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.