Nutrient Digestion & Absorption

Nutrient Digestion & Absorption


– In this video, I want to talk
a little bit about nutrient digestion and absorption. So, you should have
watched the videos and studied up on what’s going
on along the digestive tract. In this video, what
I want to do is I want to talk a little bit
about the different types of nutrients and
talk specifically about how they’re absorbed
and where they’re absorbed. You can think of this as a
micro version of a little bit of Nutrition 101. First off, I just
want to identify the different types of nutrients
that you get from food. Nutrients can be
generally classified into two broad categories. The first category are
the macronutrients. Now the prefix
macro- means large, so these are the nutrients that
you need in large quantities. These are also
the nutrients that contain calories,
the nutrients that can be used by your cells
to make energy, to make ATP. These macronutrients are the
carbohydrates, the lipids, and the proteins. They’re made primarily
of carbon, hydrogen, and oxygen. Proteins
contain nitrogen as well. And apart from being able
to be broken down by cells and used to make
ATP, these nutrients serve as the building blocks. You’re made up of the same
things as your food is. You literally are what you eat. So carbohydrates are
broken down and used to make new carbohydrates
in your cells. Lipids are broken
down and used to make the lipids your cells need. Proteins are broken down
and their amino acids are used to make new proteins
in your cells and tissues. The micronutrients
are those nutrients that are absolutely
essential for life, but you only need
very small amounts. And these are the
vitamins and minerals. The vitamins are
organic molecules. And I mean the chemical
definition of organic, not the Whole Foods definition. So these are the micronutrients
that contain carbon. Vitamins are usually
used to make part of– to make a part of
coenzyme, so molecules that are necessary for
enzyme activity even though they aren’t
actually proteins. Minerals are inorganic elements,
like calcium, iron, iodine, potassium, sodium, and
they serve a variety of different functions. Some minerals act as co-factors
that help enzymes function, some minerals contribute
to body structure, like calcium building bones.
Several of the minerals circulate in the
fluids of the body as ions that are essential
for muscle and nervous system functioning. Think about sodium,
potassium, calcium ions. So obviously with
calcium, some minerals can do multiple functions, can
contribute to body structure and serve as signaling ions. Now, all of these nutrients,
in order to actually be used by your body, they need
to get into the bloodstream, and that means that
they need to cross the intestinal epithelium. And in order to cross the
intestinal epithelium, they have to get across
the plasma membrane of the epithelial cells. This diagram summarizes
some of the ways in which substances cross
the epithelial cell plasma membrane. So the plasma membrane is
the barrier to diffusion. Remember with the
plasma membrane, you have this phospholipid bilayer. And the inner part of
this phospholipid bilayer is hydrophobic. And that’s the
important part that acts as a barrier between
the watery solution outside the cell and the watery
solution inside the cell. So some molecules
can cross directly through that
hydrophobic barrier. Water can cross, because water
is a relatively small molecule that can just slip in
between the phospholipids. Small lipids can
also cross directly. They are lipid soluble,
they’re hydrophobic, they can just
dissolve right through that hydrophobic barrier. And that’s called
simple diffusion. So simple diffusion,
substances passing straight through the plasma membrane. Other substances can get
through the plasma membrane without using any
energy, but they do require some assistance. And there are transport proteins
that basically will either act as little channels opening
up a passageway for substances to enter the cell or will
pick up a substance on one side of the cell
and then release it on the inside of the cell. This is referred to as
facilitated diffusion. To facilitate a
process basically means to help it along. So in facilitated diffusion,
it’s still diffusion, there’s no energy being
used, substances are still moving from an area
of high concentration to an area of low
concentration, but they’re getting a little bit of
help from proteins embedded in the plasma membrane. In the third form of
movement, some substances require assistance and energy. And the use of energy– and by energy I mean ATP– this use of energy is what
defines active transport. You’re still getting
assistance from proteins, but you’re now expending
a little bit of ATP to move something across. Energy is usually
required if substances are moving against their
concentration gradient. So, from an area of
low concentration to an area of high
concentration, you have to burn a
little bit of energy. Now, as far as how this applies
to the nutrients… Sugars. Sugars are the breakdown
products of carbohydrates. They’re taken up mostly
by active transport, some by facilitated diffusion. Fats are taken up
by simple diffusion, although the absorption of
fat is a little bit more complicated. We’ll talk about that in more
detail a little bit later in this video. Proteins, they’re digested
down to amino acids. And amino acids
are taken up mostly by facilitated diffusion,
some by active transport. Fat soluble vitamins diffuse
across in lipid droplets. So if the vitamins there
are both vitamins that dissolve in fat and vitamins
that dissolve in water, fat soluble vitamins are
taken up by simple diffusion. Water soluble
vitamins are mostly taken up by facilitated
diffusion, mostly. The one exception is B12. B12 needs to bind to intrinsic
factor from the stomach. And then that B12
intrinsic factor complex is taken up by
active transport. The minerals are
primarily taken up by either facilitated
division or active transport depending on the mineral. Now, once nutrients get across
the intestinal epithelium, they are taken up into either
the capillaries or lacteals. Most absorbed nutrients are
taken up into the capillaries. Capillaries are the
smallest blood vessels. These capillaries then drain
into the hepatic portal vein and carry those nutrients
directly to the liver. However, there are
some fat droplets that are made in the process of
absorbing fat that are just too big to enter the capillaries. The capillaries can really
only take in smaller molecules. Large fat droplets
enter into the lacteals. That’s why these lymphatic
vessels are called lacteals. The prefix lacto- should
make you think milk, like the sugar lactose,
which is found in milk, like lactation, the
production of milk. So basically someone,
some historical anatomist at some point, took a look at
the fluid in these vessels, saw that it was kind
of a milky white because of all the fat
droplets in it, and said, I’m going to call these
lacteals, because it looks like milk in there. So here’s what I want you
to know about the nutrients. I want you to be
able to tell me what the final products
of digestion are, where each nutrient is digested. So, where in the
digestive tract? I mean, the small
intestine, you’ve got everything being digested
in there, but in the mouth you only have carbohydrates
and a little bit of fat. In the stomach, it’s
only protein and fat. So, where does digestion happen? You should also be familiar
with what enzymes break down each type of nutrient. So if you see lipase,
you know that that’s breaking down lipids, fats. If you see protease,
or pepsin, or trypsin, you know that it’s
acting on proteins. If you see amylase,
that’s an enzyme that breaks down starch, which
is a type of carbohydrate. So, understand where the enzymes,
what enzymes break down each type of nutrient
and where they come from. I would also like you
to be able to tell me how each type of nutrient
is absorbed, especially fat, because fat
absorption is complicated because fats are hydrophobic,
and the fluids of your body are watery. So what I would
strongly recommend doing as you’re
studying this material is preparing a chart. So just basically draw up
carbs, proteins, lipids or fats. And then, one,
products of digestion, two, where digested,
three, enzymes involved, and then absorption. First off, let’s talk
about the carbohydrates. The carbohydrates are
also known as sugars, although sugars are really
the more simple carbohydrates. There are more complex
carbohydrates as well, including carbohydrates that
your body can’t break down, the fiber. So carbohydrates can
be either very simple, and these are the
monosaccharides where you have basically one
single unit, one simple sugar molecule. And there are three
types of monosaccharides. You have glucose shown here. You also have fructose
and galactose. If you put two
monosaccharides together, you get a disaccharide. This is the disaccharide
lactose that’s found in milk. And that’s made up of one
glucose molecule bonded to a galactose molecule. Sucrose is another disaccharide. And that is one glucose molecule
bound to one fructose molecule, and that’s the kind of sugar
that you find in refined table sugar is sucrose. You also have maltose, which
is two glucose molecules joined together. And you find maltose in
fermented substances. Lastly, you have
the polysaccharides. And the polysaccharides are
the more complex carbohydrates where you have many
sugars strung together. The main two
polysaccharides that you should be familiar with
are glycogen, shown here, which is a sort of hundreds
of glucose molecules all strung together
and branching. Glycogen is how glucose is
stored in your skeletal muscle cells and how glucose is stored
in your hepatocytes. Plants store glucose as starch. So starch is a large
molecule made up of hundreds of glucose
molecules strung together. But it is made by
plants, it’s not something that’s typically
found in our body. But when we consume
carbohydrates, we frequently are
consuming a lot of starch– so potatoes and bread
are very high and starch. And the fibers,
indigestible fiber, are also polysaccharides. This diagram summarizes the
digestion and absorption of carbohydrates. So, in order for
carbohydrates to be absorbed, carbohydrates must be broken
down to the monosaccharides. So no matter what form you’re
taking in a carbohydrate, it’s going to be absorbed
as glucose, galactose, or fructose. So carbs are always broken
down to monosaccharides and absorbed as monosaccharides. They’re broken down in the
mouth by salivary amylase. They’re broken down
in the small intestine by pancreatic amylase. And their brush
border enzymes that take those simpler
carbohydrates and finish the job of breaking them down
into the monosaccharides. And then glucose and galactose
are absorbed cotransport with sodium. Fructose is absorbed through
facilitated diffusion. Then they are taken up
into the capillaries and taken to the liver by
the hepatic portal vein. Proteins are polymers
of amino acids. So proteins are made up
of individual amino acids, just strung together by
the thousands, if not tens of thousands. And sometimes when you
hear the term polypeptide, you’ll hear that
referred to protein. And proteins are referred
to as polypeptides because the bond that joins
those amino acids together is called a peptide bond. So a protein and polypeptide
is the same thing, though polypeptide is
usually used to refer to a fragment of a protein. In order for protein in
your diet to be absorbed, it needs to be broken
down to amino acids. So in digestion, proteins are
broken down to amino acids and absorbed as amino acids. And there are about 20 different
amino acids that period. There are 20
different amino acids that proteins are made up of. So the process of
breaking proteins down goes something like this. Proteins are broken down into
larger polypeptides, and then smaller polypeptides,
and eventually to individual amino acids. Protein is not
digested in the mouth. There’s nothing in saliva
to break down protein, but once that food bolus
gets to the stomach, pepsin acts on the
protein and that’s with the assistance
of hydrochloric acid. Remember, the hydrochloric
acid denatures the protein, straightens it out so that
pepsin can get in there and chop up the protein. Pancreatic enzymes act on
those large polypeptides and break them down into
smaller polypeptides. And then the brush
border enzymes take those small peptides,
those small fragments, so just a few amino acids,
and break them down to individual amino acids. This process, the fact that
proteins are broken down to amino acids
before absorption, that’s why it doesn’t
really make sense to say take in a protein– a nutritional supplement
that is a protein that’s supposed to be acting directly
on your brain, or your muscles, or your kidneys, or something
like that, because all proteins are digested down
to amino acids that completely lose their
function before absorption. Now if that protein is intended
to act in the intestinal tract, then you might have some basis,
but it loses its function, and then you’re only
absorbing the amino acids. And then those
amino acids can be used to rebuild different
proteins in the body. As far as the path
of absorption, amino acids are taken up through
active transport with sodium. There are some very small
dipeptides and tripeptides absorbed and broken
down to amino acids in the epithelial cells. And then the amino acids are
taken into the capillaries and sent to the liver. Now, to talk about the
fats, we need to understand the nature of dietary fats. Most dietary fats are
these triglycerides. And they’re called
triglycerides because you have three fatty acid chains
joined to a glycerol molecule. So that’s what gives
you the triglyceride. Now, when triglycerides
are digested, those fatty acid molecules are
ripped off of the cholesterol. And then both the glycerol
and the fatty acid chains are absorbed and
used by the body. And they’re absorbed and used
a little bit differently. This diagram doesn’t
show it, but you do have a small amount of
fat digestion happening in the mouth and stomach. Because you have lingual lipase
in the mouth and gastric lipase in the stomach, you have
small amounts digested there. But you don’t have
bile coming in, you don’t have emulsification
of dietary fats until you get to
the small intestine. So you don’t really– the
majority of fat digestion happens in the small intestine. You don’t really get large
quantities of fat digestion until you have bile in there
to break up the fat droplets and make them more accessible
to the pancreatic lipases– enzymes from the pancreas
that take that triglyceride and break it down into
glycerol, and fatty acids, and some monoglycerides,
which is just that glycerol backbone with a
single fatty acid stuck to it. Now the absorption of
fat is complicated, because fats are hydrophobic. So the fatty acids and
monoglycerides, they’re taken up by simple diffusion. Then they’re recombined
into triglycerides in the epithelial cells and
packaged into these structures called chylomicrons. And then chylomicrons are
the large, fat particles that are taken up by lacteals
and enter into the lymph. There are some
smaller fatty acids, what are called short
chain fatty acids. That just means
that they have a short chain of carbon atoms. Smaller fatty acids can be
taken into the capillary blood, but the larger fatty
acids and monoglycerides, they’re packaged
into chylomicrons and taken up into the lacteals. This is another way of
looking at fat digestion and absorption. So here, we’re looking
at the rule of bile. So you’ve got
emulsification by bile. And that is forming these
little droplets called micelles. So micelles are basically
fatty acids and monoglycerides packaged together with
bile salts in the lumen of the small intestine. OK, so micelles are only in the
lumen of the small intestine. The micelles ferry those fatty
acids and monoglycerides over to the intestinal
epithelium, where they diffuse into the
epithelial cells and are recombined and packaged as chylomicrons. So chylomicrons are these
particles full of fatty acids, cholesterol, some proteins. And these chylomicrons
are exocytosed from the intestinal
epithelial cells and drawn into the lacteals,
because they are too big to go into the capillaries. This chart is just basically
one big summary chart of how carbohydrates, lipids,
and proteins are dealt with by the digestive system. So it takes each region
of the digestive tract and describes what’s going on
with each type of nutrient. And then there’s a little
bit about absorption and how they get
into circulation. So, we’ve talked about
the macronutrients, and I just want to add a
couple of small comments on the micronutrients. First off, I’m not going
to talk about digestion of the micronutrients, because
the micronutrients aren’t digested. The minerals, they
are absorbed as ions. You can’t really
break an ion down. There’s no covalent bonds
in there to be broken. The vitamins are joined
by covalent bonds all over the place, but
they aren’t digested. Those bonds aren’t broken. The vitamins are absorbed
whole and undigested. I mentioned that there
are fat soluble and water soluble vitamins. You need a little bit of fat
in your diet to absorb fat soluble vitamins. Vitamin A, vitamin D,
vitamin E, and vitamin K are all fat soluble. Vitamin C and all of the B
vitamins are water soluble. So that’s one reason
why you can’t completely eliminate fat from
your diet and still have a healthy, sufficient diet,
because you would be seriously hampering your ability to
absorb the fat soluble vitamins. These are some of the more
common ions that are absorbed, minerals that are absorbed,
how they are absorbed, and any factors that
affect their absorption. And down here, you’ve got
the water soluble vitamins. And vitamin B12, which is
a water soluble vitamin, but it’s a bit of a special
case, because vitamin B12, remember, needs intrinsic
factor from the stomach. And the fat soluble vitamins
taken up by diffusion, they’re absorbed
in those micelles that dietary lipids
are absorbed in. OK, so like I said, this is a
Nutrition 101 micro version, light version. Although, in Nutrition 101, we
do cover a lot of other things too. But after studying
this video, you should be able to talk about the
locations and enzymes involved in the digestion of
the macronutrients. You should be able to identify
the end products of digestion of macronutrients and talk
about how they’re absorbed, particularly
focusing on the fats. Make sure you draw a
distinction between the micelles and the chylomicrons. They’re both fatty particles,
but they serve different roles. Micelles transport fats in the
lumen of the small intestine. Chylomicrons transport
fats out of the intestine into the lymph. You should also be able
to discuss a little bit how the minerals and
vitamins are absorbed, including making a distinction
between the water soluble and the fat soluble vitamins.

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