photo: Carmine de Fazio, Unsplash
Did you know that 70% of our immune system is placed in the gut? This might sound surprising, but it actually makes sense, as we bring the outside world into our bodies through what we eat. The gut is the largest essential barrier between the outside world and the inside of our bodies (even larger than the skin). And our immune system within the gut, called gut-associated lymphoid tissue or GALT, is faced with the task of clearing out viruses, bacteria or bacterial fragments, parasites and toxic substances.
The small intestine is where most nutrients are absorbed into the body. Once a food is broken down into simple components, the cells of the gut transport those components from inside the gut to inside the body (and then to the blood and lymphatic vessels, which further transport them to the different tissues in our body). Impressively, the front line of this barrier is maintained by only a single layer of specialized cells – the enterocytes. The enterocytes are linked together by tight junctions to form a semi-permeable barrier that regulates what should be absorbed and what should remain in the intestines. Nutrients go through and everything else remains inside the gut and is excreted as waste. Just across the enterocyte layer lie the resident immune cells of the gut, ready to protect the body from attack.
When the intestinal barrier is compromised you have a condition called leaky gut or more technically, increased intestinal permeability. A leaky gut allows not only nutrients to go through but also lets larger particles of partially digested food proteins and other substances that are inside the intestines (like bacteria, infectious organisms, and waste products) to leak out into the body. Once inside the body, these substances come in contact with the immune system, which recognizes them as foreign invaders and mounts an immune response against them.
The destruction of the small intestine’s barrier can cause micronutrient deficiencies (vitamins and minerals) by reducing the surface area for nutrient absorption, which can then manifest in a variety of ways (micronutrient deficiencies are associated with autoimmune disease). It may result in lactose and fructose intolerance and the inability to properly digest fats and absorb fat-soluble vitamins. But most importantly, the substances that leak out of the gut stimulate the immune system causing generalized body inflammation and giving origin to many symptoms and health conditions. Some incompletely digested proteins from food stimulate the immune system, creating food allergies and food intolerances that can give origin to allergies and autoimmune disease in susceptible individuals.
A trigger to autoimmune disease
One of the mechanisms used by the immune system to recognize foreign invaders that "leak" into the body (such as bacteria fragments and food particles) is the formation of antibodies. Antibodies recognize the substances they bump into by “reading” specific proteins in these foreign substances, such as those in the cell membranes of bacteria but also in partially digested food particles that cross the gut barrier. When antibodies bind to one of these proteins, they signal to the immune system that they are foreign proteins that must be attacked.
When an antibody forms against one protein (of a bacteria, for example), there is a chance that it will also bind to another similar protein. This is a good thing because it makes the immune system recognize several types of pathogens, protecting us from infections. But with autoimmune disease, the body accidentally creates antibodies that identify not only foreign proteins but also the body’s own proteins (autoantibodies). This is called molecular mimicry or antibody cross-reaction and is one way that certain dietary proteins are believed to cause autoimmune disease. A leaky gut lets a variety of substances to come across the immune system, providing a trigger for the body to produce autoantibodies and a continuous stimulus for the immune system to attack.
If you have an autoimmune disease, you also have leaky gut. In order to develop autoimmune disease you need to meet three criteria: first you must have genetic predisposition to it, then you must be exposed to a triggering non-self antigen (infections, environmental toxins, stress and certain dietary triggers) and lastly you must have lost the protective function of the gastrointestinal barrier (leaky gut). In fact, leaky gut syndrome is present in every autoimmune disease that has been tested, including rheumatoid arthritis, ankylosing spondylitis, Crohn’s disease, celiac disease, multiple sclerosis, type 1 diabetes and systemic lupus erythematosus, and even in Alzheimer’s disease and cancer. 
A recent study from Lund University in Sweden have shown that a leaky gut not only is present in autoimmune disease but is actually a requisite to the development of these diseases. Here is a quote from this paper:
Recent observations in humans and in a variety of animal models indicate that an increased intestinal permeability (IP), often referred to as a “leaky gut”, is playing a pathogenic role not only in development of gastrointestinal disorders like inflammatory bowel disease (IBD) and celiac disease, but also in systemic autoimmune diseases, like type 1 diabetes.
So what causes leaky gut?
A leaky gut is usually provoked through diet and lifestyle factors such as some foods, alcohol, and stress, and by infections and certain medications (like NSAIDs and corticosteroids - medication such as Ibuprofen, Naproxen, and cortisone). But in the case of short-term infections or medication, what keeps the gut with increased permeability afterwards is the negative impact of diet and lifestyle.
There are a variety of foods that can damage the gut barrier, being gluten the biggest contributor to a leaky gut in people with genetic predisposition for autoimmune disease. Gluten can interfere with the normal function of the intestinal barrier, causing a leaky gut, and then directly activating the immune system. Gluten is believed to break immunological tolerance, playing a role in the development of autoimmune disease, particularly in rheumatoid arthritis.
Related: Why gluten is bad for you
The good news is that if a leaky gut is provoked by diet and lifestyle factors, it can also be healed by addressing those factors. This underlines the importance of eating to support gut health, which is precisely the goal of the autoimmune protocol. Now you understand how by eating nourishing foods that heal the gut and regulate the immune system, you can reverse autoimmune disease, regardless of which specific one it might be. There are many factors at play in the development of a leaky gut and many others that contribute to the maintenance of a healthy gut. And through this blog, I will share with you my knowledge about the foods and lifestyle factors that contribute to its development and how it can be healed. Maintaining a strong gut barrier is the best way to keep your immune system healthy!
 Susan Blum, “The immune system recovery plan”, Scribner, April 2013
 Sarah Ballantyne, “The Paleo Approach”, Victory Belt Publishing Inc., 2013
 Mu Q, Kirby J, Reilly CM and Luo, XM, “Leaky gut as a danger signal for Autoimmune Diseases”, Front Immunol. 2017; 8: 598. doi: 10.3389/fimmu.2017.00598
 Fasano A, “Leaky gut and Autoimmune diseases”, Clinic Rev Allerg Immunol (2012) 42: 71. doi.org/10.1007/s12016-011-8291-x
 Mehrnaz Nouri, et al., “Intestinal Barrier Dysfunction Develops at the Onset of Experimental Autoimmune Encephalomyelitis, and can be induced by Adoptive Transfer of Auto-Reactive T cells”, Plos one, September 2014. https://doi.org/10.1371/journal.pone.0106335
 Cordain L, Toohey L, Smith MJ and Hickey MS, “Modulation of immune function by dietary lectins in rheumatoid arthritis”, Br J Nutr. 2000 Mar;83(3):207-17.