702
UNIT FIVE
Lipids provide a variety of physiological functions; how-
ever, fats mainly supply energy. Gram for gram, fats contain
more than twice as much chemical energy as carbohydrates
or proteins.
Before a triglyceride molecule can release energy, it must
undergo hydrolysis. This happens when digestion breaks
triglycerides down into fatty acids and glycerol. After being
absorbed, these products are transported in the lymph to
the blood, then on to tissues. As
f gure 18.4
shows, some
of the resulting fatty acid portions can then form molecules
of acetyl coenzyme A (acetyl CoA) in a series of reactions
called
beta oxidation,
which occurs in the mitochondria.
In the F rst phase of beta oxidation, fatty acids are acti-
vated. This change requires energy from ATP and a special
group of enzymes called thiokinases. Each of these enzymes
can act upon a fatty acid that has a particular carbon chain
length.
Once fatty acid molecules have been activated, other
enzymes called
fatty acid oxidases
in mitochondria break
them down. This phase of the reactions removes succes-
sive two-carbon segments of fatty acid chains. In the liver,
some of these segments react to produce acetyl coenzyme A
Carbohydrate Requirements
Carbohydrates provide the primary source of fuel for cellu-
lar processes, so the need for carbohydrates varies with indi-
vidual energy requirements. Physically active individuals
require more carbohydrates than those who are sedentary.
The minimal requirement for carbohydrates in the human
diet is unknown. It is estimated, however, that an intake of
at least 125 to 175 grams daily is necessary to spare protein
(to avoid protein breakdown) and to avoid metabolic disor-
ders resulting from excess fat use. An average diet includes
200 to 300 grams of carbohydrates daily.
PRACTICE
8
Why do daily requirements for carbohydrates vary from person to
person?
9
What is the daily minimum requirement for carbohydrates?
18.3
LIPIDS
Lipids
are organic compounds that include fats, oils, and fat-
like substances such as phospholipids and cholesterol (see
chapter 2, pp. 62–64). They supply energy for cellular pro-
cesses and help build structures, such as cell membranes.
The most common dietary lipids are the fats called
triglycer-
ides
(tri-glis
er-ı¯dz) (see F
g. 2.14).
Lipid Sources
Triglycerides are found in plant- and animal-based foods.
Saturated fats (which should comprise no more than 10%
of the diet) are mainly found in foods of animal origin, such
as meat, eggs, milk, and lard, as well as in palm and coco-
nut oil. Unsaturated fats are in seeds, nuts, and plant oils.
Monounsaturated fats, such as those in olive, peanut, and
canola oils, are the healthiest. Saturated fats in excess are a
risk factor for cardiovascular disease.
Cholesterol is abundant in liver and egg yolk and, to
a lesser extent, in whole milk, butter, cheese, and meats.
±oods of plant origin do not contain cholesterol. A label on a
plant-based food claiming that it is “cholesterol-free” states
the obvious.
Be wary of claims that a food product is “99% fat-free.” This usually
refers to percentage by weight—not calories, which is what counts.
A 99% fat-free creamy concoction may be largely air and water, and
therefore in that form, fat comprises very little of it. But when the air is
compressed and the water absorbed, as happens in the stomach, the
fat percentage may skyrocket.
Lipid Use
The lipids in foods are phospholipids, cholesterol, and, most
commonly, fats (triglycerides). A triglyceride consists of one
glycerol and three fatty acids.
Heat
CO
2
+
+
+H
2
O
Energy
Citric
acid
cycle
ATP
Fats from foods
Acetyl coenzyme A
Fatty acids
Ketone bodies
Beta oxidation
Glycerol
Glucose
Fat
Hydrolysis
FIGURE 18.4
The body digests fat from foods into glycerol and fatty
acids, which may enter catabolic pathways and provide energy.
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