SIBO — Small Intestinal Bacterial Overgrowth

What is SIBO?

SIBO stands for small intestinal bacterial overgrowth.

There are a great many microorganisms in our digestive system. Most of them live in the large bowel. There are bacteria in the small bowel too, but normally their numbers and their characteristics are different.

In SIBO the presence in the small bowel of more bacteria than normal, or of unsuitable kinds, can affect the processes of digestion and absorption of food and give rise to various complaints.

SIBO may arise on its own, or it may be the result of another condition affecting the movements of the bowel or the structure of the bowel.

Why does SIBO develop?

Under normal conditions, stomach acid, the movements of the bowel, the anatomy of the bowel and the other protective mechanisms of the digestive system limit the excessive multiplication of bacteria in the small bowel.

A disturbance of these mechanisms can make it easier for SIBO to develop.

Slowing of the bowel movements

The regular onward movement of the contents of the small bowel is one of the important mechanisms preventing the excessive multiplication of bacteria.

The risk of SIBO may be increased in diseases where the movements of the bowel are markedly slowed.

Changes in the structure of the bowel

Some previous abdominal and bowel operations, blind segments in the bowel, strictures or certain anatomical changes can cause bacteria to collect in particular regions.

Certain diseases

SIBO may be associated with:

• Disorders of bowel motility,
• Certain previous bowel operations,
• Diverticula of the small bowel,
• Certain structural changes due to Crohn’s disease,
• Diseases affecting the movements of the bowel, such as systemic sclerosis.

In some people no clear cause can be found.

What symptoms does SIBO cause?

The symptoms of SIBO can vary from person to person and can resemble those of other digestive diseases to a large extent.

The commonest complaints are:

• Bloating of the abdomen,
• Wind,
• Abdominal pain or discomfort,
• Rumbling in the abdomen,
• Diarrhoea or a change in bowel habit,
• A feeling of discomfort that increases after meals.

In more marked cases the absorption of food may also be affected.

The findings that may then appear include:

• Weight loss,
• Fatty stools,
• Vitamin B12 deficiency,
• Deficiencies of vitamins and minerals.

But these are not seen in every patient with SIBO.

Why does SIBO cause bloating and wind?

The bacteria present in excess in the small bowel can ferment some of the carbohydrates eaten earlier than normal.

Gases such as hydrogen, carbon dioxide and, in some people, methane may be produced during this process.

The result, particularly after meals, may be:

bloating → wind → tightness in the abdomen → pain or discomfort.

Can SIBO cause malabsorption?

Yes. In more marked cases of SIBO in particular, the bacteria can affect the processes of digestion and absorption.

Disturbance of the function of the bile acids can reduce the digestion and absorption of fats. As a result, fatty stools (steatorrhoea) and impaired absorption of the fat-soluble vitamins may develop in some patients.

Because bacteria can also affect the use of vitamin B12, deficiency of vitamin B12 may be seen in long-standing or serious cases.

For this reason SIBO may be one of the causes of malabsorption in some patients.

Are SIBO and IBS the same disease?

No.

Irritable bowel syndrome (IBS) and SIBO are different conditions.

But because both can show:

• Bloating,
• Wind,
• Abdominal pain,
• Diarrhoea,
• A change in bowel habit,

the symptoms can resemble one another.

Some aspects of the relationship between SIBO and IBS are still being investigated. So having complaints of bloating and wind alone does not mean that a person definitely has SIBO.

What is IMO? Is it different from SIBO?

In recent years a separate concept has been used, particularly for people with a raised methane level:

IMO — intestinal methanogen overgrowth.

The microorganisms that produce methane are in fact not bacteria but different microorganisms called archaea. Nor do they have to be present only in the small bowel.

For this reason it is now considered more correct to use the term IMO rather than SIBO for a raised methane level.

IMO may be associated particularly with constipation and with slowing of transit through the bowel.

How is SIBO diagnosed?

The diagnosis of SIBO is not always easy.

The symptoms alone do not establish the diagnosis, because bloating, wind and abdominal pain can be seen in a great many digestive diseases.

In making the diagnosis, the patient’s complaints, previous operations, the medicines they take and any diseases that could predispose them to SIBO are all assessed together.

What is a breath test?

One of the methods used most often in investigating SIBO is the hydrogen and methane breath test.

The patient is generally given a liquid containing a set amount of glucose or lactulose.

Breath samples are then taken at set intervals and the amounts of hydrogen and methane are measured.

Normally these gases are produced not by human cells but by the microorganisms in the bowel. The changes in gas levels during the test give indirect information about the activity of the bowel bacteria.

But breath tests are not perfect. Factors such as the transit time of the bowel and the test substance used can affect the result.

For this reason the test result must always be assessed together with the patient’s clinical findings.

Can SIBO be diagnosed at endoscopy?

SIBO is not seen directly at routine gastroscopy or colonoscopy.

In some particular situations a fluid sample can be taken from the small bowel for microbiological assessment. But because this method is invasive and has technical limitations, it is not done in every patient in day-to-day practice.

Is all wind and bloating SIBO?

Certainly not.

Wind and bloating are extremely common in the population.

Similar complaints can also arise from:

• IBS,
• Lactose malabsorption,
• Problems with the absorption of other carbohydrates,
• Coeliac disease,
• Constipation,
• Functional bloating,
• Certain diseases of the stomach and bowel.

For this reason it is not right to diagnose SIBO simply on “I have a lot of wind” or “I swell up after eating”.

How is SIBO treated?

Treatment has two main aims:

1. To reduce the excessive load of microorganisms,
2. Where possible, to correct the condition that caused or facilitated the development of SIBO.

In suitable patients, antibiotic treatment may be used under a doctor’s supervision. The drug to be used and the length of treatment are decided according to the patient’s circumstances.

But taking an antibiotic alone is not always enough.

If the cause of the SIBO is a serious slowing of the bowel movements or an anatomical problem, for instance, SIBO can recur for as long as the underlying problem continues.

Is rifaximin used in SIBO?

Rifaximin is one of the antibiotics used often in the treatment of SIBO; very little of it is absorbed from the bowel.

But it is not right for everyone with bloating to take rifaximin on their own initiative.

The decision to treat should be made by assessing together the patient’s symptoms, the likelihood of SIBO, the test results and any underlying diseases.

The approach to treatment may differ in patients with a raised methane level in particular.

Is a diet needed in SIBO?

Adjusting the diet can help control the symptoms in some patients.

Reducing certain carbohydrates that are easily fermented by the bowel bacteria in particular can lessen wind and bloating.

A short-term low FODMAP approach may be useful in some patients.

But it is not right to follow very restrictive diets for a long time without supervision. Unnecessary restrictions can lead to nutritional deficiencies and to a reduction in the variety of the diet.

Diet should not be seen as a treatment that on its own removes the underlying cause of SIBO.

Do probiotics treat SIBO?

Although there are studies on the place of probiotics in the treatment of SIBO, the evidence is not strong or consistent enough to recommend giving a probiotic routinely to every patient.

The approach of “if there is SIBO a probiotic must be taken” is therefore not correct.

Does SIBO recur?

Yes.

SIBO can appear again after treatment, particularly if the disorder of bowel motility, the anatomical change or the other disease that caused it is still present.

For this reason, in recurrent SIBO it is important to investigate why it is recurring before using antibiotics continuously.

When should a doctor be consulted?

Long-standing or frequently recurring complaints of bloating and wind should be assessed.

Where there is:

• Weight loss of unexplained cause,
• Long-standing diarrhoea,
• Fatty stools,
• Anaemia,
• Deficiency of B12 or other vitamins,
• Blood in the stool,
• Complaints that wake the person at night,

SIBO should not be the only thing considered, and other underlying digestive diseases should be investigated as well.

Remember

SIBO is not a simple condition that can be explained merely as “there are too many bacteria in the bowel”.

Bloating, wind and abdominal pain are common in SIBO; but these symptoms alone do not establish the diagnosis.

The patient’s symptoms, their risk factors and, where needed, a breath test should be assessed together in making the diagnosis.

The aim of treatment is not only to reduce the bacteria, but to establish why the SIBO developed and, where possible, to correct the underlying cause.

It should also be remembered that SIBO and a raised methane level (IMO) are not the same condition.