparation. The bacteria ferment this substrate, producing hydrogen and/or methane gases that are absorbed into the bloodstream and exhaled through the lungs. The patient collects breath samples at regular intervals, typically every 15 to 20 minutes over three hours.
An early rise in exhaled hydrogen or methane gas indicates that fermentation is occurring too high up in the digestive tract, confirming bacterial overgrowth. The results provide data on the specific gas type and concentration, which helps guide the appropriate therapeutic approach. A full clinical assessment by a healthcare provider remains important for interpreting the results and understanding the patient’s overall health picture.