Now that we've learned about the macronutrients, let's look at nutrient
processing, how we take what we eat, break it down,
absorb it and get it into our body. When you think of the GI tract, you can
still think of this as being outside your body.
It is a system that still protects us from outside invaders.
It's very important in proper immune health, in addition to all of it's
digestive properties. So as we focus now on digestion, what is
digestion? It is the breaking down of our food into smaller pieces. It occurs both,
both mechanically and chemically. Once we've digested our food, we'll be
able to absorb it. Absorption is simply the movement of these smaller pieces from
this external portion of our body into circulation.
Finally, we can also consider transport as part of nutrient processing.
This is the movement of nutrients from one part of the body to another.
For example, from the liver out towards the body's cells.
As we review the digestive tract, we're going to start at the mouth.
For anyone interested in a career in dentistry, or practicing dentistry, or
someone that's just interested in good oral health,
a healthy mouth is key to proper nutrition, as it is the forefront of your
digestive process Without healthy teeth, its much harder to break your food down.
If we can't properly break our food down, we won't be able to absorb it into our
systems. In addition an unbalanced diet can be
detrimental to oral health. So good nutrition, good teeth, go hand in
hand. As we chew our food we'll process further
along into the stomach down to the small intestine.
and finally, the large intestine. This figure gives you an overall idea of
what's broken down where, and where things are absorbed.
It also illustrates that we have two systems for absorbing nutrients, both the
portal system, blood, and the lacteal. This is the lymph system,
and it's where we absorb our fat-based foods.
When we think about nutrient processing, the proper breakdown and absorption and
utilization of our food is key to maintaining all body systems,
the skeletal system, circulatory system, the nervous system, the respiratory
system, our muscles, and of course, the digestive tract itself.
Proper nutrition is key to a healthy digestive system.
When we talked about carbohydrates, we mentioned fiber being an important
component to digestive health. It keeps the stomach and intestinal
muscles strong and healthy. All right, so let's start from the
beginning, the mouth. Here we have both mechanical and
enzymatic digestion. Of course, the most important part of
digestion is the mechanical digestion. In the mouth, besides the teeth and the
tongue moving foods around, we also have salivary glands, producing saliva as well
as salivary amylase, lingual lipase, fluids, mucus, and
lysosomes. The enzymes, salivary, amylase, and
lingual lipase only break down a small portion of our food, but you can
understand how these work with a simple experiment.
Try biting into a slice of bread. Don't swallow it, but continue to chew
it. Lingual lipase will slowly break down
some of that carbohydrate into sugars. Over time, if you don't swallow, you will
notice this sweet taste in your mouth that didn't start out as sweet, but
started out as bread. The other components of saliva are very,
also very important. Of course, the fluid keeps the
consistency in the mouth, mucus protects the GI tract and the oral
surfaces, and lysosomes have antibacterial properties to protect the
mouth from bacterias entering the mouth. Continuing, we go to the stomach.
Between the mouth and the stomach, we do have sphincters that are helpful in
making sure food goes in the right direction,
and stays there. At the end of this lecture, we'll talk a
little bit about problems in digestion, and one of these, GIRD, can occur if
these sphincters don't work properly and some of the foods that have entered the
stomach return to the mouth. They don't return as whole foods
obviously, but they do return in acidic from, creating that acid environment in
the mouth and the upset stomach. Looking at the stomach, it's important to
understand that it is a muscular structure.
The muscles of the stomach move food around, breaking it down and allowing the
enzymes and chemicals in the stomach to work on food.
In the stomach we will find hydrochloric acid.
This is why we want to make sure that food does stay in the stomach, and
doesn't return to the esophagus. The esophagus is a far less acidic
environment than the stomach. Acid reflux, or GIRD, as I mentioned, is
when these acids go back towards the mouth.
The goal of the stomach digestion is to move food slowly, towards the bottom of
the stomach and the pyloric sphincter. This is the barrier between the stomach
and the initial part of the small intestine, the duodenum.
At the end of this lecture, we'll also talk a little bit about the impact of
gastrointestinal surgeries on nutrition. Most of these focus on changing the size
of the stomach to aid in weight loss. Once we've gone past the stomach, we do
enter the intestinal tract. There are several areas of the intestinal
tract that are important. The duodenum is the site of most
digestive processes. Further down in the intestinal tract, we'll see an increase
in absorption. In the duodenum, we see significant
enzymatic digestion. We also see a buffering of the foods
coming from the stomach. Foods that have been broken down in the
stomach, and are entering the intestinal tract, are called chyme.
It's now a mixture of what you've eaten, stomach acids and stomach fluids.
In the duodenum, bicarbonate will be released, to help bring down the pH to
protect the intestinal tract, because we don't want that acid to come into the
intestinal tract and damage the tissues. As we go on to talk about the
macronutrients individually, we'll talk about what enzymes are present
in the duodenum for digestion. As we continue down, the Jejunum and the
Ileum are where we start to see absorption take place.
This figure shows you exactly what types of nutrients are absorbed where.
There is both passive, active and facilitated transport.
We'll talk a little bit about what those are.
First off, let's look at different types of digestion.
So, let's review the Enzymatic Digestive processes.
Digestive enzymes work just like any other enzyme.
This is an example here of breaking down sucrose.
Enzymes work to facilitate chemical processes.
In this case, they facilitate the hydrolysis of the disaccharide sucrose
into independent molecules of glucose and fructose.
Enzyme function is very specific and enzymes will only target specific
compounds. As you read through this, try to learn
the different enzymes that act on specific nutrients.
In addition to chewing, physical processes of digestion that occur in the
stomach and intestinal tract include peristalsis and segmentation.
Peristalsis is the muscular movement that pushes foods forward in the GI tract
while segmentation helps breaks foods up into smaller amounts, so that as they do
enter locations where they are acted on enzymatically, we can increase the
surface area, and make digestion more efficient.
The link below is a useful animation for you to understand what this looks like,
and review the digestive process, in addition to reading your text.
In the stomach, peristalsis is very important in mixing the foods and
creating the chyme. As I have already mentioned, the goal is to move the chyme
towards the pyloric sphincter, to release the chyme, in a regulated manner, into
the intestinal tract. Again, the stomach environment is highly acidic. Look at
this chart here. You can see that gastric juice is more
acidic than lemon juice and orange juice. Of course not, as acidic as battery acid,
but nonetheless, significantly acidic. A healthy sphincter here, as well as the
upper sphincter, is very important in controlling flow into the GI tract.
Depending on what you eat, the movement of the food from the stomach into the
intestines may happen at different rates. Fiber, for example, will actually slow
down stomach emptying. This is why it's useful for people with
diabetes. In someone with diabetes, if they do have
a certain amount of carbohydrate in the diet, that is broken down to sugars.
By adding fiber, we slow the release, and help the body process the sugar a little
bit at a time. So now, let's look at some of the
enzymatic and hormonal controls of digestion.
This is a basic overview of the different hormones that control the digestive
processes, including how quickly food moves through.
When we eat, food hits the stomach wall, and gastrin is released.
Gastrin will activate the release of hydrochloric acid.
We certainly wouldn't want an acidic environment to be maintained at all times
in the stomach, even though the stomach does have mucosa to protect its cells,
the cells of the stomach, these endothelial cells are protein.
If we have acids all the time, we can actually break down the cell walls and
damage the stomach lining. Ulcers are just that.
Ulcers are when we do have damage in these stomach linings, and these damaged
areas can become infected. This is very painful and causes
significant problems. In addition to hydrochloric acid, gastrin
activates the enzyme pepsin. Just like hydrochloric acid, we don't
necessarily want proteolytic enzymes, those are enzymes that break up proteins,
sitting around active all the time. We don't want them to act on body
proteins. So pepsin sits as an inactive protein
until gastrin activates it. Gastrin will also activate stomach
emptying. That way, the stomach can rest when
there's no food to be processed. Other important regulators of, of gastric
function include Gastric Inhibitory Peptide,
secretin, and Cholecystokinin. Gastric Inhibitory Peptide, or GIP, will
actually slow stomach emptying and trigger the release of insulin.
This is important in regulating how much carbohydrate material is released into
the duodenum. We don't want too much at once,
or the body's insulin system won't be able to keep up.
Secretin will actually decrease pepsin release.
This helps slow the digestive process. And Cholecystokinin slows stomach
emptying, with a movement into the duodenum.
It's also important for releasing bile. This will be the released into the
intestinal tract. We've talked a little bit about this
already. Bile is generated in the liver and and
stored in the gall bladder. If someone does not have a gall bladder, that's
fine. They can still make bile and release it
directly from the liver. Cholecystokinin also triggers the release
of digestive enzymes. So if we review this figure, gastrin is
responsible for getting the stomach working, while the others, GIP, secretin
and Cholecystokinin, are responsible for prepping the duodenum for its work, and
for controlling the movement of chyme from the stomach into the intestinal
tract. As we move into the intestines, we'll see
that certain anatomical structures are very important to digestion.
The villi help increase surface area of our intestinal tract to make sure that we
have enough time to both digest and absorb our foods.
If we take a closer look at these finger-like projections on the surface of
our intestinal tract, we see that they are made up of even smaller villi,
and that within these villi are the actual absorptive cells, the lacteal, the
capillaries of the blood system, as well as other important components.
As we move into the intestines, we will note that the intestinal tract, in
addition to being significantly long itself, is made up of villi.
This further increases the surface area of the intestine, to ensure that we have
sufficient capacity to both digest and absorb our food.
Within each villi, are the vessels that will carry nutrients from the GI tract to
the bladder. The capillaries carry water-soluble
substances towards the liver, while the lacteal carry fat-soluble substances to
the liver. We talked briefly, when we talked about
carbohydrates and fiber, about the fact that intestinal cells turn over.
New intestinal cells will start at the bottom in the crypts.
They'll move slowly up the villi until they reach the top where they can be
sloughed off. Fiber and other substances in our
digestive product help slough these cells off.
Certain auto-immune conditions, such as Crohn's disease, can actually damage
intestinal villi and reduce absorptive capacity.
As I mentioned, there's different ways that we do absorb nutrients.
The first is facilitated diffusion. With this, we do see the importance,
important role of proteins. Proteins are the transporters.
These proteins change shape to allow for specific nutrients to cross over, and
keep things we don't want going in, like bacteria keeping those out.
Diffusion is a simpler form of substances crossing, and they can do so either
between the cell walls, between, actually, those Phospholipids, that we've
already discussed in the lipids section, or through the proteins.
Active transport requires energy. Minerals are often carried through by, by
active transport as are our proteins. So here we can see that it takes
calories, energy, ATP to digest our food. When we talk a little bit about basal
metabolism, this is part of that basal metabolsim,
the amount of energy required to keep our body functioning, even when we're really
doing nothing. Finally, endocytosis is where the cell
wall breaks itself off, engulfing a substance and bringing it into the
system. As I mentioned, most of the digestive
process really happens in the intestinal tract.
Lets look at some of what happens in the mouth and the stomach.
In the mouth as we know, lots of mechanical breakdown.
Chewing your food is very important. We also have salivary amylase and lingual
lipase working on our starches and fats. In the stomach we'll also continue fat
breakdown with gastric lipase and we'll begin protein breakdown with Pepsin.
Pepsin is, as I mentioned previously, inactive at first and activated as food
enters the stomach and hydrochloric acid increases, and is responsible for
beginning to breakdown polypeptides into smaller peptide chains.
Let's now consider each macronutrient's final digestion and absorption.
Take a moment to review this slide. Carbohydrates, such as starch, will
continue to be broken down from their long chains, until they reach
disaccharides and finally, into simple sugars.
When we reach the disaccharide form, specific enzymes, matching each
disaccharide, will do the final break down.
Lactase for lactose, maltase for maltose, and sucrase for sucrose.
Once carbohydrates are broken down into simple sugars, they can be absorbed.
Absorption of the three monosaccharides will occur through one of the mechanisms
we already discussed. Glucose and galactose are absorbed by
active transport. In fructose, you have facilitated
diffusion. The flow of these monosaccharides in
absorption goes from the lumen of the small intestine to the mucosal, or gut
cell, then towards the capillaries, or into the capillaries within the
intestinal villi. And from those capillaries,
monosaccharides move to the portal vein, and finally, the liver.
Throughout this course, as you read and learn, you will discover how important
the liver is to nutrition and overall health.
It is certainly the center of our factory, where everything is processed
and built to keep the machine running. The digestion of lipids differs from
other macro-nutrients, in that lipids, being fats, will not go into the aqueous
environment of the blood. They'll enter the body through lacteals.
These will still eventually get to the liver, however these lipids are not going
to be transported in the blood. When we think about the break down of
fats, imagine a glass of water with drops of oil added to it.
If you let it sit, the oil will separate out from the water.
These two substances certainly don't mix on their own.
As we've already discussed, bile is a very important substance for the
digestion of lipids, in that it will break lipids up into small droplets that
can be acted upon by enzymes. The result of this micelle activity and
enzymatic digestion are short chain fatty acids, medium chain fatty acids, the
glycerol backbone, as well as a some diglycerides and monoglycerides.
And as a reminder from a previous slide, Cholecystokinin will control the release
of bile from the gall bladder. As I mentioned, the fats are not going to
go into the portal system, they're going to go into the lacteal.
And this image just shows a closer picture of what's happening with these
substances. Eviction of the bile salts, the enzymes
involved in breaking down these droplets of fat, the lipase,
and how we free fatty acids from the monoglyceride, including the glycerol
backbone. If we look at what substances are
actually entering the internal site, we'll see that both monoglycerides and
free fatty acids enter. but once into the intestinal cell they
will actually reconnect as triglycerides. These triglycerides will then be further
packaged into chylomicrons. And it is the chylomicron that is going
into the lacteal to be transported further into the body.
Finally, when we digest and absorb proteins, we'll see similar patterns, as
with the carbohydrates, breaking down long chains into smaller
components. Specifically, we will digest polypeptides
into tripeptides and dipeptides and free amino acids.
Now, to get these components into the blood, we still have to break them down
further into all free amino acids. In the case of protein, unlike fats, for
example, it's the simplest component that's going to be absorbed completely.
Amino acids are going to be actively absorbed and actually couple with sodium
to process through protein transporters. Important enzymes in the breakdown of
proteins include Trypsin, Chymotrypsin and Peptidases.
An interesting note about Trypsin is that some compounds in the foods can actually
destroy Trypsin. These are termed anti-Trypsin factors.
They are actually found in beans, and if we do not cook beans properly, that
anti-trypsin factor will still be active. So in populations where cooking is done
over a fire, if it's not done sufficiently, these anti-Trypsin factors
can significantly impair digestion. To conclude, lets look at some of the
potential problems in digestion We talked a little bit about GIRD, or Gastro
Intestinal Reflux Disease already. this is an extensive form of reflux.
You can certainly have acid reflux without having GIRD but a chronic
condition of reflux could be GIRD. And in this, condition, we are seeing the
sphincter not working properly, so that the acidic substances of the
stomach, that chyme, are released back up the esophagus, potentially damaging the
lining of the esophagus, and doing damage to oral surfaces, including the teeth.
Achlorhydria is a lack of hydrochloric acid.
This is a condition that can happen in aging, and it will impair digestion.
And as hydrochloric acid is important in the breakdown of foods in the stomach
level, if we don't have enough hydrochloric
acid, we won't break down our food sufficiently to be acted upon by the
enzymes in the intestinal tract. Gastric ulcers can also cause digestive
problems, stomach upset, nausea, pain, vomiting and even blood,
or bleeding from the stomach. Gastric ulcers do happen when the mucosal
lining in the stomach is damaged, and these areas where the lining's been
damaged become infected. Continuing on condition, conditions that
affect the intestinal tract include lactose intolerance, Celiac disease and
Chrons disease. With food based conditions, such as
lactose intolerance and celiac disease, the body actually either launches against
itself, so in the Celiac disease condition, antibodies can be synthesized
that attack intestinal cells when one consumes wheat,
damaging these cells, to the extent where we can actually reduce the size of the
villa. In the case of lactose intolerance, these
individuals lack the enzyme lactase. When lactose can't be broken down into
its monosaccharide, it will simply stay in the GI tract, and causes diarrhea.
Chrons Disease is an auto-immune disease that affects the entire GI tract, but
primarily the upper part of the small intestine, and it does also damage the
villi. Ulcerative Colitis is another gastric
disorder related to Chrons disease, but this occurs in the large intestine.
Diverticulosis also occurs in the large intestine, and causes pockets to form in
the intestinal walls, where bacteria can enter and cause infection.
Lastly, in the age of morbid obesity, gastric bypass surgeries and gastric
banding can affect the digestive processes.
In the case of banding where the opening to the stomach is made smaller,
we still have an intact stomach. We still have an intact intestinal
system. This is the least severe of the surgeries
you can have, and it is highly reversible, and indeed, some individuals
even have flexible bands where they can change the size depending on the day.
Whether that's counter-intuitive or not, is up to you to decide, but again this is
one process that one can undergo. In addition to banding, there is the
process of sleeve gastrectomy. This is, has been shown to be effective
in aiding in weight loss. Here we see that the upper portion of the
stomach is stapled, and part of the stomach's removed.
The remaining portion is what we call, is what the sleeve is referring to.
So we still have a stomach, and again, we still have an intestinal tract. Very
importantly, we still have a duodenum. What this does is reduces the size of the
stomach, so someone with the surgery is going to feel full much quicker, and
therefore, eat less. The most invasive surgery is gastric
bypass surgery. In this surgery, you will actually remove
the stomach. and the initial part of the intestinal
tract. So think about what we just talked about,
how important the duodenum is for the complete digestion of our food.
Individuals with gastric bypass surgery certainly have been shown to have
significant weight loss, but after the surgery, good nutrition is essential, and
supplementation is most likely going to be needed.