Millions of Americans suffer from a wide variety of gastrointestinal (GI) symptoms ranging from mild gas and bloating, to chronic constipation and diarrhea each day. In addition to causing a great deal of discomfort to many individuals across the nation, the burden and costs of GI diseases in the United States can cost up to $136 billion each year.

In fact, healthcare expenditures for digestive diseases are greater than those dedicated to treating heart disease, trauma and mental health 1.

While the research that has linked abnormalities in the microbiome to various health conditions is still in its infancy, researchers have made significant strides in connecting intestinal microbiota imbalances to various digestive disorders like irritable bowel syndrome (IBS).

This article will provide some insight into the current understanding of one particular type of digestive disorder known as small intestinal bacterial overgrowth (SIBO). Furthermore, this article will discuss the current classifications for SIBO, the relationship of this disorder to IBS and other bowel disorders, as well as the potential future treatment options expected to emerge for both SIBO and IBS patients.

Understanding the Role of the Small Intestine

The GI tract, which is also referred to as the alimentary canal, is the pathway in which food enters our body starting at our mouth, until it is excreted by the anus. The minute we place food inside our mouths, digestion begins. Our tongue, teeth, and lips work together to break down larger food components into smaller molecules that can be more easily absorbed by the rest of our digestive system. Once food leaves the mouth, it travels down to our esophagus in the form of a bolus until it reaches the stomach. From here, food travels through the small intestine to the large intestine and rectum, until it is eventually excreted from our body by the anus.

During the process of digestion, the small intestine plays a critical role in performing as much as 90% of the digestion and absorption of food and its nutrients, whereas the remaining 10% is performed by the stomach and large intestine 2. Various different enzymes work together in the small intestine to break down carbohydrates, proteins, fats, and other food products into molecules that can be easily absorbed by the intestinal cells. To put the tremendous amount of work performed by the GI tract into perspective, it is estimated that each day the GI tract processes up to 10 liters of food, liquids and GI secretions, 90% of which will be absorbed by the small intestine and ultimately result in less than 1 liter of fluid that will actually enter the large intestine each day 3.

When present in normal quantities, the bacteria (flora) of the GI tract perform a variety of critical functions including assisting the digestive process by absorbing various vitamins, such as folic acid and vitamin K, as well as protect the small intestine from being invaded by harmful bacteria associated with diseases. In fact, the gut microbiota also assists in fermenting ingested substances that are more difficult for our bodies to digest, such as dietary fibers.

In the event that the normal function of the small intestine or any other part of the GI tract is compromised, bacterial overgrowth can occur and cause a wide range of harmful effects, such as inhibition of normal digestion processes, as well as damage to the lining of the small intestine.

What Exactly is SIBO?

small intestine bacterial overgrowth

Small intestine bacterial overgrowth (SIBO) is a condition in which there is an over-colonization of specific bacteria, typically that which is normally found within the large intestine, that is greater than 103 colonies, ultimately causing uncomfortable digestive symptoms to occur in those affected.

Since the small intestine is not typically designed to house this quantity of bacteria, these species end up interfering with the normal digestion and nutrient absorption processes that traditionally occur in the small intestine.

Under normal circumstances, the small intestine has a number of protective measures it uses to prevent the overgrowth of bacteria, some of which include:

  • Bile
  • Enzymes
  • Gut-associated lymphoid tissue (GALT)
  • Migrating Motor Complex (MMC) (click here to learn more about the role of the MMC)

SIBO is often associated with other types of illness that affect the small intestine, such as:

  • Irritable Bowel Syndrome (IBS, which is the majority of SIBO)
  • Crohn’s Disease
  • Celiac Disease
  • Achlorhydria (a chronic inflammation that prevents the stomach from producing acid normally)
  • Liver cirrhosis
  • Alcohol abuse
  • Non-alcoholic steatohepatitis (NASH) 2

Despite the known associations of SIBO with other illnesses, there remains a tremendous gap that needs to be filled in regard to understanding the pathophysiology of SIBO, as well as how its progression plays a role in its relationship to IBS.

SIBO Symptoms

SIBO Symptoms

The symptoms of SIBO are similar to those which are associated with several other digestive disorders, including IBS. This similarity in symptom presentation is most likely attributed to the documented association that exists between SIBO and IBS, which has indicated that up to 60% of IBS patients also have bacterial overgrowth as an underlying issue of their disease.

The most identifiable symptoms of SIBO are usually digestive symptoms like gas, bloating, diarrhea, and constipation; however, several other parts of the body can be affected by this condition.

Common Symptoms of SIBO and IBS:

  • Bloating
  • Gas
  • Diarrhea
  • Constipation
  • Cramps
  • Indigestion
  • Nausea
  • Belching
  • Feeling of fullness
  • Pain
  • Malnutrition and malabsorption
  • Fatigue
  • Weight loss
  • Acne
  • Depression
  • Anxiety
  • Brain fog
  • Restless legs

How Many People Does SIBO Affect?

world map

Both SIBO and IBS affect an immense amount of people around the world. In fact, it has been estimated that, as of 2017, they affect around 10% of the population in Western countries, which ultimately results in 50% of these individuals requiring periodic consultations with their physicians when their condition relapses 4.

As a result of this disease’s widespread prevalence, researchers have remained devoted to acquiring useful information on the mechanisms by which this disease arises, as well as new and improved potential treatment options for both IBS and SIBO.

Here are the numbers:

  • Around 10% of people in the world have IBS, which amounts to a total of 700 million 3
  • Around 60% of people with IBS have been found to also have SIBO
  • Therefore, roughly around 420 million people are affected by SIBO
  • In the United States alone, roughly 32 million people deal with IBS; therefore, it is estimated that nearly 19 million people also suffer from SIBO

Other Facts Which Help Explain SIBO & IBS

  • The symptoms and quality of life for each patient with either SIBO, IBS or both can greatly vary. While some people may experience mild symptoms, others may experience severe and debilitating symptoms
  • About 1 in 3 sufferers are male, whereas the remaining 2 in 3 sufferers are female 4
  • These conditions are not fatal; however, their associated symptoms can dramatically affect the patient’s quality of life.
  • Both SIBO and IBS can be chronic conditions for some patients, whereas others can heal rather quickly.

What Causes SIBO?

what causes sibo

There are two underlying mechanisms that contribute to the ability of bacteria to overgrow in the small intestine and ultimately lead to SIBO:

1. Small Intestinal Dysmotility

A healthy GI system is in constant motion; a process that is more formally referred to as peristalsis, which is the irregular and forceful movements that occur within the GI tract following a meal. This process works with the migrating motor complex (MMC) to produce coordinated movements every 90-120 minutes during times of fasting 5.

These two mechanical processes work together to move food, bacteria, and debris from the intestines to their next destination within the GI tract. Research estimates that up to 70% of people living with SIBO, particularly those with hydrogen- and/or hydrogen-sulphide SIBO, experience a disrupted MMC rhythm as a result of nerve dysfunction in the gut.

Motility disorders, excessive stress, and organ dysfunction can all play a role in SIBO.

2. Malabsorption

There are multiple ways in which nutrient malabsorption can lead to SIBO. Consider the way in which our stomach functions. The primary role of the gastric acid, which is also known as hydrochloric (HCl), in the stomach is to not only break down larger food products into absorbable molecules, but to also suppress the growth of ingested bacteria 5. Low gastric acid levels can be attributed to Helicobacter pylori (H. pylori) infection or acid-suppressing medicines like omeprazole.

Other underlying causes of malabsorption include a reduced secretion of bile acids and pancreatic elastases, both of which are important substances that play a role in maximizing nutrient absorption from food. Low levels of both bile acids and pancreatic enzymes can contribute to an imbalance in normal gut bacteria levels. Additionally, any process that damages the small bowel mucosa, which is the primary site of food absorption, can also contribute to SIBO.

Delving deeper, we find there are four main categories of diseases where dysmotility or malabsorption occurs which can lead to SIBO:

1. Motility Disorders

Motility disorders affecting the gut are a major underlying cause of SIBO. Many metabolic and organ disorders (see below) lead to motility problems. For example, scleroderma is an underlying SIBO cause in up to 62% of patients.7

Similarly, neurological disorders, such as chronic intestinal pseudo-obstruction and gastroparesis, can also slow down small intestine motility and ultimately lead to SIBO. With a slowed gut, bacteria are unable to be eliminated as fast as normal, thereby, leading to stasis and bacterial growth.

2. Malnutrition and Malabsorptive Disorders

Any disorder that disrupts normal GI absorption can become a SIBO cause. If you can’t absorb your food, the bacteria will feed on it and grow.

A classic example of a malabsorptive disorder associated with SIBO is Celiac disease. In fact, up to 2/3 of Celiac patients who experience symptoms on a gluten-free diet have been shown to also have SIBO 8. Other malabsorptive disorders that have been associated with SIBO include chronic pancreatitis, cirrhosis, and Crohn’s disease 7.

Additionally, over time, the Standard American Diet (SAD), which is high in poorly digested carbohydrates and inflammatory fats, can contribute to an unhealthy gut bacteria in both the small and large intestines.

3. Anatomic Disorders

Patients who have undergone gastric bypass surgery in an effort to lose weight, abdominal surgery or who have small intestine diverticula or outpouchings 8 are also at an increased risk of developing SIBO. The anatomical lesions that result from these types of procedures can cause a “swamp” 5 in the small intestines, which can ultimately reduce gut motility, thereby disrupting food absorption, which, as previously mentioned, can ultimately induce excess bacterial growth within the small intestine.

4. Immune and Organ Dysfunction

A reduced antibody or T-cell response has also been associated with an increase in GI bacterial overgrowth. The absence of these important immunological components can lower the ability of the immunoglobulin A (IgA) within the mucosa of the small intestine to adequately protect itself against foreign invasion,1 which can therefore invite bacteria to cling to the intestinal mucosa 5. Another side effect of reduced intestinal immunity is increased inflammation in the small intestine, which can also contribute to lowered nutrient absorption.

Organ dysfunction, such as that which occurs in diabetes, cirrhosis and renal failure, can also impair the enteric nervous system within the GI tract and subsequently slow down GI motility.

It is also worth mentioning that SIBO occurs in up to 15% of elderly and disabled adults as a result of their increased intake of different medications, a greater number of health problems, and lowered immunity 8.


A Summary of Underlying Causes in SIBO

Anatomical disorders
  • Small intestine diverticula
  • Gastric resection
  • Surgical blind loops
  • Adhesions
Motility disorders
  • Gastroparesis
  • Small bowel dysmotility
  • Celiac disease
  • Chronic intestinal pseudo-obstruction
Irritable Bowel Syndrome
  • IBS-Diarrhea
  • IBS-Mixed

*Both suspected to occur from impaired motility following food poisoning

Metabolic disorder
  • Diabetes
  • Hypochlorhydria
Organ system dysfunction
  • Cirrhosis
  • Renal failure
  • Pancreatitis
  • Immunodeficiency
  • Celiac Disease
  • Crohn’s Disease
  • Recurrent antibiotics
  • Gastric acid suppression


Learn about SIBO causes with Dr. Leonard Weinstock:


A Special Note on Food Poisoning- Leading Cause of SIBO in IBS

Meat Contamination

More than 250 different types of pathogenic bacteria and toxins are transmitted through food each day. In fact, this number continues to increase when considering the different types of bacteria that can be present in water or following exposure and/or direct contact with other animals and human beings.

Some bacteria species that are well known for their harmful effects to human health include:

  • Shigella
  • Salmonella
  • Campylobacter jejuni
  • C. difficile
  • E. coli
  • Norovirus 9

Although we may ingest minuscule amounts of bacteria within our food each day, most of the time we will not experience any severe side effects as a result of the various methods in which our GI tract protects our body against invasion from unwanted pathogens.

While this may be true the consumption of food that has not been cleaned properly or is contaminated with such a large amount of bacteria that our bodies cannot mount an adequate immune response, can prevent our GI tract from preventing illness. For example, individuals traveling to third world countries outside of the U.S. are often at a much greater risk of coming in contact with these types of situations, thereby increasing the risk of these individuals to experience the severe GI symptoms associated with a condition known as food poisoning.

The following figure demonstrates exactly what happens in our bodies when we experience food poisoning in terms of DNA damage and cell death:

sibo sequence

The following figure demonstrates exactly what happens in our bodies when we experience food poisoning in terms of our autoimmune response 10, 11:

ibs and sibo disease sequence

A significant amount of research has established the connection between an infectious event, such as that which occurs when someone experiences food poisoning, and an individual’s increased susceptibility to attain IBS.

Researchers suspect that around 10% of people who get severe food poisoning will develop IBS following an episode.

The connection between these two illnesses has been attributed to a change in the affected individual’s physiology, such as altered migrating motor complex cleaning waves (MMC), at both the anatomical (structural) and neurological (nerves) level. More specifically, the effects that food poisoning and other intestinal infections have on a patient’s intestinal motility, which is the ability of the intestines to move food, is a leading factor that promotes bacterial overgrowth, IBS-D and IBS-M.

Click here to learn about how you can protect yourself from food poisoning.


What Tests Identify the Underlying Causes of SIBO?


Before you undergo testing, consider your symptoms:

  • Do you have inflammatory symptoms, such as hives?
  • Are you currently taking any prescribed medications, or have you been previously diagnosed with any medical conditions that could affect your gut?
  • Have you recently experienced food poisoning or any other GI-related illness?

Classic SIBO Diagnostic Tests

1. Upper GI Bacteria culture

Obtaining an upper GI bacteria culture involves taking samples from your small bowel and culturing the bacteria. This type of diagnostic procedure is invasive and requires an upper GI endoscopy to be completed. Unfortunately, many endogenous bacteria do not grow well on standard culture media, thereby limiting the accuracy of this type of diagnostic procedure 7.

2. Breath testing (Most Common)

A SIBO diagnostic breath test measures the gas produced in the small intestine. After drinking a substrate (such as lactulose), a physician will analyze the patient’s breath for high levels of hydrogen and/or methane gas.

Generally, a rise of 20 parts per million (ppm) for hydrogen and 10 ppm of methane is considered to be indicative of SIBO.

Advancements in this area of diagnostic breath tests have led to a novel SIBO breath test capable of analyzing hydrogen sulfide in the breath 12. Hydrogen sulfide gas has been well documented as a more accurate biomarker of IBS- D and SIBO, especially if elevated levels of Desulfovibrio piger bacteria is suspected.

To learn more about the breath test process, click here.

Other important tests to consider:

3. IBS check test (anti-vinculin and anti-CdtB)

The IBS check test is specific for post-infectious IBS, which is a term that is commonly used to describe SIBO, that occurs after food poisoning. Recall that approximately 60% of IBS cases are estimated to occur because of a previous food poisoning event. The benefit of an IBS check test is that it provides the patient with more information as to whether he/she has developed IBS and SIBO as a result of food poisoning.

4. Stool tests (measures large intestine microbiome)

A DNA analysis of your large gut bacteria is performed through a laboratory technique known as polymerase chain reaction (PCR). This type of test can be especially useful for diagnosing large intestinal bacterial overgrowth (LIBO), and therefore help patients differentiate their condition from SIBO.

It is important to note that the science behind this specific diagnostic test is still in the early stages, as researchers are still learning new things each day about the complexity of the gut microbiome and its health implications.

Currently, a commercially available test is uBiome.

5. Other tests

Other tests that are important are:

  • Gluten sensitivity (Celiac disease)
  • Pancreatic elastase (pancreatitis)
  • Blood count and Ig panel (immune deficiency)
  • Small bowel follow through (anatomical problems)

Treatment for SIBO


SIBO is usually treated with a course of antibiotics, the elemental diet, diet changes, as well as probiotics and/or prokinetic agents.


Reducing the bacterial overgrowth in the small intestine with antibiotics is one treatment that doctors will commonly prescribe for SIBO. Some of the most commonly prescribed antibiotics for this purpose include Xifaxan and Neomycin.

Antibiotics are useful in the treatment of SIBO to help reduce bacterial overgrowth; however, if SIBO was the result of an underlying condition, such as a motility disorder or malabsorptive disorder, then the patient’s symptoms will most likely recur sometime following antibiotic treatment. This common complication emphasizes the importance of identifying the underlying causes of SIBO to reduce unnecessary and unsuccessful treatments.

Elemental Diet

The elemental diet is another treatment option that is used to eliminate SIBO and help the gut heal. SIBO patients that follow the elemental diet have been shown to experience a success rate of 80-85% in clinical studies. To learn more about the Elemental Diet, click here.


A diet that is lower in fermentable carbohydrates, such as the low FODMAP diet, can be used for treating SIBO to reduce bacterial overgrowth. The low FODMAP diet can be used alone or in combination with other treatments like antibiotics. To learn more about which foods to avoid and eat during an elimination diet, click here.


There have been studies demonstrating the ability of probiotics to help eliminate SIBO. It’s important to keep in mind that the action of probiotic agents is strain-specific, which means that different strains will often have different therapeutic effects on different individuals, depending on the specific type and amount of bacteria already present within their GI tracts.

If a patient is considering incorporating probiotic agents into their SIBO treatment, it is recommended that they find a strain that has been scientifically studied in SIBO and IBS patients to have positive effects. To learn more about the best probiotics indicated for SIBO treatment, click here.

Prokinetic Agents

Prokinetic agents can help SIBO patients who also have problems with gut motility. In the case of SIBO, dysmotility is one of the leading factors that can allow bacterial overgrowth to occur in the first place. Prokinetic agents like Prucalopride or ginger have been shown to have beneficial effects on intestinal motility. Often times, doctors prescribe prokinetic agents following antimicrobial treatment. To learn more about prokinetic agents, as well as how they are used for SIBO, click here.

How IBS is Connected to SIBO

Types of IBS and SIBO

With over 1 billion patients worldwide, IBS is one of the most commonly diagnosed GI disorders in the world. IBS can be characterized by a number of symptoms including abdominal pain or discomfort, bloating and some type of altered stool form, such as diarrhea or constipation. Patients with IBS may also experience an increase in either the number of normal bacteria present within their GI tract, such as in the case of SIBO, or a change in the diversity of bacteria present throughout the bowel, which is a condition otherwise referred to as dysbiosis.

Although IBS and SIBO are intertwined in many different aspects, it is important to realize that not all IBS is SIBO, and not all SIBO is IBS!

Below are the categories that medical science groups IBS into and their relationship to SIBO:

IBS-D (Diarrhea)

IBS Diarrhea

IBS-D is estimated to affect approximately 40% of individuals with IBS.

This type of IBS has been shown to be caused by SIBO and the autoimmune process that results from food poisoning highlighted previously.

Recent research has also suggested a link to hydrogen sulfide gas in diarrhea-predominant patients.

IBS-M (Mixed, alternating diarrhea or constipation)

IBS M or mixed

IBS-M is estimated to affect approximately 23% of individuals with IBS. This type of IBS has also been shown to be caused by SIBO and the autoimmune process that results from food poisoning.

SIBO has been shown to be present in both IBS-D and IBS-M due to the autoimmune process that results from a severe episode of food poisoning.

IBS-C (Constipation)

IBS C or Constipation

IBS-C is estimated to affect approximately 35% of individuals with IBS.

IBS-C is considered to be a separate microbial condition that is related to a bloom of methane-producing bugs called Archaebacteria in the small and large intestines.

This subset of patients has not been shown in research to have an autoimmune component like IBS-D and M patients.

Methane gas, which is produced by archaea has been shown to slow down the intestinal tract leading to constipation.


In conclusion, SIBO is the disease state that occurs when the small intestine is not functioning properly, resulting in an overgrowth of bacteria. As previously mentioned, SIBO can cause patients to experience a number of different digestive symptoms.

On the other hand, IBS is still the medical name doctors use to diagnose patients who also have SIBO, but it has been shown that about 60% of IBS patients have SIBO which is caused by the autoimmune process that happens as a result of food poisoning.

Keep in mind that SIBO will always occur as a result of an underlying condition like IBS, Crohn’s Disease, Celiac Disease, or any condition that impairs bowel motility.

SIBO is treatable with antibiotics, probiotics, diet, and prokinetics; however, it is important to remember that ignoring the underlying cause of a patient’s SIBO will likely lead to its recurrence in the future.



Submit a Comment

Your email address will not be published. Required fields are marked *

Share via