SIBO

SIBO

SIBO (small intestinal bacterial overgrowth) describes a situation where an excessive amount of bacteria is present in the small intestine. In simple terms, most gut bacteria should be concentrated in the large intestine. When bacterial load increases in the small intestine, it can contribute to digestive symptoms and may become clinically relevant.

Symptoms commonly linked with SIBO

When bacteria ferment carbohydrates in the small intestine, symptoms may include:

  • abdominal bloating
  • excessive gas and flatulence
  • abdominal pain and cramping
  • changes in bowel habits (diarrhoea or constipation)
  • early or excessive fullness
  • nausea

Why SIBO is difficult to interpret

SIBO

SIBO does not have a gold standard diagnostic criterion. As a result, prevalence estimates vary widely, and symptoms overlap substantially with IBS. Estimates discussed in the episode included:

  • healthy controls: approximately 2.5% to 15%
  • IBS-D type symptoms: around 35% as a closest-to-consensus estimate (noting a wide range reported)
  • diabetes: around 30%
  • coeliac disease: around 20%
  • IBD: around 20% to 30%, particularly where prior surgery may contribute to strictures and increased risk

Potential causes and contributing mechanisms

SIBO is often considered a secondary condition. In other words, it tends to occur when normal protective processes in the gastrointestinal tract are disrupted. Two key mechanisms include:

  • Impaired motility: slower movement through the small intestine provides more time for bacteria to persist and multiply.
  • Low stomach acid: reduced gastric acid may allow more bacteria to survive and reach the small intestine.

In addition, bacterial migration from the large intestine may contribute. Structural issues, including physical blockages and the effects of prior abdominal surgery, may also play a role.

Diagnosis

Hydrogen breath test

Because no perfect test exists, diagnosis often relies on imperfect tools and careful clinical interpretation.

Breath testing

Hydrogen-methane breath testing involves consuming glucose or lactulose, then measuring hydrogen or methane in the breath. A rapid rise may indicate bacterial overgrowth. However, results can be influenced by factors such as food intake beforehand, tooth brushing, and naturally fast transit. Importantly, lactulose can reach the large intestine quickly, even in healthy individuals, which can appear as a positive result. One review reported that 30% of healthy controls tested positive on a lactulose breath test, suggesting a substantial false-positive rate.

Small intestine aspirate

This approach involves endoscopy and sedation. A gastroenterologist suctions a small amount of fluid from the small intestine and sends it to a lab to assess bacterial colony growth.

Despite being more direct, limitations remain:

  • the sample may pick up bacteria during the procedure
  • bacterial overgrowth may not be evenly distributed, so sampling location matters
  • some gut bacteria die when exposed to oxygen, which can affect handling and results

Given the invasiveness and required sedation, clinicians often favour breath testing, while recognising its limitations.

How clinicians may improve diagnostic confidence

Rather than relying on a single test result, the episode discussed additional strategies that may improve diagnostic interpretation:

  • blood tests to assess vitamin deficiencies associated with SIBO
  • stool evaluation to assess fat malabsorption
  • imaging (X-ray or CT) to identify structural abnormalities
  • medication review, particularly where medications may contribute to low stomach acid
  • clinical evaluation, including consideration of possible post-infectious IBS

Management options discussed

Carnivore

Antibiotics as a first-line approach

Antibiotics are the main first-line treatment. Rifaximin and neomycin are options, with research suggesting around 60% effectiveness for normalising breath test results. However, approximately 45% of people may experience recurrence within nine months after antibiotic treatment. For that reason, repeated courses raise concerns, and identifying contributing factors becomes important.

Diet strategies that may support treatment

Low FODMAP diet

Reducing fermentable carbohydrates can help reduce symptoms, particularly bloating and gas. However, it is not presented as a standalone treatment. While it may reduce the “fuel” available for fermentation, there is limited evidence showing that it meaningfully reduces overgrowth on its own. Reintroduction and personalisation remain key, as it is not intended as a long-term elimination approach.

Very low carbohydrate or carnivore-style approaches

These approaches follow similar logic, aiming to reduce carbohydrate availability. However, they are not designed as long-term strategies, and reintroduction and personalisation matter for longer-term gut health.

Elemental diet

An elemental diet uses a pre-digested liquid formula for 14 to 21 days, with the goal of absorbing nutrients early in the small intestine so bacteria further down have little to ferment.

One study reported greater than 80% success for normalising breath tests. Practical barriers may limit its use, including:

  • cost
  • access and logistics
  • poor palatability
  • limited use in routine practice

Probiotics

Probiotics can be contentious in SIBO. They add bacteria in a context where bacterial overgrowth already exists.

A meta-analysis suggested specific strains may help clear SIBO and improve symptoms, and may support antibiotic effectiveness:

  • Saccharomyces boulardii
  • Bacillus clausii

At the same time, some reported that multi-strain broad-spectrum probiotics increased abdominal pain and bloating. Timing may also matter. Clinicians may consider probiotics after antibiotics to support balance and potentially reduce recurrence. However, this remains optional rather than essential.

Key takeaways

SIBO is challenging to interpret due to unclear diagnostic standards and high false-positive potential with commonly used testing. As a result, clinicians often need to combine test results with strong clinical reasoning. SIBO remains a legitimate condition. However, breath testing alone can lead to overdiagnosis when results are not interpreted in context.

A practical management sequence includes:

  • treating initially with antibiotics (or considering an elemental diet in selected cases)
  • identifying and addressing contributing factors such as low stomach acid, impaired motility, or post-surgical issues
  • considering probiotics carefully, with attention to strain selection and timing

Relevant Studies

Relevant Blog Posts