What is Small Intestinal Bacterial Overgrowth (SIBO)?

I used to believe that SIBO was a condition that only alternative medicine practitioners such as naturopaths identified & ‘treated’, and it wasn’t a real diagnosis at all and it seems I was not alone in this belief.

This is the first article about SIBO which will outline what it is, it’s symptoms and risk factors and a subsequent article will outline how SIBO is diagnosed and some of the treatment options.

Last week I asked on Twitter “Do you believe that SIBO is a credible diagnosis?” and of the sixty one people that responded, here’s what people thought;

“Do you believe that SIBO is a credible diagnosis?”

Fifteen percent of people thought SIBO wasn’t a legitimate medical diagnosis, while the remainder thought that either it was a credible diagnosis that not all doctors know about (62%), or that only Functional Medicine MDs diagnose and treat it (18%), or only naturopaths (5%) do.

My interest in searching the scientific literature about SIBO came when a rheumatologist suggested that it may be SIBO that was underlying the increase in joint pain that I was experiencing. While I had been diagnosed with osteoarthritis many years ago — which is a degenerative joint disease and not a normal part of aging (more in this article), the pain in my fingers had become excessive, even though there had not been any additional deterioration or deformation in those joints. If it wasn’t a rheumatologist that was suggesting SIBO as a possible cause, I would have discounted it without a thought but because the possibility was raised by a credible clinician, I decided to search the scientific literature to see what I could find.  To be honest, I was quite surprised to find that it was not only well-researched, but that there were academics at well-known universities that have been studying it!

What is SIBO?

Small Intestinal Bacterial Overgrowth (SIBO) is an increase in the type of bacteria present in the small intestine that are normally found in the large intestine (also called the colon) [1].

The small intestine consists of three parts; the duodenum connects to the stomach, the middle part is the jejunum and the last part called the ileum, attaches to the colon. It is called the small intestine because its diameter is smaller than the large intestine, although it is actually longer in length than the large intestine [2].

Normally, the small intestine contains very few bacteria and when it does, the type of bacteria found in the duodenum and jejunum are usually a specific type (i.e. lactobacilli and enterococci, gram-positive aerobes or facultative anaerobes) and are found in small amounts (< 104 organisms per mL)[1] and research indicates that samples taken from the jejunum of healthy volunteers found no bacteria present at all. When the bacteria that normally populate the large intestine spills over into the small intestine, it is called Small Intestinal Bacterial Overgrowth or “SIBO”.

The body has several built-in defense mechanisms for normally preventing bacterial overgrowth of the small intestine. The major defense against small intestine bacterial overgrowth is (1) the very high acid environment of the stomach (gastric acid) which kills most bacteria, as well as (2) a normally intact ileocaecal valve which is the sphincter muscle that separates the small intestine from the large intestine. In addition, there are additional defense mechanisms such as immunoglobulins in the secretions of the small intestine, as well as  secretions from the pancreas and bile-related secretions that keep bacteria from reproducing [1].

SIBO can occur for different reasons, including low stomach acid (achlorhydria), pancreatic insufficiency, as well as anatomical abnormalities including small intestinal obstruction, diverticula (more about this in this article), fistula (which is abnormal connection between an organ and the intestine which can be created after some infections), as well as slowing of intestinal movements (motility disorders) that are common in those with diabetes mellitus, and other conditions. It has been known for many years that those that consume significant amounts of alcohol are known to be at risk for SIBO [3] but a more recent study found an association between moderate alcohol consumption and SIBO [4], which was defined as up to one drink per day for women and two drinks per day for men. It is thought that alcohol consumption may cause injury to the mucosal cells of the small intestine which contributes to a slowing of intestinal contractions (i.e. motility disorder), which is associated with SIBO. In some people, a combination of the above factors may be involved.

[Note: in my case, an underlying diagnosis of SIBO was certainly possible as I had been on a long-term, high dose of H2 antihistamines due to having Mast Cell Activation Disorder (MCAD) — medications which are known to also significantly reduce stomach acid, and I had also been diagnosed with type 2 diabetes 8 years before going into remission 2 1/2 years ago.

How Common is SIBO?

The prevalence of SIBO in young and middle-aged adults appear to be between 6 and 15% , but higher in the older adults (14.5–15.6%) [5]. Perhaps this is due to decreasing amounts of stomach acid associated with aging, as well as increase prevalence of diverticulosis and type 2 diabetes, all of which are associated with SIBO risk.

What are the Symptoms of SIBO?

Many of the symptoms of SIBO are similar to those of Irritable Bowel Syndrome (you can read more about that here), including abdominal pain, bloating, gas, bouts of diarrhea or constipation or alternating diarrhea and constipation. As mentioned above, there are other lesser known symptoms of SIBO, including joint pain.

Update (September 4, 2019): In the second article (posted here), I outlined different tests used to diagnose SIBO, the difference between hydrogen-dominant SIBO and methane-dominant SIBO and why Irritable Bowel Syndrome (IBS) that does not improve despite adopting appropriate dietary changes may be SIBO.

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  1. Bures J, Cyrany J, Kohoutova D, et al. Small intestinal bacterial overgrowth syndrome. World J Gastroenterol. 2010;16(24):2978–2990. doi:10.3748/wjg.v16.i24.2978
  2. Medscape, Small Intestine Anatomy, Dec 8 2017, https://emedicine.medscape.com/article/1948951-overview
  3. Hauge T, Persson J, Danielsson D: Mucosal Bacterial Growth in the Upper Gastrointestinal Tract in Alcoholics (Heavy Drinkers). Digestion 1997;58:591-595. doi: 10.1159/000201507
  4. Gabbard SL, Lacy BE, Levine GM et al, The Impact of Alcohol Consumption and Cholecystectomy on Small Intestinal Bacterial Overgrowth, Digestive Diseases and Sciences, 2014, Volume 59, Number 3, P. 638
  5. Dukowicz AC, Lacy BE, Levine GM. Small intestinal bacterial overgrowth: a comprehensive review. Gastroenterol Hepatol (N Y). 2007;3(2):112–122.

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