SIBO refers to an overgrowth of bacteria in the small intestine and can cause symptoms such as bloating, gas, abdominal pain and changes in bowel habits, as well as a range of more general symptoms. Several factors may contribute to SIBO, including reduced intestinal motility and changes in stomach acid production. A significant prevalence of SIBO has been observed among people with IBS. Because gastrointestinal symptoms can have many different causes, it is important to consider the whole picture. Where possible, a long-term approach should not focus solely on bacterial overgrowth but also on the factors that may have contributed to its development.
SIBO stands for Small Intestinal Bacterial Overgrowth and refers to an overgrowth of bacteria in the small intestine. Under normal conditions, the small intestine contains relatively few bacteria and is covered by a thin mucus layer that helps facilitate the passage of food through the intestine.
The small intestine has a surface area of approximately 40 square metres (roughly half the size of a badminton court) and its main function is to absorb nutrients from food into the bloodstream. The large intestine, on the other hand, has a surface area of approximately 2 square metres and is primarily responsible for absorbing water and salts into the body. The large intestine also has a different structure, with two thick mucus layers that help prevent bacteria and bacterial toxins from coming into contact with the intestinal lining and causing damage.
When bacteria grow in the small intestine, the thin mucus layer may no longer provide sufficient protection against bacteria and bacterial toxins. This can damage the intestinal lining and contribute to increased intestinal permeability, sometimes referred to as “leaky gut”. It may also affect the function of immune cells located in the small intestinal wall, where approximately 70% of the body’s immune cells are found.
The presence of bacteria in the small intestine can also mean that bacteria consume nutrients while producing gases. This may contribute to symptoms such as bloating, gas, abdominal pain, diarrhoea and constipation. Other microbial by-products may also enter the bloodstream. Two such by-products have been described as potential risk factors for the development of atherosclerosis and metabolic changes such as insulin resistance, as well as abdominal obesity.
Research has shown that up to 85% of people with IBS may have SIBO. In this article, we look at what SIBO is, how common it is, which symptoms it may cause, possible contributing factors, and how SIBO can be investigated and addressed.
The most important facts about SIBO
- SIBO refers to an overgrowth of bacteria in the small intestine.
- Most of the bacteria that make up the gut microbiota are normally found in the large intestine.
- SIBO can cause gas, bloating, abdominal pain, diarrhoea and constipation.
- Several factors may contribute to SIBO, including reduced stomach acid and impaired intestinal motility.
- SIBO can affect digestion and nutrient absorption.
- SIBO may occur alongside conditions such as IBS.
- SIBO has been associated with fatigue, brain fog, difficulty concentrating, neurological symptoms, skin problems, particularly rosacea, metabolic effects and anxiety/depression.
- Because SIBO symptoms can resemble many other gastrointestinal conditions, an appropriate medical assessment is important.
How common is SIBO?
The exact prevalence of SIBO in the general population is not known because results vary depending on the testing method and diagnostic criteria.
Studies nevertheless suggest that around 5–15% of people without significant gastrointestinal symptoms may have findings consistent with SIBO.
A 2026 meta-analysis reported a pooled prevalence of approximately 11.5% among controls without significant gastrointestinal symptoms, compared with around 39% among people with functional gastrointestinal disorders.
Overall, approximately 10% is therefore a reasonable estimate for people without significant gastrointestinal symptoms, while prevalence appears to be considerably higher among groups with conditions such as IBS and other gastrointestinal symptoms.
What are the symptoms of SIBO?
SIBO can cause a range of gastrointestinal symptoms. Common symptoms include:
- Bloating
- Gas
- Abdominal pain or discomfort
- Diarrhoea
- Constipation
- Changes in bowel habits
- Digestive symptoms after meals
The symptoms experienced may partly depend on which microorganisms are present and how the intestine functions.
Because the same symptoms can occur with IBS and other gastrointestinal conditions, symptoms alone cannot determine whether someone has SIBO.
Hydrogen, methane and SIBO
Bacteria and other microorganisms in the gut produce different gases when they ferment carbohydrates.
When investigating SIBO, hydrogen and methane are primarily measured in the breath.
Hydrogen is more commonly associated with diarrhoea, while methane is more strongly associated with constipation. Methane, however, is not produced by bacteria but by microorganisms called archaea.
When methane overgrowth is present, the term IMO (Intestinal Methanogen Overgrowth) is therefore often used rather than SIBO, because methane-producing microorganisms can also be present in the large intestine, where they may contribute to constipation.
There is currently no treatment that can selectively eliminate methane-producing microorganisms in the large intestine.
What causes SIBO?
Research has shown an association between chronic stress, with or without the use of long-acting acid-reducing medications, and SIBO.
Possible contributing factors include:
- Reduced intestinal motility, often associated with chronic stress
- Changes in stomach acid production
- Previous intestinal infections, for example after traveller’s diarrhoea
- Structural changes in the gastrointestinal tract
- Certain conditions that affect intestinal movement
- Certain medications
It is therefore important not to focus solely on the bacterial overgrowth, but also to try to understand which factors may have contributed to its development.
Reduced intestinal motility and SIBO
The small intestine has a natural movement pattern that helps transport its contents through the digestive tract.
Between meals, the so-called Migrating Motor Complex (MMC) is activated. This recurring movement pattern helps move remaining food particles and microorganisms through the small intestine.
If intestinal motility is reduced, the contents may remain in the intestine for longer. This can create conditions that allow bacteria to grow in the small intestine.
Intestinal motility can be influenced by several factors, but increased stress through activation of the sympathetic nervous system — the body’s fight-or-flight system — is considered an important factor.
Can low stomach acid affect SIBO?
Stomach acid is one of the body’s natural defence mechanisms against microorganisms entering through food.
If stomach acid is low, more microorganisms may survive passage through the stomach before reaching the small intestine. Changes in stomach acid production may therefore be one of several factors affecting the risk of SIBO.
Long-acting medications that reduce stomach acid production may also be relevant when assessing possible risk factors.
This does not mean that acid-reducing medications automatically cause SIBO. The relationship needs to be assessed together with other factors.
Is there a link between SIBO and IBS?
SIBO and IBS can cause many of the same symptoms, particularly bloating, gas, abdominal pain and changes in bowel habits.
IBS is a functional gastrointestinal diagnosis based on a specific pattern of symptoms and a medical assessment.
SIBO refers to an overgrowth or altered presence of bacteria in the small intestine.
SIBO has been reported in up to 85% of people with IBS.
Can SIBO affect nutrient absorption?
More pronounced SIBO may affect digestion and nutrient absorption.
Bacteria can use certain nutrients and influence how fats and other substances are broken down. In some cases, this may contribute to nutrient deficiencies or malabsorption.
Possible signs of impaired nutrient absorption include:
- Unintentional weight loss
- Persistent diarrhoea
- Fatty stools
- Signs of vitamin or mineral deficiencies
- Anaemia
These symptoms can have many different causes and should therefore be assessed by a healthcare professional.
How is SIBO identified?
SIBO can be difficult to identify because its symptoms overlap with many other gastrointestinal conditions.
One way of investigating SIBO is through a home urine test, such as the Four-in-One Test. During the test, a solution is consumed and biomarkers are then measured in the urine.
Breath tests are also non-invasive, but test results can be affected by a number of different factors. This may help explain why the sensitivity of breath testing — the ability to correctly identify the condition — is around 55%.
More invasive methods involve taking samples from the small intestine and are used more selectively. These studies have shown, however, that SIBO can begin as early as the duodenum, at the very beginning of the small intestine.
How can SIBO be addressed?
The goal is to manage the bacterial overgrowth while also identifying underlying factors that may contribute to recurring problems.
Addressing underlying factors
If reduced intestinal motility, a previous intestinal infection, medication or another factor contributes to SIBO, it is important to identify and address the underlying factor whenever possible.
Antibiotics
Rifaximin is an antibiotic that has been studied in relation to SIBO.
Dietary changes
Dietary changes may help reduce symptoms for some people.
A low-FODMAP diet, for example, can reduce the amount of fermentable carbohydrates in the diet and thereby help reduce gas production and bloating.
It is important, however, to distinguish between symptom relief and addressing the underlying bacterial overgrowth. A dietary change may reduce symptoms without addressing the underlying reason for the bacterial overgrowth.
Plant-based antimicrobial preparations
Plant-based antimicrobial preparations such as GutClear® are used by some people as an alternative or complement to other strategies.
The evidence varies between different preparations, and their use should be adapted to the individual’s situation.
Can diet help with SIBO?
Some people experience less bloating and gas when they temporarily limit foods that are easily fermented by bacteria. A low-FODMAP diet is one example of a dietary strategy that may be used for symptom relief.
Results vary from person to person. In some people with SIBO, additional fibre intake has been found to worsen symptoms.
A highly restrictive diet should not be followed for longer than necessary. The goal should be a nutritious and sustainable diet that works for the individual. For some people, this may also be a reason to address the bacterial overgrowth itself through antimicrobial products.
Can SIBO come back?
Yes, recurring symptoms can occur.
If the underlying factor contributing to SIBO remains, the risk of recurring symptoms may be higher. It is therefore important not only to focus on the bacterial overgrowth but also to try to understand why it developed in the first place.
When should you seek medical care?
Long-lasting or recurring gastrointestinal symptoms should be assessed by a healthcare professional, particularly if they affect your everyday life.
It is especially important to seek medical advice if you experience:
- Unintentional weight loss
- Blood in the stool
- Persistent or severe abdominal pain
- Fever
- Persistent diarrhoea
- Signs of nutritional deficiency or anaemia
- New or clearly changing gastrointestinal symptoms
Because SIBO can resemble many other gastrointestinal conditions, an appropriate medical assessment is important before starting a solution.
