Small intestinal bacterial overgrowth (SIBO)

Small intestinal bacterial overgrowth, often shortened to SIBO, is a digestive condition that develops when bacteria that normally live in the large bowel build up in the small intestine instead. This shift in the usual balance of gut bacteria can disrupt digestion and lead to troublesome symptoms such as bloating, wind and changes in bowel habit. SIBO is increasingly recognised within gastroenterology, particularly in people whose digestive symptoms have not settled with standard treatment. Understanding what SIBO is, why it develops, and how it is diagnosed and managed can help you have a more informed discussion with your doctor.

What is Small Intestinal Bacterial Overgrowth?

The small intestine usually contains far fewer bacteria than the large bowel. Several natural processes work together to keep it this way. Stomach acid destroys many bacteria before they can travel further along the digestive tract. Regular muscular waves, which sweep through the small bowel between meals, clear bacteria and food debris into the large bowel. A valve between the small and large bowel, known as the ileocaecal valve, also helps stop bacteria moving backwards into the small intestine.

When one or more of these protective processes is disrupted, bacteria can build up in the small intestine in numbers, or types, that are not usually found there. This is what defines SIBO. The extra bacteria can interfere with the normal breakdown and absorption of food, which is why SIBO often causes noticeable digestive symptoms.

What causes SIBO?

SIBO develops when the usual safeguards that limit bacteria in the small intestine stop working as they should. Long term use of medication that reduces stomach acid can make it easier for bacteria to survive and multiply. Conditions that slow the movement of food through the small bowel, including diabetes affecting the nerves of the gut and connective tissue diseases such as scleroderma, can also raise the risk.

Changes to the structure of the digestive tract are another common contributor. This includes small pouches in the bowel wall called diverticula, narrowed sections known as strictures, and changes from previous abdominal surgery, such as a surgically created loop of bowel. Removal of the ileocaecal valve during earlier surgery can also allow bacteria to move more freely into the small intestine.

SIBO is also seen more often in people with certain underlying gastrointestinal conditions, including coeliac disease, Crohn’s disease and chronic pancreatitis, as well as in people with liver cirrhosis or conditions affecting the immune system. It becomes more common with age, likely reflecting a combination of these factors.

What are the symptoms of SIBO?

The symptoms of SIBO vary from person to person and can range from mild to more troublesome. The most frequently reported symptoms are bloating, abdominal discomfort and excess wind. Diarrhoea is also common, although some people experience constipation instead. Many people describe a feeling of fullness or distension, particularly after eating.

In cases where SIBO has been present for some time, the body’s ability to absorb certain nutrients can be affected. This may lead to deficiencies in vitamin B12, iron or fat-soluble vitamins, and can contribute to fatigue, unintentional weight loss or, less commonly, bone health concerns. These symptoms overlap considerably with other digestive conditions, particularly irritable bowel syndrome, which is one of the reasons SIBO can be difficult to identify without appropriate testing.

How is SIBO diagnosed?

There is currently no single test that can diagnose SIBO with complete certainty, and researchers continue to refine the most reliable approach. Your gastroenterologist will usually begin by taking a detailed history of your symptoms and reviewing any relevant medical conditions, medications or previous surgery that may be contributing.

Breath testing is the most commonly used tool to support a diagnosis of SIBO. This non-invasive test measures the hydrogen and methane gas in your breath after you drink a sugar solution, since bacteria in the small intestine produce these gases when they break down sugar. In some situations, your gastroenterologist may also consider sampling fluid from the small bowel during an endoscopy, although this is used less often because it is more invasive and breath testing is generally sufficient to guide management. Your response to initial treatment can also provide useful information to help confirm the diagnosis.

How is SIBO treated?

Treatment for SIBO generally focuses on three areas: addressing any underlying cause where possible, correcting nutritional deficiencies, and reducing the bacterial overgrowth itself.

Where an underlying cause can be identified, your gastroenterologist will work to manage this alongside your SIBO symptoms. This may involve adjusting medications that could be contributing, or managing an underlying condition such as diabetes or coeliac disease. In some cases, a structural problem such as a stricture or a surgically created loop of bowel is responsible for ongoing overgrowth. Correcting these structural issues, where appropriate, is usually undertaken by a general or colorectal surgeon, working alongside your gastroenterologist as part of your overall care.

A course of antibacterial treatment, prescribed and monitored by your gastroenterologist, is the mainstay of therapy for most people with SIBO. This aims to reduce the excess bacteria in the small intestine and improve symptoms. Because bacteria can return once treatment finishes, some people require further courses over time, particularly if an underlying cause cannot be fully corrected.

Nutritional support is an important part of ongoing care, especially where deficiencies in vitamin B12, iron or fat-soluble vitamins have developed. Your gastroenterologist may recommend supplementation where needed. If you are referred to a dietitian, we recommend using the Dietitians Australia Find a Dietitian directory to locate a dietitian who can provide tailored dietary advice and support as part of your care.

SIBO and Irritable Bowel Syndrome

There is considerable overlap between the symptoms of SIBO and irritable bowel syndrome, and research into the relationship between the two conditions is ongoing. Some studies have found higher rates of SIBO among people diagnosed with irritable bowel syndrome, while others have found little difference compared with the general population. This inconsistency reflects, in part, the challenges involved in testing for SIBO.

If you have been living with symptoms of irritable bowel syndrome that have not improved with standard management, it may be worth discussing the possibility of SIBO with your gastroenterologist. Appropriate assessment can help clarify whether bacterial overgrowth is contributing to your symptoms and guide the most suitable treatment approach.

When should you see a gastroenterologist about SIBO?

If you have ongoing bloating, abdominal discomfort, or changes in your bowel habit that are affecting your daily life, it is worth raising these symptoms with your GP. Your GP can assess your symptoms and, where appropriate, refer you to a gastroenterologist for further investigation and management of SIBO.

Our team of Gastroenterologists have experience diagnosing and managing SIBO alongside a wide range of other digestive conditions, and can work with your GP to develop a plan appropriate for your circumstances.

Once you have your referral, please contact our practice directly to arrange an appointment.

For further reading, visit:

• Coeliac disease
• Crohn’s disease
• Pancreatitis (Acute & Chronic)
• Irritable bowel syndrome (IBS)
• Endoscopy
• Our team of Gastroenterologists

References

1. Dukowicz AC, Lacy BE, Levine GM. Small intestinal bacterial overgrowth: a comprehensive review. Gastroenterol Hepatol (N Y). 2007;3(2):112-122.
2. Grace E, Shaw C, Whelan K, Andreyev HJN. Review article: small intestinal bacterial overgrowth – prevalence, clinical features, current and developing diagnostic tests, and treatment. Aliment Pharmacol Ther. 2013;38(7):674-688.

This information was prepared with reference to the sources above and reflects current Australian clinical guidance available at the time of writing. It is intended for general patient education and does not replace individual medical advice from your GP or gastroenterologist.