SIBO: bacterial overgrowth in the small intestine

SIBO: Bacterial overgrowth in the small intestine

SIBO is short for "small intestinal bacterial overgrowth" and describes a situation where more bacteria are present in the small intestine than there normally are. Overgrowth with methane-producing microorganisms is classified separately as IMO (intestinal methanogen overgrowth), because methanogens are archaea and not bacteria.

SIBO is most often regarded as a finding that occurs alongside something else, rather than as a disease in its own right. The relationship between bacterial counts, symptoms and predisposing conditions is not fully clarified, and a demonstrated overgrowth is not in itself proof that it explains the symptoms.

Symptoms

The symptoms are non-specific and overlap with many other gastrointestinal conditions:

• Bloating and a visibly distended abdomen
• Excess gas
• Abdominal pain or cramps
• Diarrhoea, constipation or alternation between them
• Fatigue
• With a longer course, weight loss or deficiency of vitamins and minerals may occur

The symptoms cannot on their own separate SIBO from other functional gastrointestinal conditions. Most people with bloating do not have SIBO, and investigation for SIBO should follow an independent clinical suspicion rather than form a fixed part of every bloating work-up.

Associated and predisposing factors

The following are described in association with SIBO. These are associations, not demonstrated causes in the individual patient:

• Altered small bowel motility, for example in diabetes, scleroderma or gastroparesis
• Structural factors after abdominal surgery, small bowel diverticula, strictures or adhesions
• Reduced gastric acid, including use of proton pump inhibitors (PPIs), which is associated with SIBO in observational studies
• Conditions that may weaken the gut's defence mechanisms, for example common variable immunodeficiency and coeliac disease
• Older age

Investigation

A breath test with lactulose or glucose may be considered when there is an independent clinical suspicion of SIBO. You drink a sugar solution, and hydrogen and methane are measured in your exhaled breath.

• Hydrogen (H₂): a rise from baseline of at least 20 ppm within 90 minutes can support the suspicion when the clinical picture fits [1].
• Methane (CH₄): at least 10 ppm at any point during the test can support a diagnosis of intestinal methanogen overgrowth (IMO) [1,2]. The criterion is an absolute value, not a requirement for a 10 ppm rise.
• Hydrogen sulphide (H₂S): there is no universally accepted clinical threshold, and this gas is not measured routinely.

Thresholds vary between laboratories and protocols, and the result is influenced by the substrate used, by intestinal transit time and by the preparation before the test. Both false positive and false negative results occur. A crossed threshold is not in itself diagnostic proof and cannot stand alone without symptoms and medical assessment. The evidence base is limited: the 2017 North American consensus rests largely on expert opinion, and the 2020 ACG guideline itself grades several of its breath-testing recommendations as low certainty [1,2].

Small bowel aspirate with quantitative culture is described as the reference method, but it is rarely performed and the cut-off values are not fully agreed. The doctor also considers whether there is reason to investigate predisposing conditions. Breath testing is not available everywhere in Denmark. If you want to read more about the test itself, EndoCap has a walk-through of hydrogen and methane breath testing, including what the numbers can and cannot show.

Treatment

Treatment options are assessed individually by the treating doctor on the basis of the overall clinical picture. There is no single fixed algorithm that applies to everyone.

Diet: A short period of low FODMAP may give some people symptom relief for a limited time. It is not documented as a treatment that removes an overgrowth, and unnecessarily prolonged or strict restriction is discouraged.

Antibiotics: Antibiotic treatment is prescribed by a doctor and is individual; there is no single fixed regimen. The 2020 ACG guideline discusses antibiotic treatment of symptomatic patients but does not point to a specific agent. Rifaximin has been used in several studies in this field, but it is not a universal first choice. In Denmark, rifaximin (Xifaxan) is approved for indications other than SIBO, so its use in SIBO is off-label and follows an individual medical assessment.

Predisposing factors: If an underlying condition is present, it forms part of the doctor's overall assessment. Prescribed medicine, including PPIs, should not be changed or stopped without agreement with the prescribing doctor.

When should you see a doctor?

Contact your own doctor if, for more than a few weeks, you have:

• Persistent bloating
• Unexplained weight loss
• Diarrhoea lasting beyond a couple of weeks
• Fatigue combined with digestive problems

Frequently asked questions

Is SIBO the same as irritable bowel syndrome? No. The symptoms overlap considerably, and the two conditions do not exclude one another. The breath test cannot separate them on its own, because it has a known margin of error and is only one element of an overall clinical assessment.

Can probiotics cure SIBO? Probiotics are not a cure. The evidence for probiotics in SIBO is limited and inconsistent across strains and studies. Decide together with a doctor or dietitian.

Do I have to avoid sugar and carbohydrates completely? No. In the short term a strict low-FODMAP period may relieve symptoms, but unnecessarily prolonged and strict restriction increases the risk of a one-sided or inadequate diet. The diet should therefore gradually be widened back to normal, with help from a dietitian if needed.

Sources

1. Rezaie A, Buresi M, Lembo A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. Am J Gastroenterol. 2017;112(5):775-784. PubMed - consensus document on breath test thresholds and interpretation, largely expert-based.
2. Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. 2020;115(2):165-178. PubMed - guideline on diagnosis and treatment; several breath-testing recommendations are graded as low certainty.
3. Quigley EMM, Murray JA, Pimentel M. AGA Clinical Practice Update on Small Intestinal Bacterial Overgrowth: Expert Review. Gastroenterology. 2020;159(4):1526-1532. PubMed - expert review noting that the definition and true extent of SIBO remain debated.
4. Sundhed.dk. Bacterial overgrowth in the small intestine (Danish physicians' handbook) - Danish clinical reference work for healthcare professionals with a general description of the condition.
5. Rao SSC, Bhagatwala J. Small Intestinal Bacterial Overgrowth: Clinical Features and Therapeutic Management. Clin Transl Gastroenterol. 2019;10(10):e00078. PubMed - review of clinical features, predisposing factors and treatment considerations; notes that the true prevalence is unknown.

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This article is general information and does not replace a medical consultation. Contact your own doctor if you have symptoms.