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.
Under normal circumstances, the environment of the small intestine — including the acidity, oxygen level, motility (the ability of muscles to contract, which helps determine how quickly the stomach empties), and immune cells — control microbial growth, while the bacteria compete for nutrients and living space. If acid-suppressing medications are introduced or bowel surgery or disease become part of the equation, microbes can rapidly divide and take up residence in previously inhospitable areas of the small intestine, resulting in SIBO.
Emerging research suggests that the composition — the variety and proportions of different microbes — of the intestinal microbiome may predict gastrointestinal (GI) symptoms better than the number of bacteria living in the small intestine.
It’s possible to have no symptoms at all, but SIBO often occurs with other GI diseases that have overlapping symptoms, which can complicate diagnosing either accurately. For example, antibiotics are more effective in cases of people who have both irritable bowel syndrome (IBS) and SIBO compared with IBS alone, which suggests that SIBO may play a direct role in some IBS symptoms. It could also explain the persistent GI symptoms in people with chronic pancreatitis who don’t respond to pancreatic enzyme replacement therapy.
Bloating, flatulence, nausea, abdominal pain, and diarrhea are some of the most common symptoms, but constipation may also occur in certain types of SIBO. Some theories suggest that SIBO may affect gut motility and induce constipation by altering the serotonin pathway that coordinates intestinal contractions or producing excess methane, which slows intestinal transit (the speed at which food moves through the intestines).
Anemia isn’t a primary symptom, but it has been identified as a predictor of SIBO. This may reflect a reduction in vitamin B12 absorption and could explain the fatigue people with SIBO sometimes have. SIBO is also associated with reduced nutritional status (the levels of nutrients in the body) in people with inflammatory bowel disease (IBD).
SIBO may contribute to malnutrition in IBD, Parkinson’s disease, and cirrhosis. It could also exacerbate liver disease by making the intestinal wall more permeable, which could allow bacterial contents to enter the bloodstream and cause inflammation. SIBO \also has the potential to increase the risk of bacterial translocation — bacteria moving from inside the intestine to outside tissue — in susceptible individuals, such as people with chronic liver disease, who would then be at risk of life-threatening infection.
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.
During a breath test, the patient ingests a carbohydrate — usually glucose or lactulose — which is fermented by bacteria in the small intestine. The bacteria produce hydrogen and methane, which are expelled and measured in the patient’s exhaled breath. (Hydrogen sulfide is also produced, but only in recent years has it been added to some tests.) The quantity and production rate of these gasses can then be used to estimate the number of microbes in the small intestine, because gut microbes are the only source of hydrogen and methane. However, many factors, including the way the test is administered, can affect the results and lead to a false positive or false negative diagnosis, so these tests are likely to be less accurate than the duodenal culture method.
Sampling contents from the duodenum, a section of the small intestine, is a more direct way to measure the bacteria, but it’s highly invasive. This test requires intubating the patient, which involves inserting a tube into their throat and routing it through their stomach to reach the first section of their small intestine, where the clinician will aspirate (suction out) multiple samples of liquid. Swabs of the liquid are applied to cell culture plates that support the growth of many (but not all) types of bacteria in that sample, and the bacteria are eventually counted. Culturing methods could lead to false positives or false negatives due to sample contamination or improper growth conditions, respectively.
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 103–105 colony forming units per milliliter (CFU/mL) in a fluid sample. However, some researchers or clinicians consider that number to be greater than 10^5 CFU/mL.
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. SIBO is generally considered to be cured (often referred to as being eradicated or decontaminated) based on normal breath test results.
Antibiotics such as rifaximin, neomycin, and metronidazole have an eradication rate of 50 to 70%, making them an effective treatment option in many cases. One pilot study found that antibiotics reduced SIBO-related flatulence incontinence (uncontrollable gas) more effectively than an over-the-counter gas pill containing simethicone and activated charcoal.
Fecal microbiota transplants (FMT) aren’t an established treatment yet, but the results of a recent clinical study were promising. In the double-blind, placebo-controlled, randomized study, 55 patients with SIBO were assigned to take 16 oral FMTcapsules (from five healthy donors) or a placebo every week for 4 weeks. SIBO symptoms — abdominal pain, acid reflux, indigestion, diarrhea, and constipation — were lower after one month of FMT and remained lower than baseline until the final follow-up at 6 months; the placebo group experienced no change in symptoms.
Breath tests at 6 months were normal in the FMT group but not the placebo group. Fecal analysis also showed that the microbiota of the recipients changed after FMT, becoming more similar to the donors’ (who had greater microbial diversity than the recipients). Larger studies are needed to confirm these findings, but the study design, confirmation of SIBO eradication via breath testing, and fecal analysis provide strong evidence that FMT changes the microbiota and may effectively treat SIBO.
Few high-quality studies have examined the effects of supplementation — with the exception of probiotics — on SIBO.
Probiotics are estimated to be about 60% more likely than a placebo to normalize breath tests and have also been shown to relieve abdominal pain and bloating (independent of SIBO eradication). Studies have used strains of Bifidobacterium, Lactobacillus, and Bacillus, as well as the yeast Saccharomyces boulardii — alone or in combined regimens — with no clear advantage to either one, but some researchers say that multiple strains may be more effective.
According to some studies, supplementing with a probiotic during or immediately following antibiotic treatment improved the eradication rates — reaching nearly 86% in one trial — of SIBO compared with antibiotics alone. When used in isolation, probiotics are often more effective than a placebo for reducing SIBO symptoms and normalizing breath tests, but they’re still less effective than most antibiotics.
Partially hydrolyzed guar gum is a prebiotic — a source of energy used by beneficial microbes — that can also stimulate gastric motility. In a randomized clinical trial, 77 patients with SIBO received the antibiotic rifaximin for 10 days, with or without 5 grams of the prebiotic per day. At the end of the treatment, breath tests revealed higher eradication rates (nearly 90%) in the group that added the guar gum compared with the antibiotic-only group (just over 60%). In patients whose SIBO resolved, clinical improvement — defined in this study as a decrease of at least 50% in GI symptom scores — was high (around 90%), regardless of the treatment. 
In a randomized, double-blind, controlled trial, 79 people with self-reported GI complaints took 500 milligrams of curcumin extract or a placebo daily for 8 weeks. Curcumin has been associated with improvements in IBS and changes to the microbiome in preliminary research. GI symptom severity scores improved slightly in both groups, but a greater reduction was seen with curcumin supplementation. However, the supplement had no effect on microbiome diversity or breath tests, indicating that it didn’t affect SIBO.
In a small pilot study of 24 individuals with SIBO and functional dyspepsia, researchers observed less methane in breath tests and lower dyspepsia scores after two months of supplementing with 300 milligrams of ursodeoxycholic acid per day. This secondary bile acid — a product of the liver that has been modified by gut microbes — has antimicrobial properties, which could explain its effects. However, the participants knew they were receiving the supplement, and there was no placebo control, so more research is needed.
Daikenchuto, a traditional Japanese herbal preparation, was studied in another small, uncontrolled study without blinding. Limited prior evidence suggested that this blend of processed ginger, panax ginseng, and Japanese pepper could stimulate gut motility and speed up the time it takes food to make its way through the gut. However, two weeks of supplementation failed to normalize breath tests in the four participants who’d tested positive at the beginning of this study. All 10 participants reported improvements in bloating, indigestion, and constipation, but more data from randomized, placebo-controlled trials is needed.
A retrospective chart review evaluated serum-derived bovine immunoglobulin/protein isolate in 24 patients with IBS who were unresponsive to standard treatments; 13 of them also had SIBO. Though a majority of the patients with or without SIBO were reported to have a “good clinical response” (as opposed to no clinical response, which was the only alternative), the improvement was only significant in patients without SIBO.
Another retrospective chart review claimed that antimicrobial herbal preparations were as effective as the antibiotic rifaximin for normalizing breath tests in patients with SIBO, but the researchers failed to adjust their statistics for the loss of participants at follow-ups, and the patients weren’t blinded or randomized, since they chose their treatment. This limits conclusions about the herbs’ causative effect on SIBO reduction.
Antibiotics are the most effective treatment option for SIBO, and certain probiotics and partially hydrolyzed guar gum may enhance the efficacy. FMT is the most promising treatment alternative, but more research is needed. Firm conclusions about other supplements can’t be made based on the quantity and quality of current evidence.
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 might reduce some of the symptoms associated with SIBO, but their effects on the condition itself are unknown.
Elemental diets consist of hydrolyzed formulas, meaning they have been somewhat broken down to make digestion easier. The formulas contain amino acids and simple sugars that require little to no digestion before they’re absorbed, which reduces the workload of a GI tract compromised by disease. Early research from the 1970s found that elemental diets led to reductions in gut microbes, likely due to the nutrients being absorbed quickly and containing no microbe-accessible carbohydrates. Recently, some researchers have proposed that an elemental diet could treat SIBO by “starving” intestinal bacteria, but this hasn’t been formally studied.
A retrospective chart review — a type of research that looks at a patient’s history before and after treatment — was used to determine the effects of an elemental diet on SIBO. After 2 weeks on an elemental diet, breath tests were normal in 74 of the 93 patients with IBS and a prior SIBO diagnosis. The remaining 19 patients followed the elemental diet for an additional week, but only five had a normal breath test afterward. Firm conclusions can’t be drawn from this information, given the lack of randomization, blinding, and control groups.
A low-FODMAP diet is thought to improve GI symptoms such as bloating, abdominal pain, and gas by reducing the content of fermentable and water-attracting carbohydrates in the intestines. This results in lower intestinal water content and diminished microbial gas production.
Several studies have shown that it leads to reductions in Bifidobacteria (largely regarded as beneficial microbes), but the clinical significance of these changes is unknown. The diet is often recommended for individuals with diarrhea-predominant IBS but hasn’t been studied as a treatment for SIBO.
Low-sugar diets have been proposed as a treatment for bacterial or fungal overgrowth, but evidence doesn’t support these claims.
An open-label pilot study (a small study where the researchers and participants know what is being tested) assessed the effects of a 4-week starch- and sucrose-reduced diet compared with the regular diets of 105 women with IBS. Participants were encouraged to increase their intake of vegetables, meat, fish, dairy, and certain fruits while avoiding all sucrose-containing foods and swapping refined carbohydrates (such as white bread) for whole grain alternatives. These dietary modifications reduced the women’s calorie, starch, and sucrose intake while upping the amount of fat and protein in their diets. They reported less abdominal pain, bloating, and flatulence at the end of the 4 weeks. However, their SIBO status was never determined.
One 24-week randomized controlled trial studied 52 participants with chronic fatigue syndrome presumably caused by a form of gut imbalance characterized by fungal overgrowth. Participants were assigned one of two diets: a low-sugar, low-yeast diet, which eliminated caffeine and all foods containing sugar, refined carbohydrates, or yeast, or a healthy eating intervention that encouraged more fruits, vegetables, and fish and fewer refined carbohydrates and fat. The diets led to similar improvements in ratings of fatigue and general health, but this study didn’t assess potential fungal or bacterial overgrowth or GI symptoms.
The Specific Carbohydrate Diet (SCD) is thought to control the growth of intestinal flora by reducing the amount of microbe-accessible carbohydrates that reach the intestines. It appears to improve the symptoms of IBD similarly to other prudent dietary patterns, such as a Mediterranean-style diet. The SCD hasn’t been studied in patients with SIBO, however, and its effects on the microbiome are unclear due to great variability between the few participants who were analyzed after following the diet.
Interestingly, a small pilot study sought to determine the effects of a low-fiber, high-sugar intervention on GI symptoms in healthy individuals, some of whom had asymptomatic SIBO. For one week, 16 participants followed a diet that was very low in fiber (<10 grams per day), with over 50% of their daily carbohydrates coming from simple sugars. Thirteen of the participants experienced GI distress that subsided within a week of returning to their habitual diets, which provided about 23–27 grams of fiber per day.) The researchers noted that the symptoms were unrelated to SIBO, however, leaving the question of a relationship between sugar intake and SIBO unanswered.
Replacing refined carbohydrates with whole grains, fruits, and vegetables could improve some GI symptoms, but it’s unclear what effect this would have on SIBO. High-FODMAP foods and sugary foods could cause digestive distress regardless of SIBO. More research is needed to determine whether an elemental diet can treat the condition, because early research was observational.
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
Functional dyspepsia (commonly known as indigestion), IBS, IBD, diabetes (types 1 and 2), dysregulated gut motility (food moving more slowly through the intestines due to abnormal muscular contractions), chronic pancreatitis, chronic liver disease, Parkinson’s disease, and systemic sclerosis (an autoimmune disorder) are all associated with greater odds of a positive SIBO diagnosis. Limited evidence also suggests that females with IBS may be more likely to have SIBO compared with males who have IBS.
A history of GI surgery (such as gastric bypass or resectioning of the intestines), the use of acid-reducing proton-pump inhibitors (PPIs), and smoking are also associated with a greater likelihood of having SIBO.
Though celiac disease isn’t linked to an increased chance of having SIBO, one analysis observed higher rates of SIBO in people with celiac disease who were unresponsive to a gluten-free diet compared with healthy controls. However, most of the analyses detected inconsistencies and issues with the quality of studies used to determine the prevalence of SIBO in these conditions.