What Is Small Intestinal Bacterial Overgrowth

Do you know that the human body contains trillions of bacteria on and in it? These bacteria are found in various parts of the body, on the skin, along the digestive tract, and in the nose. The number of bacteria on and in the human body is said to outnumber the cells that make up a human being by a ratio of 10 to 1! 1

Despite the large numbers of bacteria we carry in and on our bodies, not all bacteria are pathogenic (i.e., capable of causing disease). Most bacteria present on and in the human body are harmless and play vital roles in the everyday workings of the human body.1

So what, then, is  Small intestinal bacterial Overgrowth (SIBO)? Recall that most bacteria in and on the human body are harmless. Well, in some conditions, such as SIBO, where there is an abnormal increase in the population of certain bacteria over the levels deemed normal for maintaining bodily functions, this can become a problem, often requiring medical intervention.2

Read on to learn about SIBO, why some people get it, the signs and symptoms, and what to do when you think you may have SIBO.


Small intestinal bacterial overgrowth (SIBO) is quite a mouthful to say. Still, the name gives us a clue about what the problem is. That is bacterial overgrowth in the small intestine.

What is the small intestine's role, and why is bacterial growth in the small intestine a problem? The small intestine is a part of the digestive tract that connects the stomach to the large intestine. It is approximately 23 feet long and consists of three parts: the duodenum, jejunum, and ileum.3 

Digestion of food starts in the mouth, continues in the stomach, and is completed in the small intestine through the action of enzymes from the pancreas and bile duct.3,4 The small intestine is where the absorption of nutrients occurs on a large scale before the digested material moves to the large intestine, where reabsorption of water from the digested material occurs. This water reabsorption reduces the volume of waste material to be excreted as poo.3,4

In SIBO, it's not just the number of bacteria that becomes a problem. An increase in the population of certain types of bacteria can also be a problem.2 Remember that the small intestine is involved in the digestion of food through the action of enzymes from the gallbladder (bile) and pancreas. Bile from the gallbladder plays a role in the breakdown of fats from digested food for absorption. At the same time, pancreatic enzymes are involved in carbohydrate digestion.2

Some populations of bacteria in the small intestine can break down bile salts; therefore, an overgrowth of these bacteria can affect the digestion of fats and the absorption of fat-soluble vitamins, resulting in nutrient deficiencies.2

Additionally, the continuous movement of food through the small intestine during digestion keeps the population of bacteria in the small intestine relatively low compared to the large intestine, where indigestible material is stored for extended periods before excretion.5

The small intestine is home to about 10,000 bacteria per ml of gastric fluid in the uppermost part of the small intestine, the duodenum.2 In SIBO, the population of bacteria in the small intestine is likely to exceed 1,000,000 to 10,000,000 bacteria per ml of gastric fluid.2,6 That is 100-fold more than the number of bacteria we would expect to see in the small intestine of a healthy individual.6

Causes of SIBO

Now that we have a better idea of what SIBO is, what factors can increase your risk of developing SIBO?

Generally, it is thought that certain conditions or procedures that delay the movement of food through the small intestine during digestion can contribute to the development of SIBO.5 The stasis of digested material in the small intestine due to the delay in movement through the small intestine is thought to create a conducive environment that encourages bacteria to multiply, resulting in an overgrowth.5

Conditions that can slow down the rate of transit of digested material through the small intestine include Crohn's disease, celiac disease, diabetes, radiation-induced enteritis, and scleroderma.5

Additionally, the following factors can increase your risk of developing SIBO:5

  • Complications following abdominal surgery 
  • Structural defects affecting your small intestine, e.g., diverticulitis affecting the small intestine, an injury to the small intestine, and an abnormal connection (the medical term for this is called a fistula) between two sections of the digestive tract 

Signs and symptoms of SIBO

Signs and symptoms that may indicate that you have SIBO include:5

  • Abdominal pain
  • Bloating 
  • Nausea 
  • Constantly feeling full 
  • Diarrhea
  • Loss of appetite
  • Unintentional weight loss
  • Malnutrition - Remember we mentioned earlier that some bacteria in the small intestine could break down bile salts, which can affect the digestion and absorption of fat and, by extension, the absorption of fat-soluble vitamins? If the overgrowth of bacteria in SIBO includes the bacteria that break down bile salts, this can contribute to malnutrition.

In addition, if diarrhoea occurs with SIBO, this can also contribute to malnutrition.

Management and treatment for SIBO

The treatment goals for SIBO are to reduce the population of bacteria back to levels that are considered normal, fix the root cause of what is causing the SIBO in the individual, and correct nutritional deficiencies, if any. 7 This may involve:7

  • Surgery to fix the structural defect, if possible 
  • Long-term antibiotic therapy to reduce the population of bacteria. 
  • Nutritional support: individuals with SIBO may require supplementation with essential nutrients such as B12, calcium, and iron. In addition, a lactose-free diet is encouraged in individuals with SIBO as the damage that occurs with SIBO can affect your ability to digest lactose in milk.

Diagnosis of SIBO

To confirm if you have SIBO, your GP or healthcare professional will need to send you for tests that will check for bacterial overgrowth in the small intestine and check if your body can digest and absorb fats from the food you eat.7

These tests may include:7

  • Breath Test: This is a non-invasive test where you are given a solution of glucose and water to drink. Then, you will be asked to breathe through a device that measures the amount of hydrogen and methane in your breath. Bacteria can break down sugars such as glucose to produce hydrogen or methane; thus, a sharp rise in hydrogen and methane gas levels after drinking the glucose solution may indicate SIBO. Although non-invasive, the breath test is not specific to SIBO and is often used in the diagnosis of other conditions affecting the digestive tract, such as sugar intolerance and irritable bowel syndrome (IBS)
  • Take a sample of fluid from the small intestine and send it for culture. This process is invasive but is considered the gold standard for diagnosing SIBO. This involves using an endoscope passed down from the mouth into the small intestine, where a sample of the fluid is taken and sent off to the laboratory for culture.
  • Blood tests to check for nutrient deficiencies
  • Stool tests to check for fat malabsorption - when fats are not digested and absorbed, they are excreted in your poo.
  • Imaging tests such as Magnetic Resonance Imaging (MRI), X-ray, or Computerised Tomography (CT) scan to check for structural defects affecting the small intestine or the digestive tract. 

Risk factors of SIBO

You're more likely to get SIBO if:7

  • You're diabetic
  • You've had abdominal surgery 
  • You have a structural defect or an injury affecting the small intestine 
  • If you have an intestinal fistula, 
  • If you have certain diseases that affect the small intestine, e.g., Crohn's disease, celiac disease, scleroderma.
  • If you've had radiation therapy to the abdomen 


SIBO can result in the following:7,8

  • Poor absorption of nutrients: due to the breakdown of bile salts by bacteria, which affects fat digestion and absorption. Toxic products produced from the increase in the population of bacteria in the small intestine can cause damage to its lining, which can affect the absorption of other nutrients such as carbohydrates and proteins.
  • Vitamin Deficiency: Poor absorption of fats affects the absorption of fat-soluble vitamins A, D, E, and K. Bacteria in the small intestine secrete and use Vitamin B12; thus, bacterial overgrowth can result in B12 deficiency.
  • An increased risk of developing kidney stones. A diet low in calcium can increase your risk of developing kidney stones. In SIBO, damage to the small intestine's lining can lead to poor calcium absorption.
  • Weak bones due to a decrease in calcium absorption as a result of damage caused by bacterial products to the lining of the small intestine 


How common is SIBO

In the UK, it is estimated that approximately 13 million people are living with irritable bowel syndrome (IBS). 9 IBS is a condition that affects the digestive tract and, similar to SIBO, is also characterized by bloating, abdominal cramps and diarrhoea, and/or constipation. Of the 13 million people affected with IBS in the UK, it is estimated that between 43 - 78% have SIBO.9

How can I prevent SIBO?

You can prevent SIBO or prevent it from recurring by practising the following:10

  • Reduce your stress levels
  • Avoid being sedentary - keeping active improves digestion
  • Avoid eating between meals 
  • Avoid eating a diet high in carbohydrates and sugars 
  • Stop eating 3 hours before bedtime
  • Try intestinal massages to enhance the movement of food through your gut 
  • Consider eating a plant-based diet 

When should I see a doctor

See your GP or healthcare professional if you experience the following:5

  • Persistent diarrhoea 
  • Rapid unintentional weight loss 
  • Constant abdominal pain that lasts for more than a few days (Especially if you've had abdominal surgery)


The symptoms of SIBO are very similar to other disorders affecting the digestive tract, such as IBS and Crohn's Disease, although not as debilitating as the latter two conditions.

The treatment and management include interventions that aim to reduce the population of bacteria back to normal levels, either with antibiotics or surgical intervention. In addition, the treatment also involves correcting nutritional deficiencies if present.

The goal is to improve the quality of life of affected individuals and reverse the damage done by the overgrowth of bacteria. If you think you may have SIBO, please speak to your GP or healthcare professional to determine what testing and treatment options are available in your area.


  1. National Institutes of Health. NIH Human Microbiome Project defines the normal bacterial makeup of the body [Internet]. National Institutes of Health (NIH). 2015 [cited 2023 Apr 13]. Available from: https://www.nih.gov/news-events/news-releases/nih-human-microbiome-project-defines-normal-bacterial-makeup-body
  2. Dukowicz AC, Lacy BE, Levine GM. Small intestinal bacterial overgrowth: a comprehensive review. Gastroenterol Hepatol (N Y). 2007 Feb;3(2):112-22. PMID: 21960820; PMCID: PMC3099351.
  3. National Cancer Institute. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/small-intestine [Internet]. www.cancer.gov. 2011 [cited 2023 Apr 13]. Available from: https://www.cancer.gov/publications/dictionaries/cancer-terms/def/small-intestine
  4. Turiccki J. Small Intestine - Structure - Histology - Secretions - TeachMePhysiology [Internet]. TeachMePhysiology. 2023 [cited 2023 Apr 13]. Available from: https://teachmephysiology.com/gastrointestinal-system/small-intestine/histology-and-cellular-function/
  5. Mayo Clinic. Small intestinal bacterial overgrowth (SIBO) - Symptoms and causes [Internet]. Mayo Clinic. 2022 [cited 2023 Apr 24]. Available from: https://www.mayoclinic.org/diseases-conditions/small-intestinal-bacterial-overgrowth/symptoms-causes/syc-20370168
  6. Ahmed JF, Padam P, Ruban A. Aetiology, diagnosis and management of small intestinal bacterial overgrowth. Frontline Gastroenterology [Internet]. 2022 Jul 12 [cited 2022 Jul 14];14:flgastro-2022-102163. Available from: https://fg.bmj.com/content/14/2/149
  7. Mayo Clinic. Small intestinal bacterial overgrowth (SIBO) - Diagnosis and treatment - Mayo Clinic [Internet]. https://www.mayoclinic.org 2022 [cited 2023 Apr 24]. Available from: https://www.mayoclinic.org/diseases-conditions/small-intestinal-bacterial-overgrowth/diagnosis-treatment/drc-20370172
  8. Yale Medicine. How To Prevent Kidney Stones—Yale Medicine Explains [Internet]. Yale Medicine. 2021 [cited 2023 Apr 24]. Available from: https://www.yalemedicine.org/news/how-to-prevent-kidney-stones#:~:text=Kidney%20stones%20are%20caused%20by
  9. NHS. Remote monitoring of patients with small intestinal bacterial overgrowth, IBS, and food intolerances [Internet]. NHS Transformation Directorate. [cited 2023 Apr 25]. Available from: https://transform.england.nhs.uk/key-tools-and-info/digital-playbooks/gastroenterology-digital-playbook/remote-monitoring-of-patients-with-small-intestinal-bacterial-overgrowth-IBS-and-food-intolerances/
  10. Eckelkamp S. SIBO Treatment: How to Treat SIBO and Prevent Recurrence [Internet]. Parsley Health. 2021 [cited 2023 Apr 24]. Available from: https://www.parsleyhealth.com/blog/sibo-treatment/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Maimuna Abdurrahim

Master of Science (by distance learning), Infectious Diseases, London School of Hygiene and Tropical Medicine, U. of London

Hi! My name is Maimuna and I am a pharmacist currently practising in primary care. I have always been passionate about general wellness and enjoy participating in activities that increase awareness of how to live healthier lives.

I strongly believe that empowering individuals with information about health conditions, medicines, and how to live healthier lives results in better outcomes for their health and well-being. I hope that you enjoy reading this article and that you’re able to pick up one or two salient points that’ll be of benefit to you and your loved ones.

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