Jessica Fernandez MD, Nikhil Bhandary MD

Nutrient absorption and excretion

The gut has an incredibly important role. It is composed of both the small and large intestines, and is where all of the nutrients from the food which we eat, and liquids we drink get absorbed. There are several processes our bodies go through to break down food, making it into smaller particles of carbohydrates, proteins, and fats that can be absorbed as well as excreted. Once these small particles are broken down and absorbed by the mucosa/lining of the intestine, they reach the blood where they are carried throughout the body and stored so they can be used for energy. Water is mostly reabsorbed in the colon, any undigested products and fibers that are not absorbed through the digestive tract are then excreted.

For example, vitamin B12 gets absorbed specifically in the part of the small intestine called the ilium. When there are intestinal disorders, it can cause a disruption in the absorption of vitamin B12 that could lead to a deficiency in this type of vitamin. It is also in the intestines where we have millions of bacteria called the gut microbiome that benefit us and help us lead healthy lives.

The Immune System of the Gut

Scientists continue to explore the role of the immune system (including the spleen and lymph nodes) in chronic illnesses. Interestingly, many antigens (toxins and other foreign substances that create an immune response or in other words, that the body doesn’t recognize as its own) are actually absorbed in the mucosal surfaces of the gut. It is here that we have what is called the “GALT” or gut associated lymphoid tissue. It is the GALT’s role to “constantly distinguish harmless antigens that are present in food or on commensal bacteria from pathogenic assault by microbes. The GALT contains more lymphocytes (cells that fight off infections) than all of the secondary lymphoid organs combined”(1).

In our complex gut microflora, where more than 400 different commensal1 (normal microflora) microbial species live, the gut must learn to differentiate between good and harmful bacteria. The exact way the immune system is able to do this is an area of extensive research that focuses on the different types of cells involved in this process (T cells, B cells, M cells, dendritic cells) and regulatory cytokines (substances/ signals used by the immune system’s cells to communicate). An inflammatory bowel condition results when these signals are turned off or damaged, which inhibits our immune system from distinguishing good bacteria from bad, as well as recognizing proteins that our immune system used to tolerate but are now recognized as foreign. This induces an inflammatory/ allergic response(1).

Microbiome alterations

Our gut is composed of millions of bacteria (microflora); we have ten times more microflora in our gut than we have human cells in our bodies(2). These gut bacteria play an important role in maintaining the health of our digestive system. The bacteria are in charge of synthesizing vitamin K which is important for blood coagulation and prevents uncontrolled bleeding. The bacteria also provide essential nutrients and help to break down cellulose which is a type of sugar/insoluble fiber present in a lot of the food that we eat daily. Given this, it is important that we maintain a good balance of these bacteria that live with us and play an important role in our health.

For example, when a person experiences stress, illness, aging, and unhealthy dietary habits as well as with the administration of antibiotics (important to take to help eradicate disease when prescribed by your physician), it can have a damaging effect to the intestinal microflora. When the gut’s bacterial ecosystem (environment) is disrupted, it can have an effect on many chronic diseases such as inflammatory bowel disease, cancer and obesity(2).

Bacterial (microbiome) balance

It is important to maintain a balanced bacterial microbiome as the effect of these gut microbes can have an effect on mucosal, neuronal, and serotonin balance(3). For example, it has been shown that certain infections involving E. coli can inhibit intestinal transporter function and expression3. When the body’s microbiome is altered, it may play a role in systemic disorders(3).

Serotonin Production

Serotonin is a molecule in our bodies that helps with motility (movement) of the gastrointestinal system as well as in the secretion of substances from the intestine(3). Interestingly, since the small intestine is the major source of serotonin, there is an association between serotonin dysfunction and irritable bowel syndrome (IBS), depression and other pathological mechanisms(3,4). Our intestine is not only a major source of serotonin, but it also produces dopamine and acetylcholine. This pathway is one in which our gut may affect how we feel.

What is a “Leaky Gut?” and what causes it?

Leaky gut in the literature is mostly defined in the context of intestinal mucosa permeability. When the intestine tries to protect itself from harmful substances it develops a defensive barrier that protects it from the external environment, this is called permeability(5). Sometimes when the intestinal barrier is altered by illnesses such as inflammatory bowel disease, food allergies, celiac disease, obesity, and the use of antibiotics that change the microbiome in the gut, it can damage the intestinal barrier and increase intestinal permeability6. This change in but absorbability may lead to deficiency or disease.

Please refer to works cited section for full list of references

The ideas, concepts and opinions expressed in this article are intended to be used for educational purposes only. The author and publisher of this article are not rendering medical advice of any kind, nor is this article intended to replace medical advice, nor to diagnose, prescribe or treat any disease, condition, illness or injury. It is important that before you begin any diet or exercise program that you receive full medical clearance by a licensed physician. Author and publisher claim no responsibility to any person or entity for any liability, loss or damage caused or alleged to be caused directly or indirectly as a result of the use, application or interpretation of the material in this article(8) (modified from The Primal Blueprint book).