In this next part, we will explore the microscopic, cellular structure of the digestive tract. As was said earlier, the entire length of the digestive tract from mouth to anus is literally one cell-layer thick. These special cells are called epithelium, and they are the barrier between the outside world and you. (Although the digestive tract is inside your body, it's technically still the 'outside' world until its contents are absorbed into your bloodstream.)
The lining of the small intestine is made up of numerous folds called plicae, and each plica has numerous villi or folds. Each villus is covered by epithelial cells with protecting microvilli called the brush border. This increases the surface area for absorption by several hundred times, making this lining--which spans the entire small intestine--about the size of a tennis court. These cells are responsible for many things, including absorbing the food you just ate into your bloodstream, for protecting you from anything that was ingested that may harm you, as well as for excreting toxins out of the body.
These highly specialized cells are bound tightly together with protein bonds called zonulin. These bonds form tight junctions which create a very selective permeable tubing which only lets in what the cells allow. If the protein bonds between these tightly bound cells are broken, your digestive tract loses its selective permeability and becomes leaky. The disruption of zonulin has been linked to autoimmune disorders, diabetes, systemic inflammation and many other disorders. The only other part of the body that has these protein bonds is the brain, which helps us understand why a leaky gut also means a leaky brain.
There are many things that may disrupt these tight junctions between GI cells. One of them is the glycoprotein called gliadin that is found in wheat, and some people are more genetically predisposed to this disruption. Certain foods that are allergens or that the digestive tract does not recognize as a nutrient may also disrupt these bonds. The most common food items that are disruptors are corn, soy, wheat, sugar, alcohol, caffeine and dairy. One of the most common reasons why your digestive tract may not recognize these as food is because of the chemical processing that takes place during food manufacturing. This also explains why highly processed foods like fast foods, processed cheese foods, and canned and packaged items may also confuse the digestive tract, simply because of the way the molecule has been manipulated. We will talk more about this disease state in another session.
Once the food molecules have been broken down into the smallest size possible, they begin to transfer across the epithelial cell membrane. Carbohydrates pass through easily by simple osmosis (remember these molecules have all become hydrolyzed in the duodenum). Fats that are surrounded by bile are very lipophilic, as are the cell membranes, so when they join together, fats are able to pass through readily. Fats that are not surrounded by bile not only do not get absorbed into the cell membrane, but they also act as irritants and may also cause cell bonding disruption.
Proteins, hopefully, have been broken down in the duodenum into small amino acids or dipeptides or tripeptides, and these molecules are now small enough to be carried through the gut mucosal lining by special protein channels. These channels work like a revolving door that allows the large protein molecules to be absorbed and broken further down so that they may be utilized in the bloodstream.
All of these absorbed, broken-down molecules travel through the epithelial cells and are deposited into the special blood vessels in the omentum. They are then carried by portal circulation directly to the liver, to once again be analyzed to make sure they are safe to be used by the cells all around your body. In this way, the meal you just ate is broken down molecule by molecule and absorbed into portal circulation, sent to the liver, and then distributed throughout the rest of your body to be used to build new cells, help detoxify, be burned as energy, and to help your body function as the miraculous machine that it is.
In the lumen (the tubing of the small intestine) and also in the large intestine, we find a virtual village or neighborhood full of microorganisms, also known as bacteria, flora, probiotics, the microbiome-whatever you like to call them. There are literally more cells and organisms that live in our gut with more genetic diversity than there are in the entire body! This flora is extremely important for a number of reasons, and there are many we probably have not even discovered yet. Firstly, they help us absorb special nutrients like vitamins and minerals, and many of them actually make vitamins for us. Secondly, they act as garbage collectors and scavengers, traveling along the gut mucosa and gobbling up bad bacteria and other items that shouldn't be there.
One of the more recent discoveries is that these organisms actually help us process and digest nutrients, and even contribute to our DNA. Disruptions in this 'village' have been linked to obesity and many different disease states, so the better care you take of the 'butchers, bakers, and candlestick-makers' in your microbiome, the healthier and less obese you will be.
