Did you know that the most important phase of digestion is if and how nutrients are absorbed? If this step doesn't happen then all of that energy digesting was for nothing. You could be absorbing anywhere from 10-90% of your nutrients, that is a really big discrepancy.
Some cases of obesity are even a result of poor absorption because if you're not absorbing your food properly your body will want to eat more in order to feed your starving cells even when you have had more than enough calories. This is where the calories listed on the package could lead you and your clients astray. Also, remember that calories are also always an estimate. No one actually goes in and chemically measures exactly how many carbs are in your slice of bread.
When you took at a calorie count it's just an estimate of an average piece of bread that's roughly the same size pulled from some data base.
When it comes to supplementation herbs or medicines remember these have to be absorbed through the digestive system too? So, absorption is really the root of health and disease.
From our Introduction to Digestion, now that you know that the digestive tract is a tube that exists outside your body you can understand that just because you put something in your mouth doesn't necessarily mean you'll be able to make use of it in your body. Let's look at what this means by taking a closer look at absorption.
Absorption is regulated by:
1. Secretion of enzymes, mucous and other fluids and;
2. Motility – the rate at which the digestive process takes place
Digested foods that have been through the process should be able to pass through the blood vessels behind the intestinal walls either by diffusion, when nutrients pass through the mucosal wall of the small intestine or by active transport, when one molecule attaches to another in order to be transported through the circulatory or lymphatic system. When the cells receive nutrients in a usable form, they can metabolize them.
Throughout the entire process the tube inside your digestive tract known formerly as the lumen takes in 9 liters of liquid and a total of 9 liters has to be removed. When this balance is out of whack you can end up with things like dehydration since 7 liters of this fluid comes from within your body.
This is 1/6 of your total body fluid. This liquid comes in the form of saliva, bile, water, electro-lights, mucous and enzymes.
For now, know that your body fills the lumen with what it needs to breakdown food and then absorbs the nutrients & fluids back into the system leaving only waste behind. Nutrients must cross the intestinal wall and a layer of interstitial fluid via capillaries or lymph vessels to reach the blood or lymph system.
The majority of nutrient & fluid absorption takes place in the small intestine which is divided into 3 parts. The Duodenum, Jejunum & Ileum
The Duodenum, is the first section of the small intestine. This is where the pancreas and liver secrete enzymes for digestion when chyme is present. The Jejunum, the middle section of the small intestine takes on the bulk of the absorption responsibility and, the Ileum, the last section of the small intestine functions mainly to absorb vitamin B12 and bile salts. The cells on the wall of the Ileum also secrete more enzymes to keep the digestive juices flowing. Some of these enzymes stay on the wall and are reused every time.
Lactase is an example of an enzyme that lives on the wall of the intestines for the few of us that are lucky enough to have it although most of us don't.
Almost 7-8 liters of fluid are absorbed in the small intestine leaving only about 1 1/2 liters to pass along into the large intestine. That's a lot of absorption. This is why the small intestine has so many folds and flaps for the purpose of more nutrients for absorption. The intestine increases its surface area for absorption even more through Villi, wave like hairs on the Intestinal Wall that increase surface area for absorption, and every Villi has hairs called Microvilli. All together this provides enough surface area to fill an entire tennis court. You can tell our bodies didn't miss a thing!
Each Villi or hair like structure on the intestinal wall contains capillaries and lymphatic tubes to absorb nutrients and carry them through the intestinal wall. Let's take a closer look at the intestinal wall which is the surface area that brings in all of these nutrients.
There are four layers of mucosal lining along the alimentary canal or digestive tract. These are the same four layers of tissue that line all the cavities or lumen, which just means inside of a tube, in our bodies.
The four layers of the Mucosal Lining are:
The sub mucosa
We said before that the gut is lined with a layer of epithelial cell just like all parts of the body that come in contact with the outside world. Epithelial cells have two functions, to absorb nutrients and to keep out harmful substances. The epithelial layer is lined with tight junctions, which are protein structures in between epithelial cells that seal them up. Tight junctions must be tight in some areas to keep potential irritants or allergens from passing through. And they need to be loose in other areas to let the nutrients through.
The mucosal layer is the first layer surrounding the lumen. This layer is regenerative. These cells renew every 3-4 days. They have to deal with so many toxins, these cells have a really short life. This layer also secretes mucous to prevent acid and enzymes from digesting themselves.
The second layer, the sub mucosa is below the mucosal layer. This layer has folds with larger blood vessels and lymphatic vessels which provide a means of absorption. This layer also contains the sub mucosal plexus one of the two major nerve networks in the inherent nerve center of the nervous system that controls the gut. That's right our gut has its own nervous system!
The third layer is the smooth muscle called the muscularis. Smooth muscle means it is not under conscience control. This is the muscle responsible for peristalsis motion that happens in the esophagus and small intestine. It contracts in a way that decreases the diameter of the lumen as well as the length to shorten the tube. These two actions keep things moving along.
The second nerve network of the enteric nervous system, the myenteric plexus lies between the muscles in this layer. For now, just know that these nerves are in the gut lining.
The Serosa, the last layer, is connective tissue that attaches the organ to the abdominal wall. The Serosa adds a little bit of stability, so your organs just don't float around still providing mobility. Sounds like a perfect formula for life.
Most nutrients travel through the capillaries to the blood stream, but fat cannot be directly absorbed into the blood or else it would clog our arteries. Instead fat is transported via the lymphatic system. A couple notes about the lymphatic system, unlike blood it’s not attached to a pump so to move about it mostly relies on gravity. Our muscles in the legs for example pump the fluid in the lymph back up, but we’re still fighting gravity. Ever have swollen ankles after a long day? That’s the fluid pooling and this is the same reason we often wake up with puffy eyes. We didn’t have gravity working with us all night to drain the fluid. Nurses who are on their feet all day often wear compression socks to keep the lymph fluid up by constriction.
Unlike blood vessels the lymphatic system isn’t filtered through the liver. Any nutrients that enter go straight into our bodies without the final stages of filtering the liver provides. This may be something to consider when consuming fats. It’s a good reason to make sure they are of the highest quality. In addition, for being a channel for fat the lymphatic system also provides transportation for vitamins that are not water soluble. These vitamins attach to a fat and travel through the lymph. Also, when the body is unable to fully breakdown a protein, it can enter the lymphatic system as well. Since this molecule is unfamiliar the immune system may attack it causing an allergy. This is one reason why some people are allergic to certain proteins like peanuts.
When it comes to absorption of vitamins some of them need a carrier. Vitamin B12 for example must be coupled with intrinsic factor or protein in order to be broken down in the stomach and absorbed in the Ileum of the small intestine.
Now let’s talk about how inflammation can play a role in absorption. Behind the layers of the intestinal wall is interstitial fluid. When there is inflammation in the gut there is essentially more liquid between your mucous wall and the capillaries. This makes it harder for nutrients to get across the mucosal wall.
So, someone may be eating all the right food, they may even be digesting the right nutrients, but they are having absorption problems because the nutrients are having trouble making it through the extra fluid.
These are all important things to consider because if you can’t absorb nutrients, you won’t be receiving the building blocks you need to function optimally and then it doesn’t really matter how much veggies you eat.
To recap, the main function of digestion is to break particles down small enough so that they can be absorbed in the blood or the lymphatic system and travel to where they are needed most. Protein and carbs are absorbed via capillaries to the blood vessels and fats are absorbed through the lymphatic system. The intestinal walls are structured in little folds and with little hairs to have the most surface area possible for maximum absorption. All of this is in place to help us get the most out of our food.
Now that you know how hard the body is working pumping fluids in and out does that change your desire for what you want to put in?
Next Monday we continue with Digestion & Health: Hormones in Digestion