Small intestinal bacterial overgrowth (SIBO) is a form of dysbiosis (or abnormal gut microbiota) characterized by an excessive amount of bacteria in the small intestine, sometimes due to elevations in bacteria that are usually found only in the large intestine. It is considered both a clinical syndrome and a potential mechanism that could contribute to the progression of other diseases.^[1]

Normally, the environment of the small intestine — including the acidity, oxygen level, motility (the ability of muscles to contract), and immune cells — control microbial growth. If this environment is modified by acid-suppressing medications, surgery, or disease, microbes can rapidly divide and take up residence in previously inhospitable areas of the small intestine, resulting in SIBO.^[2]

SIBO may have no symptoms, but commonly include:^[1][3]

• Bloating
• Flatulence
• Nausea
• Abdominal Pain
• Diarrhea/constipation^[3]

SIBO often occurs with other GI diseases that have overlapping symptoms, which can complicate diagnosing either accurately.^[4]

SIBO can be diagnosed with breath tests or a duodenal aspirate culture (a lab test done on a small fluid sample from the small intestine), but the lack of standardization makes accurate diagnosis challenging. Breath tests are an indirect way to measure bacteria in the small intestine, but their ease and low invasiveness make them a more common diagnostic tool than the fluid test.^[5][6]

Clinicians have yet to reach a consensus on diagnostic criteria for a positive result of either test, but it’s generally accepted that SIBO is present when there is a concentration of 10³ to 10⁵ colony forming units per milliliter (CFU/mL) in a fluid sample. ^[5][6]

SIBO is conventionally treated with certain classes of antibiotics that are poorly absorbed — unlike systemic antibiotics, which easily enter circulation — so they act primarily in the intestines. This leads to fewer side effects and lower chances of antimicrobial resistance, which leads to potentially deadly microbes becoming unaffected by antibiotics.^[7] SIBO is generally considered to be cured (often referred to as being eradicated or decontaminated) based on normal breath test results.^[8][5]

Fecal microbiota transplants (FMT) aren’t an established treatment yet, but there is promising preliminary research. ^[9]

Probiotics may be useful for treating SIBO, but it’s not clear which strains (or combinations of strains) are optimal.^[8][10] Some research has also shown good results from probiotics used alongside during or immediately after antibiotic treatment.^[11][8]

Other treatments, such as prebiotics, curcumin, ursodeoxycholic acid, daikenchuto (a traditional Japanese herbal preparation), Serum-derived bovine immunoglobulin/protein isolate, and some antimicrobial herbs have also been studied, but there isn’t enough evidence to recommend their use.

Very little research has been done on dietary interventions for SIBO specifically, so most recommendations are based on weak or anecdotal evidence. Some dietary patterns, such as the low-fodmap, low-sugar or specific carbohydrate, or elemental diets may reduce some of the symptoms associated with SIBO, but their effects on the condition itself are unknown.^{[12][13] [14][15]}

Although SIBO hasn’t been identified as a definitive cause or consequence of any other disease, it is associated with a number of conditions that create an abnormal intestinal environment, which could include:

• A less acidic (more hospitable) small intestine
• Slower (more accessible) transit of food through the GI tract
• Abnormal muscular contractions that push contents (and microbes) backward, from the large to the small intestine
• Reduced immune activity that would normally regulate microbial growth

It is thought that these factors create a nutrient-rich environment that lacks the normal mechanisms for controlling microbial growth, and SIBO develops as a result.^[2][16]