Digestion and Absorption
Kinesiology 380, Fall 2007, Lecture 8-9
Overview of Gastrointestinal System
Kinesiology 380, Fall 2007, Lecture 8-9
Energy Storage
Overview of the gastrointestinal system:
GI tube is OUTSIDE the body.
Goal is to extract nutrients and get them into the body
while keeping the filthy mess (bacteria, viruses) out.
Kinesiology 380, Fall 2007, Lecture 8-9
How your insides are actually your “outsides”
In the same way that you
are not actually “in” the
water as you drive through
the Chunnel, food is not
actually “in” your body as
it passes through your GI
system. Just as it would
not be a good thing to
allow your car to cross
from inside the Chunnel to
swimming with the fishes
in the English Channel, the
body prevents passage of
dangerous materials from
inside your GI system to the
general circulation.
England
Chunnel
France
Kinesiology 380, Fall 2007, Lecture 8-9
General scheme for digestion of food
H2O
H2O
H 2O
H2O
H2 O
H2O H O
2
Kinesiology 380, Fall 2007, Lecture 8-9
In the mouth: liquification and mechanical
disruption of solid food. Not really necessary - can
digest food added directly to the stomach but
speeds up the process.
Enzyme salivary amylase begins process of digesting
starches to simple sugars.
In the stomach, presence of HCl lowers pH and
inactivates amylase. Instead, pepsin is activated and
begins protein digestion
Kinesiology 380, Fall 2007, Lecture 8-9
How are carbohydrates absorbed?
Once ingested carbohydrate is digested into
monosaccharides, it is transported into absorptive cells in
the lining of the small intestine. The monosaccharides
then exit these cells and diffuse into capillaries and enter
the general circulation.
Absorptive Cell
Arteriole
Capillaries
Absorptive Cells
Goblet Cells
Venule
Lacteal
Artery
Lymph Duct
Vein
Kinesiology 380, Fall 2007, Lecture 8-9
Upon arrival via the portal vein non-glucose sugars are
converted to glucose. Glucose from portal vein is
not stored but passes through.
glucose 
fructose 
galactose 
GLUCOSE
general
circulation
In general circulation, most glucose is taken up and
stored by insulin-sensitive tissues, mainly in skeletal
muscle as glycogen.
Kinesiology 380, Fall 2007, Lecture 8-9
Kinesiology 380, Fall 2007, Lecture 8-9
GLYCOGEN
portal vein
glucose
glucose
to general
circulation
hepatic artery
Some glucose returns to liver via
hepatic artery. Unlike glucose entering
via portal vein (through liver to general
circulation) glucose entering via
hepatic artery is stored
as glycogen.
Kinesiology
380, Fall 2007, Lecture 8-9
Kinesiology 380, Fall 2007, Lecture 8-9
Protein Digestion and Absorption
Kinesiology 380, Fall 2007, Lecture 8-9
What exactly happens to that
steak you ate for dinner?
In the stomach: the digestion of proteins begins in the
stomach. The low pH of the stomach activates the enzyme
pepsin which begins breaking down protein into smaller
peptide fragments. The pepsin becomes inactive once it
enters the small intestine due to the more neutral pH
environment.
In the small intestine: the digestion continues by other
proteases such as trypsin and chymotrypsin.
Kinesiology 380, Fall 2007, Lecture 8-9
How are proteins absorbed?
The absorptive cells of the small intestine can only
absorb individual amino acids, dipeptides or tripeptides.
Once transported inside the cells all di/tripeptides are
broken down into individual amino acids by proteases
within the cells. Like monosaccharides, the amino acids
are transported out of the absorptive cells and diffuse into
the capillaries to move into general circulation.
Kinesiology 380, Fall 2007, Lecture 8-9
Fat Digestion and Absorption
Kinesiology 380, Fall 2007, Lecture 8-9
Fat-soluble compounds are handled very
differently. Key processes are:
1. emulsification of lipid droplets to keep them
suspended in solution (think detergent).
2. action of colipase to drill holes into bile acid coating
to allow access by:
3. Lipases, which act to digest TG ---> two FFA +
mono-glyceride.
4. After absorption (diffusion?), TG is reassembled,
combined with cholesterol and fat sol. vitamins, and
coated with protein to form a chylomicron
Kinesiology 380, Fall 2007, Lecture 8-9
In the stomach: the digestion of most lipids does
not begin until they enter the small intestine (butterfat is
one of the few lipids that is broken down by an
enzyme – gastric lipase - secreted by the stomach).
Lipids are hydrophobic and therefore do not mix with
the rest of the food beginning its journey through your
digestive system.
Rather, they form large droplets on the surface of the
chyme (mixture of food and digestive enzymes). As the
chyme leaves the stomach most ingested lipids are on
the surface massed together as globules.
Kinesiology 380, Fall 2007, Lecture 8-9
A conundrum arises because the lipases are
water soluble and thus can only act on the surface of
the globules – therefore the lipases can not actually
act on most of the lipid molecules in the globule. An
easy way to visualize this is a bottle of oil and
vinegar. Imagine that the enzymes that could digest
the oil were dissolved in the vinegar – the only oil that
could be digested would be that on the border
between the oil and vinegar.
Globule
Lipase
Lipid “core” that
is inaccessible to lipases.
Kinesiology 380, Fall 2007, Lecture 8-9
In the small intestine: lipid digestion is able to
occur because of the action of bile. Bile is a substance
produced by the liver and stored and released from
the gall bladder. Bile does not actually digest fat, rather
after the churning action of the stomach breaks apart
the fat globules into fat droplets, bile acts to keep the
droplets from reforming into globules. This process is
called emulsification.
Globule
Churning
Droplets
Bile salt
Kinesiology 380, Fall 2007, Lecture 8-9
Bile salts are responsible for emulsification of
fats. They are amphipathic, meaning that one
side of the molecule is hydrophilic (water-loving)
and the other side is hydrophobic (water-hating)
Hydrophobic
Bile Salt
Hydrophilic group
When the bile salts come into contact with a fat droplet
their hydrophobic side faces inwards towards the fat
and their hydrophilic side faces outwards into the water.
This provides a coating around the droplets which
keeps them in solution and prevents them from
reaggregating into larger droplets.
Kinesiology 380, Fall 2007, Lecture 8-9
Pancreatic lipase, an enzyme for fat digestion, is
active in the duodenum of the small intestine. Lipases
act on triglycerides (TG) to break the bonds that link the
fatty acid chains to the glycerol backbone of the TG
molecule. The result are two fatty acid chains and
a monoglyceride (fatty acid + glycerol).
FA
glycerol
As the globules continue to move through the small
intestine they are all eventually broken down by the
action of lipases before reaching the colon. The bile
salts are reabsorbed in the ileum (terminal portion of
small intestine) and recycled in the liver to be reused.
Kinesiology 380, Fall 2007, Lecture 8-9
The first step of fat absorption occurs in the
small intestine. Monoglycerides and fatty acids are
absorbed into intestinal cells via simple diffusion. (Lets
briefly revisit HS biology – cell membranes are
composed of a lipid bilayer which is hydrophobic. CHO
and protein are hydrophilic and thus do not pass through
the cell membrane unaided. However, lipids are
hydrophobic and can pass through the cell membrane.
Kinesiology 380, Fall 2007, Lecture 8-9
Chylomicrons are too big to pass through capillary
beds in the small intestine. Instead, they are secreted
into the LYMPH system and go to the thoracic duct
where they enter the sub-clavian vein.
In arterioles, LPL binds chylomicrons and hydrolyzes
TG  FFA + glycerol.FFA taken up by fat cells
(muscle as well) and re-esterified to TG for storage.
TG
FFA
TG
Kinesiology 380, Fall 2007, Lecture 8-9
Total energy stores:
CHO:about 1500 kcal (1200 muscle + 300 liver)
FAT: 50,000 to > 400,000 kcal
PRO: no true storage but >10,000 kcals are “accessible”.
Energy Density:
Person weighs 167 pounds (lb.) with 15% body fat, of
which 3% is “structural” and unavailable for energy.
167 lbs. * 12% = 20 lb. fat
At 3500 kcal/ lb. how much energy stored in body fat?
How many pounds of glycogen would be required to
store the same amount of energy? (the energy density
of stored glycogen is about 600 kcal/lb).
How much would the person weigh?
Kinesiology 380, Fall 2007, Lecture 8-9