Lipids are large molecules and generally are not
water-soluble. Like carbohydrates and protein,
lipids are broken into small components for
absorption. Since most of our digestive enzymes
are water-based, how does the body break down
fat and make it available for the various functions it
must perform in the human body?
From the mouth to the stomach
The first step in the digestion of triacylglycerols and
phospholipids begins in the mouth as lipids
encounter saliva. Next, the physical action of
chewing coupled with the action of emulsifiers
enables the digestive enzymes to do their tasks.
The enzyme lingual lipase , along with a small
amount of phospholipid as an emulsifier, initiates the
process of digestion. These actions cause the fats
to become more accessible to the digestive
enzymes. As a result, the fats become tiny droplets
and separate from the watery components.
In the stomach, gastric lipase starts to break down
triacylglycerols into diglycerides and fatty acids.
Within two to four hours after eating a meal, roughly
30 percent of the triacylglycerols are converted to
diglycerides and fatty acids. The stomach’s
churning and contractions help to disperse the fat
molecules, while the diglycerides derived in this
process act as further emulsifiers. However, even
amid all of this activity, very little fat digestion
occurs in the stomach.
As stomach contents enter the small intestine, the
digestive system sets out to manage a small hurdle,
namely, to combine the separated fats with its own
watery fluids. The solution to this hurdle is bile . Bile
contains bile salts, lecithin, and substances derived
from cholesterol so it acts as an emulsifier. It
attracts and holds on to fat while it is
simultaneously attracted to and held on to by water.
Emulsification increases the surface area of lipids
over a thousand-fold, making them more
accessible to the digestive enzymes.
Once the stomach contents have been emulsified,
fat-breaking enzymes work on the triacylglycerols
and diglycerides to sever fatty acids from their
glycerol foundations. As pancreatic lipase enters the
small intestine, it breaks down the fats into free
fatty acids and monoglycerides . Yet again, another
hurdle presents itself. How will the fats pass
through the watery layer of mucous that coats the
absorptive lining of the digestive tract? As before,
the answer is bile. Bile salts envelop the fatty acids
and monoglycerides to form micelles. Micelles have
a fatty acid core with a water-soluble exterior. This
allows efficient transportation to the intestinal
microvillus. Here, the fat components are released
and disseminated into the cells of the digestive tract
lining.
Just as lipids require special handling in the
digestive tract to move within a water-based
environment, they require similar handling to travel
in the bloodstream. Inside the intestinal cells, the
monoglycerides and fatty acids reassemble
themselves into triacylglycerols. Triacylglycerols,
cholesterol, and phospholipids form lipoproteins
when joined with a protein carrier. Lipoproteins
have an inner core that is primarily made up of
triacylglycerols and cholesterol esters (a
cholesterol ester is a cholesterol linked to a fatty
acid). The outer envelope is made of phospholipids
interspersed with proteins and cholesterol.
Together they form a chylomicron , which is a large
lipoprotein that now enters the lymphatic system
and will soon be released into the bloodstream via
the jugular vein in the neck. Chylomicrons transport
food fats perfectly through the body’s water-based
environment to specific destinations such as the
liver and other body tissues.
Cholesterols are poorly absorbed when compared
to phospholipids and triacylglycerols. Cholesterol
absorption is aided by an increase in dietary fat
components and is hindered by high fiber content.
This is the reason that a high intake of fiber is
recommended to decrease blood cholesterol. Foods
high in fiber such as fresh fruits, vegetables, and
oats can bind bile salts and cholesterol, preventing
their absorption and carrying them out of the colon.
If fats are not absorbed properly as is seen in some
medical conditions, a person’s stool will contain
high amounts of fat. If fat malabsorption persists the
condition is known as steatorrhea. Steatorrhea can
result from diseases that affect absorption, such as
Crohn’s disease and cystic fibrosis.
Storing and using of body fats
Before the prepackaged food industry, fitness
centers, and weight-loss programs, our ancestors
worked hard to even locate a meal. They made
plans, not for losing those last ten pounds to fit into
a bathing suit for vacation, but rather for finding
food. Today, this is why we can go long periods
without eating, whether we are sick with a vanished
appetite, our physical activity level has increased,
or there is simply no food available. Our bodies
reserve fuel for a rainy day.
One way the body stores fat was previously
touched upon in Chapter 4 "Carbohydrates" . The
body transforms carbohydrates into glycogen that
is in turn stored in the muscles for energy. When
the muscles reach their capacity for glycogen
storage, the excess is returned to the liver, where it
is converted into triacylglycerols and then stored as
fat.
In a similar manner, much of the triacylglycerols the
body receives from food is transported to fat
storehouses within the body if not used for
producing energy. The chylomicrons are
responsible for shuttling the triacylglycerols to
various locations such as the muscles, breasts,
external layers under the skin, and internal fat
layers of the abdomen, thighs, and buttocks where
they are stored by the body in adipose tissue for
future use. How is this accomplished? Recall that
chylomicrons are large lipoproteins that contain a
triacylglycerol and fatty-acid core. Capillary walls
contain an enzyme called lipoprotein-lipase that
dismantles the triacylglycerols in the lipoproteins
into fatty acids and glycerol, thus enabling these to
enter into the adipose cells. Once inside the adipose
cells, the fatty acids and glycerol are reassembled
into triacylglycerols and stored for later use.
Muscle cells may also take up the fatty acids and
use them for muscular work and generating energy.
When a person’s energy requirements exceed the
amount of available fuel presented from a recent
meal or extended physical activity has exhausted
glycogen energy reserves, fat reserves are
retrieved for energy utilization.
As the body calls for additional energy, the adipose
tissue responds by dismantling its triacylglycerols
and dispensing glycerol and fatty acids directly into
the blood. Upon receipt of these substances the
energy-hungry cells break them down further into
tiny fragments. These fragments go through a
series of chemical reactions that yield energy,
carbon dioxide, and water.
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