What is SIBO?

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Small Intestinal Bacterial Overgrowth (SIBO) occurs when normal gastrointestinal tract bacteria abnormally overgrow in the small intestine (or large intestine), causing symptoms such as abdominal bloating, pain/discomfort, and constipation, diarrhea or a mixture of both.[1] It's typically caused by a slowing of small intestine motility or alteration of normal intestine anatomy from various diseases, injury/surgery, or medications.[1] ​

Site author Dr. Allison Siebecker with a brief video explanation of SIBO.

The Problem

​The overgrown bacteria interfere with the normal digestion and absorption of food, and damage the lining of the small intestine (the brush border).

• They consume some of the food we eat, causing malabsorption, which over time, can lead to deficiencies in nutrients such as iron and B12, which can cause anemia or chronic low ferritin.[1] [2]
• Food unable to be absorbed due to brush border damage is consumed by the bacteria, allowing them to proliferate (overgrow) causing a vicious cycle.[1]
• After eating, they produce gas (via fermentation) within the small (or large) intestine, which causes many symptoms, especially abdominal bloating, abdominal pain, constipation, diarrhea or both (the symptoms of IBS).[1]
• They decrease proper fat absorption by deconjugating bile, which can lead to fatty stools (steatorrhea) and deficiencies of fat soluble vitamins A, D and K.[1] [2]
• The increased number of bacteria, causes increased endotoxin in the small intestine (bacterial cell wall/LPS) which the immune system can react to causing fatigue, body pain, anxiety, depression, and can burden the liver.[3]
  
• The damage to the brush border can create Leaky Gut Syndrome, allowing larger food particles to enter into the body, which the immune system can react to.[1] This can cause food allergies/sensitivities with non-digestive symptoms: headache, nasal mucus, skin rashes, joint pain, etc...
• Finally, the bacteria excrete acids which may, in high amounts, lead to neurological and cognitive symptoms, such as brain fog.[4] [5] 
  

The small intestine (pictured in yellow) fills the abdomen. The large intestine (pictured in pink) surrounds the small intestine.

Types of SIBO

The different types of SIBO are based on the type of gas produced by the overgrown microbes.[6] Different microbes produce different gases, and these gases correlate with bowel movement symptoms (see box).[9]

Hydrogen is correlated with diarrhea, and is caused by overgrown E. coli and Klebsiella. [7]  It's International Diagnosis Code (ICD-10) is K63.8211.

Methane is correlated with constipation, and is caused by overgrown M. smithii and other methanogens.[6]  ICD-10: K63.829

Hydrogen Sulfide is correlated with diarrhea or constipation
(diarrhea is shown in SIBO studies but clinically, constipation occurs in many cases), and is caused by overgrown Proteus mirabilis (small intestine), and Fusobacterium varium and Desulfovibrio piger (large intestine). This type is less common.[8]  ICD-10: K63.8212

Mixed gases are most often a mix of hydrogen and methane which correlates with a mix of both diarrhea and constipation. Hydrogen sulfide can also occur in mixed SIBO.

Note: gas and bowel movement patterns don't always correlate as expected. We frequently see exceptions to the patterns.

New Names
Both methane and hydrogen sulfide SIBO can overgrow in the large intestine too, thus the "SI" was changed to "I" for intestinal.
Methane SIBO = Intestinal Methanogen Overgrowth (IMO) [1] [6]
Methanogens are archaea, not bacteria, thus the "B" in SIBO for bacteria was changed to "M" for methanogen. Technically, this is no longer considered a type of "SIBO" but a related condition.
Hydrogen Sulfide = Intestinal Sulfide Overproduction (ISO) [8] [9]
Humans make hydrogen sulfide gas which is beneficial, but in excess amounts it's harmful, thus "O" now stands for overproduction.

On this site "bacteria" includes methanogens (archaea) and "SIBO" includes IMO and ISO.

Learn more about
SIBO Types
in these podcasts

• Hydrogen SIBO with Dr. Allison Siebecker
  
• Methane SIBO with Dr. Allison Siebecker
• Hydrogen Sulfide SIBO with Dr. Allison Siebecker

​SIBO is diagnosed by both symptoms and testing in combination. Several treatment options are available, see an overview of SIBO treatment here.

Further Information

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SIBO Podcasts

• Almost all of my podcast interviews give a full description of SIBO, including causes and treatment 
  
• See the latest SIBO podcasts by various speakers 
  

SIBO Overview Articles

• Modern concepts of small intestinal bacterial overgrowth (2025) - Drs. Barlow and Pimentel 
  
• Small Intestine Bacterial Overgrowth, the case of the perpetual patient - Drs. Siebecker & Sandberg-Lewis
• Nutritional Consequences of Small Intestine Bacterial Overgrowth - Dr. DiBaise
• Small intestinal bacterial overgrowth syndrome - Dr. Burres et. al.

See all SIBO Research Studies Here

   Learn more

SYMPTOMS ASSOCIATED DISEASES CAUSES SIBO & IBS TESTING TREATMENT SIBO DIET COURSES

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