Understanding Nutrition, 8e

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Understanding Nutrition, 8e

• Chapter 5 - The Lipids Triglycerides,
  Phospholipids, and Sterols

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Glycerol
When glycerol is free, an OH group is attached to
each carbon. When glycerol is part of a
triglyceride, each carbon is attached to a fatty
acid by a carbon-oxygen bond.
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Stearic Acid
Stearic acid is 18 carbons long, and each atom
meets the rules of chemical bonding described in
Chapter 4. A fatty acid like stearic acid that
contains only single bonds between its carbon
atoms is a saturated fatty acid.
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Stearic Acid Zigzag
In this simplified depiction of stearic acid,
each "corner" on the zigzag line represents a
carbon atom with two attached hydrogens.
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Acetic Acid
Acetic acid is a two-carbon organic acid.
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Condensation of Glycerol and Fatty Acids
Water is removed from the glycerol and the fatty
acids, forming a bond between the O on the
glycerol and the C at the acid end of each fatty
acid. Three fatty acids attached to a glycerol
form a triglyceride and yield water. In this
example, all three fatty acids are stearic acid,
but most often triglycerides contain mixtures of
fatty acids.
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An Impossible Chemical Structure
This compound cannot exist because two of the
carbons have only three bonds each, and nature
requires that every carbon have four bonds. In
this example, all three fatty acids are stearic
acid, but most often triglycerides contain
mixtures of fatty acids.
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Oleic Acid
Two carbons form a double bond to create oleic
acid, an 18-carbon monounsaturated fatty acid
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Oleic Acid Simplified
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18-Carbon Fatty Acids
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A Mixed Triglyceride
This mixed triglyceride includes a saturated
fatty acid, a monounsaturated fatty acid, and a
polyunsaturated fatty acid. Sometimes the
chemical structure of a triglyceride is drawn
with the second fatty acid to the left of the
glycerol.
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Dietary Fats
At room temperature, unsaturated fats (such as
those found in oil) are usually liquid, whereas
saturated fats (such as those found in butter)
are solid.
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Comparison of Dietary Fats
Most fats are a mixture of saturated,
monounsaturated, and polyunsaturated fatty acids.
See p. 131 for information on omega-6 and omega-3
fatty acids the table on p. 132 lists good
sources of these omega fatty acids.
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Hydrogenation
Hydrogenation yields a product that is more
saturated, more spreadable, and more resistant to
oxidation.
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cis- and trans-Fatty Acids Compared
Manufacturers rarely use total hydrogenation
most often a fat is partially hydrogenated,
yielding a trans-monounsaturated fatty acid. This
example shows the cis configuration for oleic
acid and its corresponding trans configuration
(elaidic acid).
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Omega-3 and Omega-6 Fatty Acids
The omega number indicates the position of the
first double bond in a fatty acid, counting from
the methyl (CH3) end. Thus an omega-3 fatty
acids first double bond occurs three carbons
from the methyl end, and an omega-6 fatty acids
first double bond occurs six carbons from the
methyl end. The members of a given family may
have different lengths and different numbers of
double bonds, but the first double bond occurs at
the same point in all of them.
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A Lecithin
This is one of the lecithins. Other lecithins
have different fatty acids at the upper two
positions. Notice that a molecule of lecithin is
similar to a triglyceride but contains only two
fatty acids. The third position is occupied by a
phosphate group and a molecule of choline. Thus
an omega-3 fatty acids first double bond occurs
three carbons from the methyl end, and an omega-6
fatty acids first double bond occurs six carbons
from the methyl end. The members of a given
family may have different lengths and different
numbers of double bonds, but the first double
bond occurs at the same point in all of them.
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The Lipid Family
Triglycerides (fats and oils), phospholipids
(such as lecithin), and sterols (such as
cholesterol).
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Cholesterol
The fat-soluble vitamin D is synthesized from
cholesterol notice the many similarities. The
only difference, in fact, is vitamin Ds open
ring, which accounts for its vitamin activity.
Notice, too, how different cholesterol is from
the triglycerides and phospholipids.
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Cholesterol in Selected Foods
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Emulsification of Fat by Bile
Detergents are emulsifiers and work the same way,
which is why they are effective in removing
grease spots from clothes. Molecule by molecule,
the grease is dissolved out of the spot and
suspended in the water, where it can be rinsed
away.
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Digestion (Hydrolysis) of a Triglyceride
(a) The triglyceride and two molecules of water
are split, and the pieces combine to give two
fatty acids and a monoglyceride. (b) These
products may pass into the intestinal cells, but
sometimes the monoglyceride is split with another
molecule of water to give a third fatty acid and
glycerol. Fatty acids, monoglycerides, and
glycerol are absorbed into intestinal cells.
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Enterohepatic Circulation
The recycling of cholesterol and bile through the
intestine and liver is known as the enterohepatic
circulation of bile. Fatty acids,
monoglycerides, and glycerol are absorbed into
intestinal cells.
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Sizes and Compositions of the Lipoproteins 1 of 3
This solar system of lipoproteins shows their
relative sizes. Notice how large the fat-filled
chylomicron is compared with the others and how
the others get progressively smaller as their
proportion of fat declines and protein increases.
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Sizes and Compositions of the Lipoproteins 2 of 3
A typical lipoprotein contains an interior of
triglycerides and cholesterol surrounded by
phospholipids. The phospholipids' fatty acid
"tails" point toward the interior, where the
lipids are. Proteins near the outer ends of the
phospholipids cover the structure. This
arrangement of hydrophobic molecules on the
inside and hydrophilic molecules on the outside
allows lipids to travel through the watery fluids
of the blood.
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Sizes and Compositions of the Lipoproteins 3 of 3
Chylomicrons contain so little protein and so
much triglyceride that they are the lowest in
density. Very-low-density lipoproteins (VLDL) are
half triglycerides, accounting for their low
density. Low-density lipoproteins (LDL) are
half cholesterol, accounting for their
implication in heart disease. High-density
lipoproteins (HDL) are half protein, accounting
for their high density.
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An Adipose Cell
An adipose, or fat, cell seems to expand almost
indefinitely. The more fat it stores, the larger
it grows.
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Low-Fat Food for Good Health
Enjoy low-fat foods for good heart health.
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Cutting Fat Cuts kCalories 1 of 6
Pork chop with a half-inch of fat (275 kcal and
19 g fat).
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Cutting Fat Cuts kCalories 2 of 6
Pork chop with fat trimmed off (165 kcal and 8 g
fat).
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Cutting Fat Cuts kCalories 3 of 6
Potato with 1 tbs butter and 1 tbs sour cream
(350 kcal and 14 g fat).
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Cutting Fat Cuts kCalories 4 of 6
Plain potato (220 kcal and lt1 g fat).
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Cutting Fat Cuts kCalories 5 of 6
Whole milk, 1 c (150 kcal and 8 g fat).
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Cutting Fat Cuts kCalories 6 of 6
Nonfat milk, 1 c (90 kcal and 1 g fat).
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Eating Well-Balanced Meals
Even well-balanced, healthy meals provide some
fat. In this chicken stir-fry, only two teaspoons
of oil were used in preparation, but 30 percent
of the kcalories come from fat. The chicken and
sesame seeds also contribute some fat.
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Lower Fat Intake by Food Group
Salad dressing can add more than 20 grams of fat
to an otherwise low-fat salad.
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Estimating Fat
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