SIBO and IBS: Is Bacterial Overgrowth Really the Cause?
TL;DR
SIBO is real, and it turns up more often in people with IBS than in healthy people. What does not follow is the popular claim that IBS is usually undiagnosed SIBO. The association is genuine but the tests used to measure it are not accurate enough to support that diagnosis in most IBS patients, and improving on an antibiotic does not prove bacterial overgrowth was the cause. SIBO becomes a serious consideration when there is a specific reason bacteria are not being cleared from the small intestine.
SIBO stands for small intestinal bacterial overgrowth. It helps to start with what normally keeps bacteria out of the small intestine, because SIBO is a failure of those defences rather than an invasion.
The colon is meant to be crowded. The small intestine, particularly its upper sections, is meant to be relatively sparse. Several things maintain that gap. Stomach acid cuts down the microbes arriving from above. Bile and pancreatic secretions make the environment inhospitable. The ileocaecal valve limits colonic contents washing backwards. And normal intestinal movement keeps everything travelling downstream.
Between meals a further mechanism takes over. The migrating motor complex sends waves of contraction through the stomach and small intestine during fasting, sweeping residual food and bacteria towards the colon. It is sometimes described as the gut’s housekeeping wave, and when it fails, material sits still long enough for bacteria to accumulate.
Small intestinal bacterial overgrowth is what happens when excessive numbers, or an abnormal composition, of bacteria in the small intestine start causing problems.
Those bacteria ferment carbohydrate earlier in the journey than they should. That produces gas and other microbial by-products inside the small bowel, which can register as bloating, distension, discomfort and diarrhoea. In more severe cases the effects go past symptoms: bacteria alter bile acids and damage the absorptive surface, so some people develop fat malabsorption, weight loss or vitamin B12 deficiency.
Which creates the problem this whole article is about. Every symptom in that list also appears in IBS.
Can you tell SIBO and IBS apart from symptoms?
No. There is no symptom pattern, stool test or microbial signature that reliably separates SIBO from IBS in an individual patient.
Bloating, abdominal discomfort, gas and altered bowel habits occur in both. SIBO is associated more with diarrhoea, while excess methane is associated with slower transit and constipation. IBS itself can present as diarrhoea, constipation or a mixture. There is no clean dividing line to be drawn from what a person reports.
Terrible bloating does not mean SIBO. Pain and diarrhoea do not rule it out.
A 2024 study from Peking University reported that the IBS group had more severe abdominal pain and more frequent diarrhoea. The faecal microbiome differed between groups too, with opportunistic bacteria enriched in the IBS group correlating with pain and bloating, and gas-producing bacteria more prominent in the SIBO group.
Interesting, and not yet usable.
Two things limit it. The SIBO group was defined by a lactulose breath test, which produces false positives when the test solution reaches the colon quickly. And the microbiome analysis came from stool, which describes the colon rather than the small intestine. The authors list both as limitations of their own study.
So the two conditions may not be biologically identical. But there is still no symptom pattern, stool test or microbial signature that separates them in an individual patient.
So is SIBO the cause of IBS?
For some people SIBO may be a real contributor. For IBS as a whole, the evidence does not support SIBO as the cause.
A meta-analysis by Uday Ghoshal and colleagues (Journal of Gastroenterology and Hepatology, 2020) found that 36.7% of IBS patients tested positive for SIBO, and that IBS patients were 2.6 times more likely than healthy controls to test positive on a glucose breath test, and 8.3 times more likely on jejunal aspirate culture. That association is solid enough that the 2023 Indian consensus statements on IBS list SIBO among the peripheral mechanisms that may contribute.
But an association leaves several explanations standing. A previous gut infection could have altered motility, immunity and the microbiome in ways that produce both. Or the two could simply coexist without either explaining the other.
When SIBO becomes genuinely plausible
SIBO becomes genuinely plausible when there is an identifiable reason bacteria are not being cleared from the small intestine.
Classic SIBO is associated with anything that slows or disrupts intestinal movement, or with anatomy that lets contents stagnate. Small bowel surgery, strictures, resection of the ileocaecal valve, scleroderma and other motility disorders all qualify.
This version of SIBO is not controversial. It has a mechanism, a plausible symptom set including signs of malabsorption, and a diagnosis that predates the IBS debate by a century.
What is contested is the extension of that diagnosis to large numbers of otherwise typical IBS patients on the strength of a breath test. The 2024 clinical practice update from Purna Kashyap and colleagues, endorsed by both the European Society of Neurogastroenterology and Motility and the American Neurogastroenterology and Motility Society, is blunt about the consequences: after two decades the SIBO-IBS hypothesis remains unproven.
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How SIBO is treated
Treatment has two jobs: reducing the overgrowth and working out why it developed.
The first job is reducing the overgrowth, usually with antibiotics, most often rifaximin. Other regimens are chosen depending on the clinical picture and the gas pattern, particularly where methane is involved.
Studies differ in who was enrolled, how SIBO was diagnosed, which antibiotic and dose were used, and whether success meant a better breath test or a person feeling better. Across the antibiotic literature collected by Kashyap and colleagues, eradication or breath-test normalisation rates range from 7% to 100%, with symptom response rates just as scattered, and two placebo-controlled trials found no benefit over placebo at all.
That inconsistency is itself the finding. Some studies do show breath-test normalisation tracking symptom improvement; others show the two coming apart completely. The literature does not currently support a confident statement in either direction.
The second job is working out why the overgrowth developed. If impaired motility or an anatomical problem created the conditions, killing the bacteria once does not remove the reason they gathered. That is a large part of why SIBO recurs.
Does a low-FODMAP diet treat SIBO?
A low-FODMAP diet treats symptoms. Whether it treats the overgrowth is a separate question with much weaker evidence behind it.
Reducing fermentable carbohydrate reduces the substrate available for fermentation, so gas, bloating and diarrhoea can all improve.
So when someone with suspected SIBO feels better on a low-FODMAP diet, the honest description is symptom improvement, not bacterial eradication. And as covered in the separate low-FODMAP article, the restriction phase is not meant to become permanent.
What to take from all this
SIBO is real. The overlap with IBS is real. The leap from those two facts to “IBS is usually undiagnosed SIBO” is where the evidence stops supporting the story.
The reason that leap keeps getting made is the breath test, which produces a number that looks like a diagnosis and often is not. That is worth understanding properly before building an identity around a result.
A SIBO test should answer a clinical question. It should not become an explanation for every bloated stomach.
Frequently asked questions
Is SIBO the same as IBS?
No. IBS is a disorder of gut-brain interaction diagnosed from a recurring pattern of abdominal pain or discomfort and altered bowel habits. SIBO is small intestinal bacterial overgrowth: excessive or abnormal microbial growth in the small intestine. They produce similar symptoms and can coexist, but they are not interchangeable diagnoses.
What are the main symptoms of SIBO?
Bloating, excess gas, abdominal discomfort and diarrhoea are most common. Constipation occurs too, particularly where methane production is involved. More severe SIBO can contribute to malabsorption, weight loss and deficiencies such as vitamin B12, but these are not present in everyone.
What causes SIBO?
Anything that stops bacteria being cleared from the small intestine. Impaired motility, small bowel surgery, strictures, scleroderma and removal of the ileocaecal valve are the classic causes. Reduced stomach acid, including from long-term acid-suppressing medication, appears in parts of the literature.
Does rifaximin working prove I had SIBO?
No. Rifaximin improves IBS-D symptoms in people who were never shown to have SIBO, and in at least one randomised trial it worked in patients who had all tested negative. Symptom improvement after an antibiotic does not establish what the antibiotic acted on.
Can SIBO come back after treatment?
Yes, particularly if whatever allowed the overgrowth in the first place is still there. Recurrent symptoms should prompt a review of the diagnosis as well as the underlying mechanism, rather than another automatic course of antibiotics.
Does a low-FODMAP diet cure SIBO?
No. There is good evidence it reduces symptoms in IBS and much weaker evidence that it eradicates bacterial overgrowth. Feeling better on the diet is not proof the overgrowth has gone.
Internal links
- Down to: SIBO breath tests, why lactulose and glucose disagree (/sibo-breath-test/), linked from the “When SIBO becomes genuinely plausible” section and the closing
- Sideways: low-FODMAP diet for IBS (/low-fodmap-diet-ibs/), linked from the diet section
- Up to: the IBS pillar page
- Sideways: IBS trigger foods, and IBS medication, both from the treatment section
Image briefs
Lead image. Cross-section of the small intestine and colon shown as two zones of very different microbial density, with the four defences labelled: gastric acid, bile and pancreatic secretions, the ileocaecal valve, and motility. Forest background, Emerald and Gold, flat vector, no anatomical realism.
Alternative lead. Two overlapping circles of symptoms, IBS on one side and SIBO on the other, with the shared centre taking up most of the area. The visual argument that symptoms cannot separate them, made before the reader has finished the first paragraph.
In-body diagram. The migrating motor complex as a sweeping wave, paired with a second panel showing what stagnation looks like when the wave fails. Placed in the “What SIBO actually is” section.
