Human intestinal microbiota create a complex polymicrobial ecology. This is characterised by its high population density, wide diversity and complexity of interaction. A balanced microbiome is essential for overall health. Therefore, it’s crucial to focus on the microbiome’s equilibrium in the colon while taking into account the quantity and variety of bacteria, yeast, and other microorganisms. But what occurs when these organisms move from the colon to the small intestine? This results in a condition known as Small Intestinal Bacterial Overgrowth (SIBO).

WHAT IS SIBO?
SIBO (Small intestinal bacterial overgrowth) is a very heterogeneous syndrome characterised by an increased number and/or abnormal type of bacteria in the small bowel. SIBO refers to an excess of bacteria in the small intestine. Normally, the small intestine contains only minimal bacterial amounts, but when this quantity rises, the bacteria ferment dietary carbohydrates, leading to gas production.

HOW IS SIBO DIAGNOSED

If you have ongoing digestive symptoms, SIBO may not be at the top of your healthcare provider’s mind. That’s because SIBO symptoms often overlap with many other digestive conditions such as IBS. Before your healthcare provider makes a diagnosis, they will review your symptoms and medical history. If they suspect SIBO, they will likely recommend a breath test to measure the gases produced by the microbes in your small intestine. Laboratory tests usually find anaemia, low serum vitamin B₁₂ levels and laboratory signs of malnutrition (lymphopenia, low serum prealbumin and transferrin). Hydrogen and methane breath tests are currently the most important diagnostic methods. The hydrogen breath test is considered to be more accurate for the diagnosis of SIBO compared to the methane breath test. However, hydrogen and methane breath tests have some drawbacks with possible false results and difficulties in their interpretation. Very rapid absorption of glucose in the proximal jejunum can be responsible for a false negative result. In the case of bacterial overgrowth in the terminal ileum, it might be difficult to distinguish a pathological ileal peak from a “normal peak” after the caecum is reached. In short bowel syndrome (with intact large bowel), the test substrate may reach the colon very quickly and cause a false positive result. In the case of a low density of anaerobes, breath can be false negative.

SIBO symptoms can encompass:

• Abdominal discomfort
• Bloating
• Gas
• Constipation, diarrhea, or a combination of both
• Heartburn
• Nausea
• Intolerance to carbohydrates
• Malabsorption issues
• Deficiencies in nutrients
• Stress and anxiety
• Mental cloudiness
• Joint pain

Similar to how SIBO symptoms can differ among individuals, the underlying causes can also vary.

Factors that can contribute include:

• Low levels of stomach acid
• Impaired gastrointestinal motility
• Previous food poisoning
• Certain medications

People with autoimmune conditions like Crohn’s disease and celiac disease are more vulnerable to developing SIBO.

FIVE DIFFERENT TYPES OF SIBO

HYDROGEN DOMINANT SIBO
Hydrogen dominant SIBO is defined by an increase in bacteria in the small intestine that generate hydrogen gas. It is the most common form of SIBO. What’s more, hydrogen SIBO can cause damage to your gut’s mucosal lining, creating a lactase deficiency. Lactase is the enzyme we use to break down and digest lactose. This is another reason why sudden food intolerances are a sign of an underlying gut infection.

METHANE DOMINANT SIBO
Methane dominant SIBO involves an overproduction of methane-generating bacteria within the small intestine. This type of SIBO is commonly linked to constipation and is also known as intestinal methanogen overgrowth (IMO). As the hydrogen feeds single-cell organisms in your gut called archaea, they produce methane. This can reduce your hydrogen levels which explains why you can have a false negative hydrogen breath test result and still have methane SIBO.

MIXED SIBO
Mixed SIBO is characterized by heightened levels of both hydrogen and methane gases on a SIBO breath test, indicating the presence of both hydrogen-producing and methane-producing bacteria.

HYDROGEN SULFIDE DOMINANT SIBO
This type of SIBO occurs when small intestine bacteria utilize hydrogen produced by other bacteria to create hydrogen sulfide, a gas that has a rotten egg odor. Hydrogen sulfide SIBO is often associated with diarrhea.

SIFO
SIFO, or small intestinal fungal overgrowth, happens when fungal organisms—not bacteria—invade the small intestine. SIFO shares similar symptoms and triggers with SIBO, and ongoing research is helping to enhance our understanding of this condition.

SYMPTOMS OF SIBO VS. IBS
Small intestinal bacterial overgrowth (SIBO) is one manifestation of gut microbiome dysbiosis and is highly prevalent in IBS (Irritable Bowel Syndrome).  You might have observed that the symptoms of SIBO are quite comparable to those of irritable bowel syndrome (IBS), which includes abdominal pain and alterations in stool frequency and consistency. IBS is typically diagnosed when other gastrointestinal disorders do not account for the symptoms.  Numerous pathogenic organisms have been shown to be increased in subjects with SIBO and IBS, including but not limited to Enterococcus, Escherichia coli, and Klebsiella. In addition, Methanobrevibacter smithii, the causal organism in a positive methane breath test, has been linked to constipation predominant irritable bowel syndrome.

SIBO AND HISTAMINE INTOLERANCE

Histamine is a natural substance produced by the immune system in its defense against pathogens. Histamine intolerance arises when histamine is released more quickly than the body can metabolize it. For instance, seasonal allergies often correspond with elevated histamine levels.

Symptoms of histamine intolerance include:

• Itchy skin and hives
• Nasal congestion
• Headaches and migraines
• Dizziness
• Rapid heartbeat
• Anxiety
• Abdominal cramps and nausea

There is a connection between SIBO (Small Intestinal Bacterial Overgrowth) and histamine intolerance, as SIBO may be a contributing factor to symptoms of histamine intolerance. Some bacteria produce histamine, and an overgrowth of these bacteria can lead to increased histamine levels. Furthermore, conditions like SIBO, infections, and digestive problems can hinder the body’s ability to break down histamine.

SIBO AND AUTOIMMUNE DISEASE

In certain instances, autoimmunity might be a fundamental cause of SIBO. To comprehend this link, it is important to consider the migrating motor complex (MMC), which involves the muscle contractions that help clear the intestines between meals and facilitate the downward movement of contents. If the MMC is not functioning properly, SIBO may occur.

There are multiple reasons for MMC dysfunction, one of which is elevated antibody levels following a food poisoning incident. Certain antibodies can instigate an autoimmune reaction against the MMC muscles. This pattern is commonly seen in autoimmune diseases, where an environmental trigger initiates the autoimmune response.

DIET RECOMMENDATIONS

It is important to be careful when prescribing restrictive diets, since the most severe cases of SIBO can cause malabsorption syndrome, which can lead to nutritional risk, and a healthy diet rich in vegetables and whole foods seems the best option to obtain a balanced microbiota. Dietary changes Implementing dietary changes can help eliminate foods that worsen symptoms or reduce the intake of fermentable carbohydrates that feed bacteria in the small intestine. Some dietary options that your healthcare provider may suggest include:

• low FODMAP diet
• low-fermentation diet
• elimination diet
• elemental diet
• specific carbohydrate diet

My Thoughts

An overgrowth of bacteria in the small intestine is often due to strains like lactobacillus or bifidobacterium. Since many probiotic supplements contain these species, they can worsen symptoms in individuals with small intestinal bacterial overgrowth (SIBO). Therefore, it’s essential to consult an expert in microbiota to find the appropriate treatment for balancing your ecosystem, as the rapidly evolving science presents ongoing challenges for specialists. I hope this overview of SIBO has been informative. In my next article in this series of a Healthy Gut , I will explore treatment options from the perspective of functional therapies.