• Brian Jerby MD

SIBO 1: What is it?

Updated: Apr 6, 2019



SIBO (pronounced See-Bo) is a condition where there are abnormally large populations of bacteria that are present in the small intestine. As you might expect, this can cause a number of problems. This series of articles is made available to you with the hope of increasing your understanding of chronic gut problems and how they may or may not relate to SIBO.


Normal Function of the Small Intestine

In order to better understand the problems that SIBO causes, let’s look first at some of the normal functions of the small intestine.The small intestine (also known as the small bowel) is around 20 feet long and is responsible for the majority of food digestion and nutrient absorption. The small intestine normally has bacteria present in it, but a lot fewer than does the large bowel (also known as the colon). In fact the colon has 10,000 times more bacteria in the normal setting! These normal bacteria play a significant role in the production of vitamins and other key nutrients as well as some neurotransmitters that affect mood and cognitive abilities! Another very important fact about the small intestine is that around 70% of the immune system is located here. Over the past 5 years or so there has been an explosion of research that is further defining the important relationship between normal populations of bacteria in the intestine and how this helps regulate the immune system.

So, in summary, here are the key functions of the small intestine with normal types and amounts of bacteria:

Digestion and absorption of nutrientsProduction of some nutrientsImmune system regulation and functionProduction of some neurotransmitters

Understanding the normal function of the small intestine is crucial in understanding the abnormal conditions that SIBO creates. So let’s move on to answer the question, “What is SIBO?”


SIBO Defined

Certain conditions may allow the normal bacteria in the small intestine to multiply to abnormal concentrations. In addition, abnormal types of bacteria may take up residence in the small bowel. These abnormal numbers and types of bacteria in the small intestine create the condition of Small Intestinal Bacterial Overgrowth (SIBO). The presence of abnormal types and numbers of bacteria in the small intestine can wreak havoc with normal function. Not only can the alteration of the environment result in abnormal digestion and prevent proper absorption of nutrients, but the existing bacteria may actually consume many of the nutrients that you need, including vitamin B12 and amino acids, among others. These bacteria also ferment carbohydrates which produce gaseous distension (bloating) of the small intestine. Furthermore, the overgrowth can damage the single cell barrier layer called the mucosa that lines the inside of the small intestine. When the integrity of this layer is compromised, contents of the bowel that would normally be excluded may cross the barrier and stimulate the immune system. This condition, called increased intestinal permeability (a.k.a. “leaky gut”), is widely implicated in chronic inflammatory conditions, autoimmune diseases, brain/cognition impairment, and many others. Last but not least is that this excess amount of bacteria can impact the sensory and motor functionality of the small intestine which results in abdominal pain/discomfort and impaired transit through the gut.

Now that you know the basics about the function of the small intestine and the problems that bacterial overgrowth causes, you will be able to better understand the risk factors for SIBO, how it’s diagnosed, and the treatments. These topics will be addressed in coming posts. Stay tuned!

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