The gut (a.k.a. digestive tract) is not just a tube that absorbs nutrients and gets rid of waste - it’s a complex alive system that’s a huge foundation of health. And not just gut health, but the overall health of our bodies and minds. We know how important it is to get all of our essential nutrients from food - and this is a big part of what our digestive tract does. But, there is way more to it than just that.
When the gut is not working properly, symptoms can appear. Yes, typical gut and abdominal symptoms, but also other seemingly unrelated symptoms. Did you know that things like allergies, autoimmunity, and mental health have been linked with gut problems?
Let’s look at one gut problem in particular (you may have heard about this lately) - leaky gut. This literally involves tiny “leaks” in our gut lining that can allow more than just needed nutrients and water into our bodies.
What is “leaky gut” linked with?
The “gut” is part of the digestive system, mainly the intestines, which are located in the abdomen. It’s an alive and very complex “tube” that acts as a gateway deciding what will enter the internal circulation of the body, and what must not get by. It digests and absorbs nutrients and water. It prevents toxins and “bad” microbes from being absorbed. And it shuttles all the waste to continue on and be eliminated.
You may think that symptoms of a leaky gut (a.k.a. “intestinal permeability”) are felt in the gut, and you’re right...to a point. Would you be surprised to know that lots of other symptoms and conditions are linked with leaky gut?
Leaky gut has been associated with:
Autoimmune diseases (e.g. Type I diabetes, celiac disease, etc.)
Inflammatory Bowel Diseases (e.g. ulcerative colitis, Crohn’s)
Psychological stress and mental health
Researchers are still figuring out the exact role that leaky gut plays in these conditions. Either way, the connections are there, and there are things that you can definitely do to improve your gut health. But first, how is our gut structured, and what can promote it to leak?
Gut structure - Three layers of our gut lining
Our guts have a three-layer lining that helps to allow things we need in, and keep harmful things out.
The first (outermost) layer is just one-cell thick. It’s a barrier that absorbs the nutrients and water we need, and physically prevents undigested compounds, toxins, and bacteria from getting in. Laid out flat, this layer makes up the largest surface area between the internal circulation of our bodies and the outside world (i.e. what we eat and drink).
This layer has at least seven different types of cells, and 90% of them are one type called “enterocytes.” These enterocytes actively absorb what we need and keep out what we don’t. They also help to create and regulate the other two layers.
FUN FACT: Most enterocytes are replaced with new ones every 3-5 days or so.
Enterocytes are held together with different types of bonds. The one most studied is called a “tight junction.” These tight junctions are made up of several types of protein. When they loosen, it creates tiny holes (or permeation's) in this first layer since the cells are not “stuck” together as much as they should be.
The second layer is mucus. This mucus provides physical separation between the outermost enterocyte layer and the microbes and food that are inside the centre, or “lumen,” of the gut. It also contains special proteins that help fight against invaders. This mucus and its special compounds are produced by the enterocytes.
We want that mucus layer to be nice and thick to provide a better barrier between the one-cell layer of enterocytes and protect them from “bad” bacteria that can get in there.
FUN FACT: Animal studies show that mice fed a diet low in fibre had thinner mucus barriers.
The third (innermost) layer inside our gut lining is our friendly resident gut microbes. Our guts contain billions of microbes - over 1 kg worth. Taken together, they’re sometimes referred to as a “superorganism.” These microbes include bacteria as well as other types of friendly microbes.
This layer of gut microbiota has two major functions to help promote a healthy gut lining:
They crowd out “bad” bacteria by taking up space and eating the “good” food (i.e. fiber and resistant starch, which we’ll get into in a bit).
They help to regulate the digestion and absorption of nutrients to nourish the first-layer enterocytes. One of the types of compounds they produce are called “short chain fatty acids” (SCFAs). These are considered to be anti-inflammatory and are also used as fuel for the enterocytes.
When the three layers aren’t working optimally, the tight junctions loosen, and leaks occur. This allows unwanted things to enter into the body’s circulation. This is how gut health affects our overall health.
Leaky gut and our gut microbes
Our friendly gut microbes, the third innermost layer of our gut, include hundreds of types of microbes. Some of the main types of bacteria are Bacteroidetes and Firmicutes (e.g. Lactobacillus). We think problems with our gut microbes might actually begin the whole process of leaking guts.
According to Sturgeon and Fasano, 2016:
“It is now clear there is a symbiotic relationship between the microbiome and the host. As early as 2001, it was described that commensal bacteria have an effect on intestinal permeability.”
Here’s how we think this happens, based on the current research:
The third innermost layer of the gut lining, the microbiota, get out of balance.
Inflammatory molecules (including zonulin) are released, and fewer anti-inflammatory ones like SCFAs are available.
This inflammation disturbs the tight junctions in first layer of enterocytes, hence creating tiny leaks which allows passage of harmful compounds into our bodies.
It starts when the gut microbiota are in dysbiosis (an “imbalance” of “good” and “bad” microbes). This promotes an inflammatory response because some of the “bad” microbes are pushing out the “good” ones that produce the anti-inflammatory short chain fatty acids (SCFAs). SCFAs are anti-inflammatory and are also used as fuel by the enterocytes. Some of these SCFAs promote the production of the mucus layer (the second layer), and even help to improve the tight junctions in the enterocytes in the first layer. They produce the SCFAs when they eat fibre and resistant starch.
FUN FACT: One study looked at children who were at risk of developing type 1 diabetes (which is an autoimmune condition). Researchers found that some who had an increase in one of the “bad” microbes went on to develop autoimmunity months later which led to type 1 diabetes.
Another possibility that researchers are looking at is that some of these “bad” bacteria produce a toxin that mimics zonulin.
Zonulin is a protein naturally released by our enterocytes when they’re exposed to certain things we eat, like “bad” bacteria on our food and gliadin (part of the gluten protein found in wheat and other grains). Blood levels of zonulin tend to be higher in people with autoimmune conditions like celiac disease and type 1 diabetes.
All of this increased inflammation then irritates the gut, which can result in loosening of those tight junctions.
Based on the research so far, this is the way we think we develop leaky guts. But, how does this relate to autoimmunity?
Leaky gut, allergies, and autoimmunity
Allergies and autoimmunity are directly linked to our immune system. They result when our immune system works a bit too hard - when our immune cells become a little too active.
Allergies occur when our immune system is activated to fight things that are not harmful, like certain foods, pollen, or pet dander. The body thinks they’re dangerous invaders that must be fought, and sends out immune cells that cause inflammation to try and eliminate the allergen.
Autoimmunity, on the other hand, is when our immune system is activated to fight our own cells and tissues. The immune system becomes “intolerant to self.” For example, type 1 diabetes (an autoimmune disease) occurs when our immune system fights the insulin-producing cells in our pancreas. After continued inflammation, enough of these cells die and we eventually need to start monitoring our own blood sugar levels and provide our bodies with external insulin. This occurs more often in people who have type 1 diabetes in their families.
Many things can contribute to autoimmunity, and leaky gut may be a bigger factor than we once thought. This is because of the impact of allowing undigested food, bacteria, etc. enter our bodies and how our immune system tries to fight them. A large part of our immune system is located just on the other side of that one-cell thick layer of enterocytes.
When our bodies detect things in our internal circulation that don’t belong (like undigested food or bacteria) our immune system kicks in. This immune response to things that “leaked” into our bodies can cause the release of even more inflammatory compounds this time inside our bodies and bloodstreams (i.e. on the other side of the first layer of enterocytes). The allergic and inflammatory responses that happen around our guts may affect the gut directly. But, once these are absorbed into the bloodstream, they can affect other parts of the body too.
This is the connection we see between leaky gut, allergies, and autoimmunity. It’s not just the leaky gut, it’s the interactions between what leaks into our bodies and our immune system’s response to them.
Having a healthy gut microbiota plays an important role in how our immune systems mature from when we were infants. Dysbiosis in our gut at an early age can promote changes in our immune response, and increase the risk of allergic and autoimmune diseases.
It seems that gut dysbiosis and “leaky gut” might be part of the chain of reactions that lead our immune cells to start attacking things they really don’t need to.