Understanding SIBO: Causes, symptoms and treatment

If you’ve been struggling with ongoing bloating, discomfort, or unpredictable digestion, you might have encountered the term SIBO — Small Intestinal Bacterial Overgrowth. But what exactly is SIBO, and why does it cause such troublesome gut symptoms?

In this article, gut specialist Dr Emily Porter (RD, PhD) breaks down everything you need to know about SIBO, from its causes and symptoms to how it’s diagnosed and treated. You’ll learn about bacteria’s role in digestion, why an overgrowth in the small intestine can lead to issues like bloating, gas, and even nutrient deficiencies, and how SIBO is linked to conditions like Irritable Bowel Syndrome (IBS). We’ll also explore breath testing methods used for diagnosis, the latest research on treatment options — including antibiotic therapy and dietary strategies — and why a personalised approach is key to managing symptoms and preventing recurrence.

What is SIBO?

SIBO, or Small Intestinal Bacterial Overgrowth, is a condition in which excessive bacteria grow in the small intestine, causing various gastrointestinal symptoms. 

Typically, the small intestine has fewer bacteria compared to the large intestine, and these bacteria play a crucial role in digesting certain carbohydrates through fermentation. However, when bacteria migrate into the small intestine and ferment food there instead, it can lead to discomfort.

The small intestine is typically more sensitive to the fermentation process, so an overgrowth of bacteria here can disrupt digestion, cause discomfort, and potentially lead to nutrient deficiencies if left untreated, although this is rare.

Key symptoms of SIBO:

• Abdominal pain and cramps
• Bloating (mainly upper abdominal) 
• Foul-smelling gas
• Diarrhea 
• Nausea

What causes SIBO?

Several mechanisms are responsible for maintaining the relatively sterile environment of the small intestine. If any of these are disrupted, it can result in the development of SIBO. The following are now recognised, amongst others, as potential risk factors:

• gastric surgery, including bariatric surgery
• certain medications or chemotherapy for cancer treatment
• a gut motility disorder
• chronic pancreatitis
• reduced acid secretion (this is common with long-term PPI use)
• immune-related conditions (e.g. immunoglobulin A deficiency, coeliac disease, HIV)
• Irritable Bowel Syndrome (IBS)
• Crohn’s disease

How do you diagnose SIBO?

SIBO is typically diagnosed through:

• Breath tests (using a glucose or lactulose solution) measure gases like hydrogen and methane produced by bacteria fermenting carbohydrates in the small intestine. These gases are absorbed into the bloodstream and transported to the lungs, where they are exhaled and collected for a breath test. 
• Small bowel aspirate and culture (though the gold standard, these are more invasive and less commonly used).

It’s worth mentioning at this point that the presence of hydrogen is diagnostic of SIBO, which has been associated with diarrhoea-predominant IBS and IBS with mixed bowel habits. The presence of methane, however, has been reclassified as Intestinal Methanogen Overgrowth (IMO) or methane-predominant SIBO and is more often associated with constipation. 

Breath tests can indicate the presence of excess bacteria, but they have limitations. For example, they might not effectively detect SIBO at the far end of the small intestine, and the lactulose breath test is limited by its potential false positive in those with rapid gut motility. 

So, it’s still early days for SIBO and the research, and controversy surrounds the overlap between IBS and SIBO. Whilst practitioners (including us here at The Gut Health Clinic) do refer for the breath tests, it is worth noting that the methods for measuring SIBO still need further optimisation and standardisation. In practice, the breath tests are used alongside your clinical history, lifestyle and symptoms to help diagnose SIBO.

How is SIBO treated?

Antibiotic therapy is the best evidence-based approach to treating SIBO. Some alternative practitioners may recommend herbal antimicrobials, which sound appealing instead of antibiotics. However, there’s limited evidence to suggest that these are effective. 

The treatment can get expensive, as antibiotics aren’t always readily available on the NHS for SIBO. That’s why it’s important to confirm a diagnosis, establish the underlying cause, and treat that to prevent a recurrence. 

• Antibiotics: The first line of treatment typically involves antibiotics like rifaximin, which targets bacteria in the gut without being absorbed into the bloodstream. For some cases, several courses of antibiotics may be necessary. The good news for the gut microbiome is that these aren’t antibiotics that develop a clinical resistance and are usually poorly absorbed from the gastrointestinal tract. 
• Dietary changes: A diet low in fermentable carbohydrates, known as FODMAPs, can help manage symptoms alongside antibiotic therapy. However, much of the data on diet for SIBO treatment are extensions of data from IBS studies. 
• Meal timing: Allowing sufficient time between meals (e.g., at least 4 hours between meals) helps maintain the small bowel’s natural cleansing mechanism called the migrating motor complex, which helps prevent bacterial overgrowth.
• Probiotics: While probiotics are often used for gut health, most of the studies on them for SIBO have been small and of poor quality, with mixed results. In fact, there’s research to suggest that they may cause SIBO or worsen symptoms, so they aren’t currently recommended. Probiotics may be used after successful treatment to maintain gut health and prevent recurrence. 
• Other supplements: Certain fibre supplements can improve the effectiveness of antibiotics; a gut specialist dietitian can advise further on which one to take. 

Summary of key concepts 

• The most common symptoms of SIBO are diarrhoea/loose stools and bloating. 
• The most common symptom of IMO is constipation. 
• Vitamin deficiencies are possible but not common – unexplained elevated serum folate may indicate SIBO.
• The causes(s) are multifactorial, and each needs to be determined and addressed to prevent recurrence and avoid repeated courses of antibiotics. 
• Breath testing helps identify SIBO non-invasively before antibiotic treatment but has limitations. 
• There’s often an overlap of IBS and SIBO, or other gut health conditions, so treatment should involve a coordinated plan to look at diet and lifestyle as well as breath test results. 

There is a lack of consistent data to support recommending specific probiotics in treating SIBO.

Takeaway

SIBO can be challenging to diagnose and treat due to its overlap with other conditions, especially IBS. Effective management typically requires a multi-faceted approach, including proper diagnosis through breath tests, targeted antibiotic treatment, dietary modifications, and addressing underlying causes. Consulting with a gastroenterologist or trained healthcare professional is crucial for successful treatment.

If you’re dealing with SIBO or suspect you might be, it’s a good idea to work with a gut health specialist or dietitian to create a tailored treatment plan.

Our dietitians at The Gut Health Clinic can help support you through the process, from referring you for breath testing (via reputable labs; beware of some online commercial products) to managing symptoms and optimising gut health. It is all based on the best available research on dietary management and targeting the gut-brain axis, which has been shown to improve symptoms.

References:

Pimentel M, Saad RJ, Long MD, Rao SSC. ACG clinical guideline: small intestinal bacterial overgrowth. Am J Gastroenterol. 2020;115(2):165-178. https://doi.org/10.14309/ajg.0000000000000501.

Takakura W, Pimentel M. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome – An Update. Front Psychiatry. 2020 Jul 10;11:664. https://doi.org/10.3389/fpsyt.2020.00664