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CHAPTER EIGHTEEN
Nutrition and Metabolism
apolipoproteins, can combine with receptors on the mem-
branes of speciF c target cells. Lipoprotein molecules vary in
the proportions of the lipids they contain.
Lipids are less dense than proteins. As the proportion of
lipids in a lipoprotein increases, the density of the particle
decreases. Conversely, as the proportion of lipids decreases,
the density increases. Lipoproteins are classiF
ed on the basis
of their densities, which reflect their composition.
Very-
low-density lipoproteins
(VLDL) have a high concentration
of triglycerides.
Low-density lipoproteins
(LDL) have a high
concentration of cholesterol and are the major cholesterol-
carrying lipoproteins.
High-density lipoproteins
(HDL) have a
relatively high concentration of protein and a lower concen-
tration of lipids.
In addition to regulating circulating lipids, the liver con-
trols the total amount of cholesterol in the body by synthesiz-
ing and releasing it into the blood or by removing cholesterol
from the blood and excreting it into the bile. The liver uses
cholesterol to produce bile salts. Cholesterol is not used as
an energy source, but it does provide structural material for
cell and organelle membranes, and it furnishes starting mate-
rials for the synthesis of certain sex hormones and hormones
produced by the adrenal cortex.
Adipose tissue stores excess triglycerides. If the blood
lipid concentration drops (in response to fasting, for example),
some of these stored triglycerides are hydrolyzed into free
fatty acids and glycerol and released into the bloodstream.
Lipid Requirements
The lipid content of human diets varies widely. A person
who eats mostly burgers, fries, and shakes may consume
50% or more of total daily calories from fat. ┬▒or a vegetar-
ian, the percentage may be far lower. The American Heart
Association advises that the diet not exceed 30% of total
daily calories from fat.
molecules. Excess acetyl CoA molecules react to form com-
pounds called
ketone bodies,
such as acetone, which later
may react to form acetyl coenzyme A once again. In either
case, the citric acid cycle can oxidize the acetyl coenzyme A
molecules. The glycerol parts of the triglyceride molecules
can also enter metabolic pathways leading to the citric acid
cycle, or they can be used to synthesize glucose.
When ketone bodies form faster than they can be decomposed, some
of them are eliminated through the lungs and kidneys. When this
happens, the ketone acetone may impart a fruity odor to the breath
and urine. This can happen when a person fasts, forcing body cells to
metabolize a large amount of fat, and in persons suF
ering from dia-
betes mellitus who develop a serious imbalance in pH called acidosis,
which results when acetone and other acidic ketones accumulate.
Glycerol and fatty acid molecules resulting from the hydro-
lysis of fats can also combine to form fat molecules in anabolic
reactions and be stored in fat tissue. Additional fat molecules
can be synthesized from excess glucose or amino acids.
The liver can convert fatty acids from one form to another.
However, the liver cannot synthesize certain fatty acids, called
essential fatty acids.
Linoleic acid,
for example, is an essen-
tial fatty acid required to synthesize phospholipids, which,
in turn, are necessary for constructing cell membranes and
myelin sheaths, and for transporting circulating lipids. Good
sources of linoleic acid include corn oil, cottonseed oil, and
soy oil.
Linolenic acid
is also an essential fatty acid.
The liver uses free fatty acids to synthesize triglycerides,
phospholipids, and lipoproteins that may then be released
into the blood
(f g. 18.5)
. These lipoproteins are large and
consist of a surface layer of phospholipid, cholesterol, and
protein surrounding a triglyceride core. The protein constitu-
ents of lipoproteins in the outer layer, called
apoproteins
or
Fatty acids
(except essential
fatty acids)
Cholesterol
Phospholipids
Lipoproteins
Triglycerides
Triglycerides
Fatty acids + Glycerol
Digestion
FIGURE 18.5
The liver uses fatty acids to synthesize a variety of lipids.
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