Bloating: Could It Be Small Intestinal Bacterial Overgrowth?

Lilian White, MD
Posted on September 9, 2024

The gut microbiome is one of the burgeoning frontiers of medical knowledge. The composition of the microbiome affects several facets of health, including mood, weight, cardiovascular health, and others. As more research focuses on this prolific area of study, it has been confirmed that even when addressing the microbiome, it’s possible to have too much of a good thing. Small intestinal bacterial overgrowth (SIBO), although initially considered an uncommon diagnosis made largely by alternative practitioners, has tentatively made its way into mainstream gastroenterology guidelines.

SIBO occurs because of excess levels of bacteria in the small intestine. SIBO is thought to develop through risk factors such as reduced intestinal motility, hypochlorhydria, structural abnormalities, and probiotic use, among other factors, that allow or encourage more bacteria to reside in the small intestine than would under otherwise healthier conditions.

Symptoms of SIBO typically include postprandial bloating, abdominal discomfort, nausea, and a change in bowel habits (e.g., diarrhea, constipation). Of course, the differential diagnosis of postprandial bloating is broad, as outlined in this AFP article on the evaluation of gas, bloating, and belching.

Testing for SIBO is generally recommended in those at risk of SIBO before treatment, although some clinicians may choose to treat SIBO empirically. Breath tests (e.g., hydrogen breath test) are used to confirm the diagnosis. During a breath test, patients ingest a form of carbohydrate (e.g., lactulose, glucose) that is then metabolized by the gut bacteria into hydrogen gas. Patients exhale this gas into a pouch, which is evaluated for relative levels of hydrogen to diagnose SIBO. The methane breath test is another option to diagnose SIBO because of predominantly methane-producing bacteria.

Treatment of SIBO is an area of continuing development. The American College of Gastroenterology recommends the use of various antibiotics as the cornerstone of the treatment. Additionally, underlying causes of SIBO (e.g., hypochlorhydria) may be addressed as part of treatment. Treatment options such as dietary changes, probiotics, and fecal microbiota transplantation may be helpful for some patients, but more evidence is needed.

Although the American College of Gastroenterology has published a guideline on SIBO, for some gastroenterologists controversy remains over the existence of SIBO. Access to breath testing may also be limited or of uncertain quality. The diagnosis of SIBO is likely just the beginning of diagnoses and treatments related to the gut microbiome.