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SIBO

Nov 9, 2023 18 min readBy Entegro Health

Bacteria are supposed to live in your large intestine. When they set up shop further upstream, in the small intestine, you get SIBO — and a set of symptoms that look like half a dozen other things. Here is what it is, how it is actually diagnosed, and what the evidence says works.

Woman sitting on the edge of a bed in soft morning light, curled forward with her arms pressed against her stomach

Small intestinal bacterial overgrowth is one of those conditions that sounds self-explanatory and turns out not to be. The name says bacteria in the wrong place. What it does not say is that the tests disagree with each other, that half of what gets blamed on it is not it, and that some of the gas on a breath test is not made by bacteria at all.

This is the long version. Some of it is unflattering to the supplement category this company sells in.

Why the small intestine is supposed to be nearly empty

Green plants crowding up through the square openings of a paving-stone grid, growing where they were never meant to

The small intestine is the longest stretch of the digestive tract — roughly six meters of it — and yet very few bacteria live there. A healthy upper small intestine carries fewer than 1,000 bacteria per milliliter. The colon carries hundreds of billions per milliliter.^1,2 That is a difference of about a billion-fold, across a single valve.

Several defenses keep it that way:

Stomach acid kills most of the bacteria that arrive in food.
Bile acids — made in the liver, stored in the gallbladder, released into the duodenum — behave like a detergent on bacterial membranes.
Pancreatic enzymes break down bacterial products.
The migrating motor complex, a wave of muscular activity that sweeps the small intestine roughly every 90 to 120 minutes between meals, clearing out debris. It is sometimes called the housekeeper wave, which is exactly what it does.
The ileocecal valve at the junction with the colon, which stops colonic bacteria washing backwards.

Take away enough of those — usually by slowing things down — and bacteria that behave perfectly well in the colon start colonizing a stretch of gut that was never built for them.

It is the most beautiful plant in the garden. It is still a weed if it is growing in the wrong bed.

Once they are established, the trouble comes from three directions. Bacteria strip the chemical conjugation off bile acids, and the resulting free bile acids can no longer package fat for absorption — and are themselves irritating to the intestinal lining. Biopsies in bacterial overgrowth show blunted villi, a thinned mucosa and increased intraepithelial lymphocytes.² Bacteria also consume nutrients before you can absorb them. And separately from all of that, fermenting carbohydrate produces gas, which is what actually causes the bloating.

Hydrogen, methane, and why the name is changing

A breath test looks for two gases. They come from two different kinds of organism, and that distinction is not a technicality.

Hydrogen is produced by bacteria. That is SIBO proper.

Methane is produced by archaea — a separate domain of life from bacteria — chiefly a species called Methanobrevibacter smithii. The American College of Gastroenterology puts it bluntly: "Methane is produced not by bacteria, but by archaea. Archaea are prokaryotic organisms and represent the third domain in the 3-domain system of life." And so: "The term IMO may be more appropriate than ‘SIBO’ or ‘methane-SIBO.’"¹

IMO — intestinal methanogen overgrowth

Methane-predominant overgrowth is a distinct condition with a distinct clinical signature: it is associated with constipation, where hydrogen-predominant SIBO is more associated with diarrhea.¹ It also responds to different treatment, which is the practical reason the distinction matters.

More recently a third pattern has been described, driven by hydrogen sulfide-producing bacteria such as Fusobacterium and Desulfovibrio, and associated with diarrhea-predominant IBS.³ Three-gas breath testing is now commercially available, though it post-dates the guideline.

How common is it?

Nobody has a reliable general-population figure, and that is itself the honest answer. The estimate depends enormously on which test you use.

What we do know is that it clusters in particular groups:

Irritable bowel syndrome: about 38% pooled, with roughly a five-fold higher odds than healthy controls.⁴ A second meta-analysis put it at 36.7%.⁵
Inflammatory bowel disease: about 31%, with a five-fold higher odds than controls.⁶
Parkinson’s disease: about 46%.⁷

The number that should make you cautious about all the other numbers

In the same population of IBS patients, SIBO prevalence was 40% by breath test and 19% by culture.⁴ Same people, same condition, half the answer. Any prevalence figure you read is partly a statement about the test that produced it.

What raises the risk

A doctor reviewing a patient’s medical file with her during an appointment

The guideline frames this in a way worth quoting, because it reorganizes how you should think about the whole condition: "In almost all instances, SIBO is an epiphenomenon related to something else, usually a condition that leads to stasis in the small intestine."¹

In other words, SIBO is usually a symptom of something upstream. Anything that slows transit, obstructs flow, or removes a defense is a candidate:

Motility disorders and gastroparesis — gastroparesis carried odds of 3.4 in one large consecutive cohort.⁸
Previous abdominal surgery, including blind loop syndrome and bowel resection. The guideline names it explicitly as a reason to test.¹
Structural problems — strictures, adhesions, small bowel diverticulosis. (Note: small bowel diverticulosis, which is rare. Colonic diverticulosis, the common kind, is not an established risk factor.)
Thyroid disease — hypothyroidism carried odds of 2.6, and levothyroxine therapy odds of 3.0, the single strongest contributor in that cohort’s simplified model.⁸
Medications that slow the gut — opioids, antidiarrheals, anticholinergics.¹
Diabetes with autonomic neuropathy, scleroderma, cirrhosis, chronic pancreatitis and Crohn’s disease — all classic causes of stasis or altered anatomy.^1,6
Advanced age and female sex are both associated with higher likelihood.¹

The acid question

Low stomach acid, including from long-term acid-blocking medication, appears on every risk-factor list on the internet. The evidence is genuinely split, and the split is instructive.

One meta-analysis of 11 studies found PPI users roughly twice as likely to have SIBO — but only in studies diagnosing by culture (odds 7.6). Studies using the glucose breath test found no relationship (odds 1.93, not significant), and the authors flagged possible publication bias.⁹ A larger meta-analysis of 8,398 IBS patients found no association at all (odds 1.1).⁴

The ACG’s own position is to suggest against breath testing for SIBO in asymptomatic people on PPIs.¹ Which is a reasonable place to land: do not stop a prescribed acid-blocker on the strength of a contested association.

Symptoms — and the caveat that comes with all of them

Woman curled up on a sofa in a blue knit sweater, holding her stomach through a wave of cramping

The guideline is specific: "abdominal pain, bloating, gas, distension, flatulence, and diarrhea are the most common symptoms described in patients with SIBO and prevalent in more than two-thirds of patients." And the headline finding: "The most common symptom of SIBO is bloating."¹

Constipation belongs on the list too, but attached to the right condition — it is associated with methane and IMO rather than hydrogen-predominant SIBO.¹

No single symptom is specific to SIBO

This is the guideline’s own caveat, and it is the reason this article cannot be used as a self-diagnosis checklist.¹ Bloating, pain and altered bowel habit are the common currency of a dozen conditions, several of them more serious than SIBO. Testing exists precisely because the symptoms do not settle the question.

Nutrients: what actually happens

Two mechanisms, both worth getting right.

Fat and the fat-soluble vitamins. Bile acids arrive conjugated to glycine or taurine, which is what lets them form the micelles that carry fat to the absorptive surface. Bacteria strip that conjugation off. The free bile acids can no longer do the packaging job, so fat passes through — pale, greasy, foul-smelling stools — and vitamins A, D, E and K go with it.²

B12 down, folate up. Vitamin B12 is absorbed at the very end of the small intestine. Bacteria sitting upstream take it up competitively before you can.² Folate does the opposite: small-bowel bacteria make folate, so folate is often normal or raised.² Low B12 alongside high folate is a pattern clinicians recognize, and it is the neatest demonstration that this is competition rather than general malfunction.

Worth keeping in proportion, though. The guideline states that "vitamin deficiencies in SIBO are not common and are usually seen in patients with an iatrogenic or structural abnormality" such as blind loop syndrome.¹ In most cases the deficiencies are subtle or undetectable.

Testing

A laboratory scientist handling test tubes beside an analysis machine

Two methods, and neither is as good as you would like.

Breath testing

You drink a measured dose of sugar and your breath is sampled over roughly two hours. The consensus doses are 75 g of glucose or 10 g of lactulose — dose standardization matters more than people assume.^1,10

The thresholds:

SIBO: a rise in hydrogen of ≥20 ppm above baseline within 90 minutes.^1,10
IMO: methane at ≥10 ppm at any point during the test.^1,10

Both guidelines accept either sugar. The performance data favor glucose:

Glucose breath test — pooled sensitivity 54.5%, specificity 83.2%.¹¹
Lactulose breath test — pooled sensitivity 42.0%, specificity 70.6%.¹¹

Read those numbers again. Both tests miss roughly half of true cases. A negative breath test does not clear you.

Lactulose has a specific weakness. Because it is not absorbed, it travels the entire length of the gut to the colon, where colonic bacteria ferment it and produce a hydrogen rise. If your transit is fast, that colonic peak arrives inside the diagnostic window and gets read as small-bowel overgrowth. The effect is measurable: in Parkinson’s patients, prevalence came out at 51% by lactulose against 35% by glucose in the same disease.⁷ In one IBS meta-analysis, patients were significantly more likely than controls to test positive by glucose and by culture — but not by lactulose, which failed to discriminate cases from controls at all.⁵

Aspirate and culture

The more direct method is sampling fluid from the small intestine during an endoscopy and culturing it. The diagnostic threshold has moved: ≥10³ CFU/mL is now considered diagnostic, replacing the older 10⁵ standard, which the guideline describes as too stringent and lacking validation.¹ It is harder to do, and contamination on the way in and out is a real problem.

Breath tests are offered both through gastroenterology clinics and by standalone commercial laboratories that sell direct-to-consumer kits.²² If you go the direct route, take the result to a clinician rather than acting on it alone — given the sensitivity figures above, interpreting one in isolation is how people end up on unnecessary diets. Mayo Clinic keeps a readable patient-facing overview of the condition if you want general background.²¹

Preparing for a breath test

No antibiotics for 4 weeks. No prokinetics or laxatives for at least 1 week. No fermentable foods the day before. Fast for 8 to 12 hours. During the test, no smoking and minimal physical exertion.¹

Probiotics are also generally paused beforehand — check with whoever is running your test.

Treatment

The ACG guideline makes exactly one graded treatment recommendation, and it is for antibiotics: "We suggest the use of antibiotics in symptomatic patients with SIBO to eradicate overgrowth and resolve symptoms."¹

The usual agent is rifaximin, and it is worth understanding why it is not the blunt instrument people picture. Less than 1% of it is absorbed into the bloodstream — it works locally in the gut lumen rather than systemically. A meta-analysis of 32 studies covering 1,331 patients found an eradication rate of 70.8%, with symptom improvement in 67.7% of those eradicated and adverse events in 4.6%. The authors are candid that the underlying study quality is generally poor and well-designed trials are still needed.¹²

For IMO, rifaximin alone performs badly — 28% methane eradication — while rifaximin combined with neomycin reached 87%, against 33% for neomycin alone.^13,1 That study was a small retrospective review, so treat the precision with appropriate suspicion, but the direction has held up.

Diet

A simple plate of chicken meatballs with roasted zucchini, cherry tomatoes, eggplant and red pepper

Diet is a supporting measure here, not the primary treatment — the guideline makes no dietary recommendation at all, and is open about why. The dominant principle is reducing fermentable carbohydrates, but the data are "principally extensions of the data from IBS," where low-FODMAP diets themselves rest on what a meta-analysis called "very low quality evidence."¹

The one dietary approach with a serious eradication number is an elemental diet — an exclusive liquid formula. In a study of 93 patients, 80% had a normal breath test after 14 days, rising to 85% by day 21, and those who normalized reported a 66% improvement in bowel symptoms against 12% in those who did not.¹⁴ That is the highest eradication figure in the SIBO literature, higher than rifaximin. It also came from an open-label study with no control arm, and two to three weeks on an exclusive liquid formula is genuinely hard. It belongs under medical supervision.

And the thing that actually prevents recurrence

The guideline is direct about this: "A focus on prevention of SIBO is important to avoid the need for repeated courses of antibiotics. Treatment of the underlying cause represents the primary mode of prevention."¹

Since SIBO is usually downstream of something else, the way to stop it coming back is to identify and address that something — the motility problem, the stricture, the thyroid disease, the medication slowing your gut. General advice to eat more fiber is not evidence-based here, and while an overgrowth is active, fermentable fiber is substrate for the bacteria causing the problem.

A caution about probiotics and SIBO

Read this before you take a probiotic for SIBO

The ACG guideline’s position on probiotics in SIBO is not an endorsement. It states: "There is a lack of consistent data to support recommending specific probiotics in the treatment of SIBO." Probiotics receive no recommendation of any grade.¹

The guideline also carries a harm signal: probiotics "may cause SIBO and D-lactic acidosis leading to gas and bloating."¹

The study behind that: 38 patients evaluated for brain fog, gas and bloating. All 30 in the brain-fog group were taking probiotics. SIBO was present in 68% of them versus 28% of those without brain fog, and D-lactic acidosis in 77% versus 25%. After stopping probiotics and a course of antibiotics, symptoms resolved in 77%.¹⁵

The proposed mechanism is that Lactobacillus species ferment carbohydrate into D-lactate, an isomer humans clear poorly.

Keep the size of this in view: 38 patients, observational, uncontrolled, and its conclusions were publicly contested by other researchers. It is not proof of harm. But one of its authors co-wrote the ACG guideline, and the guideline carries the warning forward — so it is not fringe either.

Practically: if your bloating or mental clarity got worse after starting a probiotic, stop and raise it with your doctor rather than pushing through.

What the probiotic evidence does show

The best available synthesis is a 2017 meta-analysis pooling 18 studies. Its findings, in full rather than in part:

Decontamination: the pooled SIBO clearance rate among people given probiotics was 62.8% (95% CI 51.5–72.8), significantly higher than the non-probiotic group (RR 1.61).¹⁶
Breath hydrogen: significantly reduced, by 36.35 ppm.¹⁶
Abdominal pain: decreased — though the confidence interval runs to −0.04, which is as close to the line as a significant result gets.¹⁶
Stool frequency: no significant effect.¹⁶
Prevention: no significant effect. The authors’ own conclusion is that probiotics "were ineffective in preventing SIBO."¹⁶

Three caveats that change how that 62.8% should be read. Eight of the 22 included records were conference abstracts rather than full peer-reviewed papers. The 62.8% is a pooled single-arm rate, which is inflated by spontaneous resolution and regression to the mean. And in several of the included trials the probiotic was given alongside antibiotics, not instead of them — in one Lactobacillus trial, every participant received three weeks of broad-spectrum antibiotics first, and the comparison was probiotic-plus-antibiotics against antibiotics alone.¹⁷

The nearest thing to a clean test of a Lactobacillus product on its own is a double-blind crossover trial which found "no significant difference compared to placebo" on breath testing, stool frequency or symptom score.¹⁸ And a trial that specifically measured intestinal permeability found that short-term probiotic therapy improved SIBO but did not improve permeability.¹⁹

So: a real signal on decontamination and breath hydrogen, mostly from studies where antibiotics were also involved; nothing on prevention; nothing on stool frequency; nothing on permeability; and a monotherapy trial that came back negative. That is the honest shape of it.

Where Flourish fits

Bottles of Entegro Health Flourish Original and Flourish Junior living probiotic resting in a woven basket

Some practitioners who treat SIBO have used Flourish with their patients. We want to be precise about what that is and what it is not.

It is practitioner experience. It is not a clinical trial. Flourish has not been studied in people with diagnosed SIBO, and we make no claim that it treats, prevents or eradicates it. The FTC’s guidance on health products is explicit that anecdote — including a professional’s — is never sufficient to substantiate a claim about what a product does.²⁰ We agree with that standard, and we are not going to work around it.

Flourish is a living liquid probiotic intended to support digestive and microbiome health in general. If you have SIBO, or think you might, the useful sequence is: get tested, find out which gas is involved, and treat the underlying cause with a clinician. A probiotic is not a substitute for any part of that, and the section above explains why it may not be a neutral addition either.

The short version

A woman walking along a quiet tree-lined path through a green park on a mild day

SIBO is bacteria growing where they should not, usually because something slowed the gut down first.
Hydrogen comes from bacteria. Methane comes from archaea — that condition is called IMO and it goes with constipation.
Bloating is the most common symptom, and no symptom is specific to SIBO.
Breath tests miss about half of cases; glucose outperforms lactulose.
Antibiotics are the only treatment the guideline recommends. Rifaximin clears about 71%.
Fixing the underlying cause is what stops it coming back.
The guideline gives probiotics no recommendation and notes they may cause SIBO in some people.

If you recognize yourself in this article, the next step is a clinician and a test — not a supplement aisle. That includes ours.

About this article

Written by: Lorilyn Van Dyke and Kelsy Armstrong, Entegro Health | Originally published: November 2023

Important information

This article is for general education and is not a substitute for professional medical advice, diagnosis, or treatment. SIBO is a medical condition requiring diagnosis and management by a qualified clinician, and its symptoms overlap with conditions that are more serious. Always seek the advice of your physician or another qualified health provider, and never start or stop a prescribed medication based on what you read here. Guideline recommendations are summarized in brief; the linked documents carry the full wording and the strength of each recommendation. Probiotic effects are specific to the strain, the dose and the population studied, and evidence for one product does not transfer to another. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Sources

Pimentel, M., Saad, R. J., Long, M. D., & Rao, S. S. C. (2020). ACG clinical guideline: small intestinal bacterial overgrowth. American Journal of Gastroenterology, 115(2), 165–178 — definition, IMO terminology, breath-test and culture thresholds, risk factors, antibiotic recommendation, and the guideline’s statements on probiotics. Source
Dukowicz, A. C., Lacy, B. E., & Levine, G. M. (2007). Small intestinal bacterial overgrowth: a comprehensive review. Gastroenterology & Hepatology, 3(2), 112–122 — bile acid deconjugation, mucosal changes, competitive B12 uptake and elevated folate. Predates the current terminology and diagnostic thresholds, and is cited here for mechanism only. Source
Villanueva-Millan, M. J., Leite, G., Wang, J., et al. (2022). Methanogens and hydrogen sulfide producing bacteria guide distinct gut microbe profiles and irritable bowel syndrome subtypes. American Journal of Gastroenterology, 117(12), 2055–2066. Source
Chen, B., Kim, J. J., Zhang, Y., Du, L., & Dai, N. (2018). Prevalence and predictors of small intestinal bacterial overgrowth in irritable bowel syndrome: a systematic review and meta-analysis. Journal of Gastroenterology, 53(7), 807–818 — 50 studies, 8,398 patients; 40% by breath test versus 19% by culture; PPI use not associated. Source
Ghoshal, U. C., Nehra, A., Mathur, A., & Rai, S. (2020). A meta-analysis on small intestinal bacterial overgrowth in patients with different subtypes of irritable bowel syndrome. Journal of Gastroenterology and Hepatology, 35(6), 922–931. Source
Feng, X., Hu, J., & Zhang, X. (2024). Prevalence and predictors of small intestinal bacterial overgrowth in inflammatory bowel disease: a systematic review and meta-analysis. Frontiers in Medicine, 11, 1490506. Source
Li, X., Feng, X., Jiang, Z., & Jiang, Z. (2021). Association of small intestinal bacterial overgrowth with Parkinson’s disease: a systematic review and meta-analysis. Gut Pathogens, 13(1), 25 — 46% pooled prevalence; 51% by lactulose breath test versus 35% by glucose. Source
Brechmann, T., Sperlbaum, A., & Schmiegel, W. (2017). Levothyroxine therapy and impaired clearance are the strongest contributors to small intestinal bacterial overgrowth. World Journal of Gastroenterology, 23(5), 842–852 — 1,809 consecutive breath-tested patients; retrospective and associative. Source
Lo, W. K., & Chan, W. W. (2013). Proton pump inhibitor use and the risk of small intestinal bacterial overgrowth: a meta-analysis. Clinical Gastroenterology and Hepatology, 11(5), 483–490 — association present in culture-based studies, absent in breath-test studies; possible publication bias noted. Source
Rezaie, A., Buresi, M., Lembo, A., et al. (2017). Hydrogen and methane-based breath testing in gastrointestinal disorders: the North American Consensus. American Journal of Gastroenterology, 112(5), 775–784 — consensus doses and diagnostic thresholds. Source
Losurdo, G., Leandro, G., Ierardi, E., et al. (2020). Breath tests for the non-invasive diagnosis of small intestinal bacterial overgrowth: a systematic review with meta-analysis. Journal of Neurogastroenterology and Motility, 26(1), 16–28 — glucose 54.5% sensitivity / 83.2% specificity; lactulose 42.0% / 70.6%. Source
Gatta, L., & Scarpignato, C. (2017). Systematic review with meta-analysis: rifaximin is effective and safe for the treatment of small intestine bacterial overgrowth. Alimentary Pharmacology & Therapeutics, 45(5), 604–616 — 32 studies, 1,331 patients; 70.8% eradication, 4.6% adverse events; authors note generally poor study quality. Source
Low, K., Hwang, L., Hua, J., Zhu, A., Morales, W., & Pimentel, M. (2010). A combination of rifaximin and neomycin is most effective in treating irritable bowel syndrome patients with methane on lactulose breath test. Journal of Clinical Gastroenterology, 44(8), 547–550 — retrospective, with small and unbalanced treatment arms. Source
Pimentel, M., Constantino, T., Kong, Y., Bajwa, M., Rezaei, A., & Park, S. (2004). A 14-day elemental diet is highly effective in normalizing the lactulose breath test. Digestive Diseases and Sciences, 49(1), 73–77 — 93 patients; open-label, no control arm. Source
Rao, S. S. C., Rehman, A., Yu, S., & Andino, N. M. (2018). Brain fogginess, gas and bloating: a link between SIBO, probiotics and metabolic acidosis. Clinical and Translational Gastroenterology, 9(6), 162 — 38 patients; observational and uncontrolled, and its conclusions were contested in published correspondence. Source
Zhong, C., Qu, C., Wang, B., Liang, S., & Zeng, B. (2017). Probiotics for preventing and treating small intestinal bacterial overgrowth: a meta-analysis and systematic review of current evidence. Journal of Clinical Gastroenterology, 51(4), 300–311 — 18 studies pooled; decontamination 62.8% (95% CI 51.5–72.8); prevention RR 0.54 (0.19–1.52), not significant. Source
Khalighi, A. R., Khalighi, M. R., Behdani, R., et al. (2014). Evaluating the efficacy of probiotic on treatment in patients with small intestinal bacterial overgrowth — a pilot study. Indian Journal of Medical Research, 140(5), 604–608 — all participants received three weeks of broad-spectrum antibiotics before randomisation. Source
Stotzer, P. O., Blomberg, L., Conway, P. L., Henriksson, A., & Abrahamsson, H. (1996). Probiotic treatment of small intestinal bacterial overgrowth by Lactobacillus fermentum KLD. Scandinavian Journal of Infectious Diseases, 28(6), 615–619 — double-blind crossover; no significant difference from placebo. Source
Sanchez, E., et al. (2014). Short-term probiotic therapy alleviates small intestinal bacterial overgrowth, but does not improve intestinal permeability in chronic liver disease. European Journal of Gastroenterology & Hepatology, 26(12), 1353–1359. Source
Federal Trade Commission (December 2022). Health Products Compliance Guidance — the "competent and reliable scientific evidence" standard, and the statement that anecdotal evidence is never sufficient to substantiate claims about the effects of a health product. See also 21 CFR 101.93 on disease claims. Source
Mayo Clinic. Small intestinal bacterial overgrowth (SIBO) — diagnosis and treatment — general background for readers. Note that Mayo and the ACG guideline differ on the role of aspirate culture; diagnostic specifics above follow the ACG guideline and the North American Consensus. Source
SIBO Center for Digestive Health, National University of Natural Medicine. SIBO test overview — retained from the original version of this article. It is a naturopathic teaching clinic and a commercial SIBO testing laboratory rather than a peer-reviewed source; the testing details above are sourced to references 1, 10 and 11. Source