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Part One – Overview of Small Intestinal Bowel Overgrowth (SIBO)

By Karen Miller-Lane, ND, L.Ac.

This is the first part in a series that examines a common, but often misdiagnosed or underdiagnosed gastrointestinal condition called small intestinal bowel overgrowth or SIBO for short. It is considered a common cause of irritable bowel syndrome or IBS. This is meant to be a brief, initial overview.

In the language of the professional literature:

The pathogenesis of irritable bowel syndrome (IBS), once thought to be largely psychogenic in origin, is now understood to be multifactorial. One of the reasons for this paradigm shift is the realization that gut dysbiosis, including small intestinal bacterial overgrowth (SIBO), causes IBS symptoms.[1]

What is SIBO?

As the following excerpt from the Metagenics Institute so succinctly portrays the basics of SIBO, I’ve included it below. I will be going into more detail on the effects of SIBO in future articles.

SIBO stands for Small Intestinal Bacterial Overgrowth. Normally, the bulk of our gut microbiome resides in the large intestine. In SIBO, we see an overgrowth of bacteria (even otherwise healthy bacteria) in the small intestine, where they are not usually present to such a high degree. Those bacteria are then able to act on poorly-digested, fermentable carbohydrates, producing uncomfortable symptoms such as bloating, gas, abdominal pain, diarrhea and/or constipation.

The effects of SIBO can extend beyond direct gastrointestinal symptoms. The excess levels of bacteria excrete acids that can underlie some neurological symptoms including brain fog and fatigue. Increased gut permeability can occur causing translocation of bacteria and insufficiently digested food particles that trigger immune activation that can lead to pain and other symptoms. It also predisposes an individual to food sensitivities. Nutrient deficiencies can arise as the bacteria consume some of the ingested foods; B12 and iron, for example, which can lead to anemia, and deconjugation of fatty acids from bile that reduces absorption of fat-soluble vitamins. Patients with SIBO also tend to have altered secretory IgA values, demonstrating that SIBO directly alters immune activity in the gut.[2]

Symptoms of SIBO include:

• bloating/ abdominal gas

• flatulence, belching

• abdominal pain, discomfort, or cramps

• constipation, diarrhea, or a mixture of the two

• heartburn

• nausea

• malabsorption: steatorrhea, anemia

• systemic symptoms: headache, joint/muscle pain, fatigue, rosacea

Factors that put someone at risk for SIBO include:

Hypochlorhydria (low production of hydrochloric acid in the digestive track)

Chronic antacid therapy (especially with PPIs or H2-blockers)

Gastroparesis (secondary to diabetes type I or II); motility of the stomach is abnormal or absent

Gallbladder/bile dysfunction

Pancreatic enzyme deficiency

Slow-transit constipation

Disruption of the migrating motor complex (MMC), as may be caused by radiation

History of bulimia

Multiple courses of antibiotics

Ileocecal valve incompetence

Diet rich in sugars and simple carbohydrates

Heavy alcohol use[3]


SIBO can be tested a number of ways, the one I recommend is a non-invasive breath test. It has its shortcomings, but it is adequate, simple and non-invasive. The gold standard has been the small bowel aspirate, however, there are also several challenges to this method. Small bowel aspirate may not capture enough of the bacteria, or capture the level of overgrowth, and certain species of bacteria may not be able to be effectively cultured and therefore identified.[4][5] As you can see, testing for SIBO has its limitations but it is still an important tool in the diagnosis and treatment of SIBO.

Treatment approaches

Systemic review and meta-analysis in 2022 showed that 49% of IBS cases were SIBO. A conservative estimation is that anywhere from 16 to 25 million people in US could have SIBO and there is a 67% relapse rate after undergoing antibiotic treatment.[6] This will be explored in future articles.

Treatments include one or more of the following: antibiotics, an elemental or FODMAP diet, herbal antibiotics and antimicrobials, and other nutrients that address the root cause. It is important that individuals with SIBO work with their physician as the symptoms of SIBO are varied and may not fall under the common symptom profile. It is also important to create a treatment plan to address the varied underlying conditions that may exacerbate SIBO and identify and treat other systems in the body that are affected. Also given SIBO’s relapse rate, it is important to follow up with your doctor to ensure an effective, long term treatment.

Coming up in Part Two

The purpose of Part One in this series is to include a brief overview of one of the causes of IBS. For those interested, in the articles to follow I will go in more depth as to SIBO’s risk factors, pathophysiology, and the various conditions that are associated with it. We will touch into how skin, vascular, liver, nutrient, and immune health are altered by SIBO. My hope is to illuminate this otherwise underdiagnosed condition.

[1] [2] [3] [4] [5] [6]



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