SIBO: Symptoms, Causes, Diagnosis and Treatment
A specialist dietitian's guide to Small Intestinal Bacterial Overgrowth — what it is, why it happens, how it's tested for, and what actually helps.
SIBO, or Small Intestinal Bacterial Overgrowth, is a digestive condition where too many bacteria grow in the small intestine.
It can cause symptoms such as bloating, abdominal pain, gas, diarrhoea, constipation and changes in bowel habits.
Understanding what causes SIBO, how it is diagnosed and the available treatment options can help you manage symptoms and improve your digestive health.
What Is SIBO?
SIBO, or Small Intestinal Bacterial Overgrowth, is when too many bacteria grow in the small intestine — the main part of your digestive system responsible for absorbing nutrients.
Normally, most bacteria live in your large intestine, or colon. However, when bacteria multiply where they should not, SIBO can develop. The small intestine is quite sensitive and is not built to handle a bacterial overload.
This can lead to uncomfortable symptoms such as:
- Bloating
- Tummy or abdominal pain
- Foul-smelling gas
- Diarrhoea
- Constipation
- Alternating bowel habits
If untreated, SIBO may also interfere with how well your body digests food and absorbs nutrients, although nutrient deficiencies caused by SIBO are not particularly common.
What Causes SIBO?
There are several reasons why bacteria may build up in the small intestine. Normally, the small intestine has mechanisms in place to keep bacteria under control. When these mechanisms are disrupted, SIBO can develop.
Some recognised causes and associated conditions include:
- Gastric surgery, such as weight-loss surgery or cancer operations
- Certain medications, including some used during cancer treatment
- Cancer treatments such as bowel surgery, pelvic radiotherapy, chemotherapy and immunotherapy
- Gut motility disorders that affect how food moves through the digestive system
- Neuroendocrine neoplasms or tumours and their treatment
- Chronic pancreatitis
- Low stomach acid, often associated with long-term use of proton pump inhibitors such as omeprazole and lansoprazole
- Immune-related conditions such as coeliac disease, HIV or immunoglobulin A deficiency
- Irritable bowel syndrome, or IBS
- Crohn's disease
What Are the Symptoms of SIBO?
Symptoms vary from person to person, but some of the most common symptoms associated with SIBO include:
- Bloating
- Excess gas
- Abdominal discomfort or pain
- Diarrhoea
- Loose stools
- Constipation
- Alternating bowel habits
SIBO is commonly associated with diarrhoea or loose stools. A related condition called Intestinal Methanogen Overgrowth, or IMO, is more commonly associated with constipation.
How Is SIBO Diagnosed?
The gold standard for diagnosing SIBO is a small bowel aspirate and culture — taking a fluid sample from the small intestine during an upper endoscopy.
However, because this method is invasive, breath tests are more commonly used in practice, measuring hydrogen or methane levels in your breath after drinking a glucose or lactulose solution.
Human cells do not produce hydrogen or methane. Therefore, if these gases are detected during the test, it suggests that bacteria in the gut are fermenting the solution, which may indicate SIBO.
Associated with diarrhoea-predominant IBS or IBS with mixed symptoms.
Now classified as Intestinal Methanogen Overgrowth, more commonly associated with constipation-predominant IBS.
How Accurate Are SIBO Breath Tests?
Breath tests are useful, but they are not perfect. For example, a glucose breath test may miss SIBO that is present further along the small intestine. A lactulose breath test may also give false-positive results in people with faster gut motility.
For this reason, diagnosing SIBO usually involves looking at breath test results alongside:
- Your symptoms
- Your medical history
- Your lifestyle
- Other relevant health conditions
How Is SIBO Treated?
The most effective treatment for SIBO is antibiotics. These help reduce bacterial overgrowth. Unlike some other types of antibiotics, those commonly used for SIBO do not typically lead to antibiotic resistance.
Depending on whether you have SIBO or IMO, your doctor may prescribe either a single antibiotic or a combination of two antibiotics. Sometimes, one course of treatment is not enough, particularly in more stubborn cases. This can also make treatment expensive because these antibiotics are not always available on the NHS.
For this reason, identifying and treating the underlying cause of SIBO is important to reduce the risk of it returning.
Diet and SIBO
While antibiotics are used to treat bacterial overgrowth, dietary changes may help some people manage their symptoms. However, research in this area is still limited.
For some people, a Low FODMAP diet may help reduce symptoms. Others may benefit from more specific dietary advice based on their main symptoms, such as dietary support for constipation.
A restrictive diet should ideally be followed with professional support to ensure you are still meeting your nutritional needs.
Lifestyle Tips for Managing SIBO Symptoms
Diet and lifestyle changes may help support digestive function alongside medical treatment. Useful strategies may include:
- Eating regular meals
- Eating slowly and mindfully
- Chewing food thoroughly
- Staying well hydrated
- Identifying foods that worsen your individual symptoms
- Avoiding unnecessarily restrictive diets
- Seeking professional dietary advice if symptoms continue
What About Probiotics for SIBO?
Research into probiotics for SIBO is mixed. In some cases, probiotics may actually worsen symptoms. For this reason, probiotics are not usually recommended during active SIBO treatment.
However, once SIBO has been treated, a short course of probiotics may sometimes help restore balance in the gut and potentially reduce the risk of recurrence. It is best to seek professional advice before using probiotics for SIBO.
Can SIBO Cause Nutritional Deficiencies?
Nutritional deficiencies caused by SIBO are possible, but they are relatively uncommon. In some cases, unusual blood test results may provide clues — for example, unexplained high folate levels may sometimes be associated with SIBO.
However, symptoms and blood tests should always be interpreted alongside your wider medical history and other investigations.
Can SIBO Come Back?
SIBO can return after treatment, particularly if the underlying reason for the bacterial overgrowth has not been addressed. Long-term management may therefore involve identifying factors such as:
- Gut motility problems
- Previous surgery
- Medication use
- Digestive conditions
- Cancer treatment
- Other underlying health conditions
Successful treatment is not only about reducing bacterial overgrowth. It is also important to understand why the SIBO developed in the first place.
SIBO and IBS
There is an association between SIBO and Irritable Bowel Syndrome. Hydrogen-positive SIBO is more commonly associated with diarrhoea-predominant IBS or mixed-type IBS. Methane-positive cases, generally referred to as Intestinal Methanogen Overgrowth, are more commonly associated with constipation-predominant IBS.
Because symptoms can overlap, it is important not to assume that digestive symptoms are caused by SIBO without appropriate assessment.
SIBO: Key Takeaways
- SIBO symptoms: Bloating, diarrhoea, loose stools, gas and abdominal discomfort.
- IMO symptoms: More commonly associated with constipation.
- Nutritional deficiencies: Possible but relatively uncommon.
- Breath tests: Helpful but not foolproof — interpret alongside other assessments.
- Treatment: Antibiotics currently have the strongest evidence.
- Underlying cause: Long-term success depends on identifying why SIBO developed.
- Diet & lifestyle: Regular meals, mindful eating, hydration and personalised support may help.
When to Seek Professional Support for SIBO
Managing SIBO can be complicated, so it is important to work with a gastroenterologist or appropriately trained healthcare professional.
Gina, The Rehab Dietitian, can support you by:
- Referring you for reputable breath testing and helping you avoid unreliable online tests
- Signposting you to a gastroenterologist where appropriate
- Creating personalised dietary plans to help manage digestive symptoms
- Supporting your long-term gut health
- Helping you reduce the risk of SIBO recurrence
If you are struggling with digestive symptoms or suspect you may have SIBO, professional support can help you better understand your symptoms and decide on the most appropriate next steps.
A tailored approach can make a significant difference to managing your gut health and improving your quality of life.
Gina specialises in gastrointestinal and oncology nutrition, helping people navigate complex digestive conditions like SIBO, IBS and post-treatment bowel changes with evidence-based, personalised care.
Think You Might Have SIBO?
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