This question has been posed a lot recently and as a nutritional therapist who specialises in IBS I believe that SIBO or some degree of bacterial overgrowth is present in the majority of “IBS” patients I see. For some people a bacterial overgrowth is the predominant factor in their IBS whilst other people have what I have termed as more of a “SIMO” or Small Intestine Microbial Overgrowth. Its worth remembering that if you have a motility issue with your small intestine then all forms of microbes, not just bacteria, can overgrow – yeasts and parasites too. In any event, it pays to undergo some clinical tests to identify the type and severity of your own form of SIBO and, once known, this information can guide as to the type of diet and supplements which are most appropriate.

SIBO – is it auto-immune?

Awareness of SIBO has grown enormously in the past 4 years and research in the field has been led by Dr Mark Pimentel of Cedars Sinai Hospital in California and Dr Allison Siebecker of

At the 2014 SIBO symposium, Dr Mark Pimentel explained the rationale for considering SIBO to be auto-immune in origin and gave the all-important mechanism of action: It is common for IBS symptoms to be experienced following acute episodes of infectious gastroenteritis or food poisoning, common bacterial causes being Campylobacter, Salmonella, E. coli, and Shigella. This is considered to be IBS-PI or post infectious IBS. Many patients link their IBS symptoms with being ill on holiday or following a gastro bug or make the link when prompted.

When infected with a gastro bug, the bug creates toxins, and in some instances a person’s immune system makes antibodies to attack and neutralise them. Once the infection has cleared, the antibodies remain in circulation in the body in case they are needed at a later time. Unfortunately however, the toxin antibodies can mistake a special protein in the wall of the small intestine called “vinculin” needed for normal GUT nerve and MMC function for the bug toxins. The immune system then sees the special vinculin proteins as the enemy and attacks them. The antibody attack on the vinculin affects normal GUT function and can cause IBS pain and symptoms. Impairment of the MMC and the loss of its cleansing wave like action can give rise to SIBO.

Further research is underway to create a blood test for the vinculin antibody, as this will be a huge help in identifying Post infectious IBS/SIBO.

How Does SIBO Affect Us?

  • The bacteria interfere with normal digestion and absorption of food can damage the intestinal lining causing “leaky gut”.
  • The bacteria consume our food which, over time, leads to deficiencies in nutrients such as iron and B12, causing anaemia.
  • The bacteria consume food unable to be absorbed (due to the damaged gut lining) and this creates more bacterial overgrowth (a vicious cycle).
  • Large volumes of gas are produced in the small intestine or upper gut. This causes abdominal bloating, pain, constipation, diarrhoea or both (the symptoms of IBS), belching and flatulence.
  • The bacteria also interfere with fat absorption by de-conjugating bile acids, this in turn leads to deficiencies of vitamins A & D and gives the sufferer fatty stools.
  • As the gut lining is damaged, larger food particles (not able to be fully digested) enter the body, and the immune system reacts to them. This causes food allergies/ sensitivities.
  • Bacteria themselves can also enter the body/bloodstream. The subsequent immune reaction to bacteria and their cell walls (endotoxin) causes chronic fatigue and body pain and burdens the liver.
  • The bacteria excrete acids which at high levels can cause neurological and cognitive symptoms.

The entire gastrointestinal tract, including the small intestine, normally contains bacteria. The number of bacteria is greatest in the colon (at least 1,000,000,000 bacteria per millilitre (ml) of fluid) and much lower in the small intestine (less than 10,000 bacteria per ml of fluid). The types of bacteria within the small intestine should differ to the types of bacteria within the colon. Small intestinal bacterial overgrowth (SIBO) is also known as small bowel bacterial overgrowth (SBBO).

What causes SIBO?

The gastrointestinal tract is a continuous muscular tube through which digesting food is transported on its way to the colon. The coordinated activity of the muscles of the stomach and small intestine propels the food from the stomach, through the small intestine, and into the colon. Even when there is no food in the small intestine, muscular activity sweeps through the small intestine from the stomach to the colon. The muscular activity that sweeps through the small intestine is important for the digestion of food, but it is also important because it sweeps bacteria out of the small intestine and thereby limits the numbers of bacteria in the small intestine. Anything that interferes with the progression of normal muscular activity through the small intestine can result in SIBO. We now know that in many cases SIBO is as a result of acute gastroenteritis or food poisoning but equally it can also be caused post surgery (blind loop or ileocecal valve removal), use of PPI’s or alongside celiac disease or IBD. Simply stated, any condition that interferes with muscular activity in the small intestine allows the bacteria to stay longer and multiply in the small intestine. The lack of muscular activity also may allow bacteria to spread backwards from the colon and into the small intestine.

SIBO Symptoms

The main symptoms of SIBO are typically those of IBS: abdominal bloating (gas, belching or flatulence), abdominal pain, constipation and/or diarrhoea.

An emerging new train of thought is that SIBO might underlie Irritable Bowel Syndrome and there are some recent studies which appear to confirm this theory, a study by Pimentel et al (2003) found SIBO to be present in 84% of patients tested, although other studies have suggested that this figure may be lower and closer to 50%. In my clinical experience around 60 – 65% of IBS patients also have SIBO.

It is also possible that SIBO could be associated with other disorders either as a cause or as an effect of another disease or condition, such conditions includes:

  • Heartburn
  • Nausea
  • Leaky Gut
  • Food Intolerance
  • Joint Pain
  • Fatigue
  • Eczema
  • Asthma
  • Depression
  • Steatorrhea
  • Anaemia (Iron or B12)
  • Acne Rosacea
  • Celiac Disease
  • Chronic Prostatitis
  • CLL (Chronic Lymphocytic Leukemia)
  • H pylori Infection
  • Hypochlorhydria
  • Hypothyroid/ Hashimoto’s Thyroiditis
  • Restless Legs Syndrome
  • Crohn’s Disease
  • Ulcerative Colitis
  • Interstitial Cystitis

As a general rule, if the symptoms of IBS are present, as well as one of the above conditions, then SIBO should be considered.

SIBO Tests

Hydrogen Breath test

There is a simple, inexpensive and non-invasive test which can be used to identify SIBO which is the Hydrogen Breath Test. Breath testing measures the hydrogen and methane gas produced by the bacterial fermentation of special types of sugars which are not absorbable by humans, only by bacteria. The gases produced diffuse into the blood, and then into the lungs, for expiration. Hydrogen and Methane are only produced by bacteria, not by humans. The gas is measured over a 2-3 hour period (the typical small intestinal transit time) and compared to the starting level.

Lactulose Breath Test (LBT)

Lactulose cannot be digested by humans; we are dependent on bacteria to do this. When bacteria digest lactulose, they produce gas. The levels of the gases created (Hydrogen and Methane) indicate bacterial overgrowth.

The lactulose test can diagnose overgrowth in the distal end of the small intestine which is the portion closest to the colon and thought to be most prevalent. However the test is not as sensitive as the glucose breath test.

Glucose Breath Test (GBT)

Glucose is usually absorbed within the top two feet of the small intestine; so if Hydrogen and methane is found in this test it indicates an overgrowth in upper end of the small intestine, closest to the stomach.

This test has its advantages; it is better at diagnosing bacterial overgrowth of the top portion of the small intestine but cannot identify an overgrowth further along the gut.

Treatment Approaches

There are 4 main treatment approaches for SIBO:

  1. Antibiotics, prescribed by a GP. Typical course is 2 weeks, the antibiotic used in much of the US research is not approved for use in the UK but may be obtained by private prescription and is somewhat costly. SIBO can and often does rebound so antibiotics by themselves can be ineffective in resolving SIBO permanently. It is imperative to combine antibiotics with a low carb style diet to be effective and to achieve a permanent resolution.
  2. Herbal antibiotics which may include Enteric coated peppermint, Golden seal, Oregano or Grapefruit Seed Extract. A combination of these is recommended and is very effective. These herbal antibiotics should be taken for a longer period than the antibiotics and can be safely continued for longer periods of time. Many of my patients follow a course of GP prescribed antibiotics with my herbal protocol and this works well.
  3. Specific Carbohydrate Diet – This starves the bacteria and takes longer than antibiotics. The length of time needed on the SCD diet varies enormously and can be a number of weeks to several months depending on symptom severity.
  4. Elemental Diet – This involves the use of a liquid diet containing predigested food which is rapidly absorbed in the upper part of the small intestine and in so doing feeds the host and not the bacteria. The diet needs to be taken for 14 days and is considered to be very effective. Unfortunately the shakes are not very pleasant tasting and the cost is very prohibitive for many people.

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