Nutrition and Metabolism
c. The liver and adipose tissue control triglyceride
d. Liver enzymes can alter the molecular structures of
e. Linoleic acid and linolenic acid are essential fatty
f. The liver regulates cholesterol level by
synthesizing or excreting it.
3. Lipid requirements
a. Humans survive with a wide range of lipid intakes.
b. The amounts and types of lipids needed for health
c. Fat intake must be suf±
cient to supply fat-soluble
PROTEINS (PAGE 704)
Proteins are organic compounds that serve as structural
materials, act as enzymes, and provide energy. Amino
acids are incorporated into various structural and
functional proteins, including enzymes. During starvation,
tissue proteins may be used as energy sources; thus, the
tissues waste away.
1. Protein sources
a. Proteins are mainly obtained from meats, dairy
products, cereals, and legumes.
b. During digestion, proteins are broken down into
c. The resulting amino acids can be used to form
new protein molecules such as enzymes, clotting
factors, keratin, elastin, collagen, actin, myosin,
hormones, and antibodies, or can be used as
d. Before amino acids can be used as energy sources,
they must be deaminated.
e. The deaminated portions of amino acids can be
broken down into carbon dioxide and water or
used to produce glucose or fat.
f. Eight amino acids are essential for adults, whereas
ten are essential for growing children.
g. All essential amino acids must be present at the
same time for growth and repair of tissues to take
h. Complete proteins contain adequate amounts of all
the essential amino acids needed to maintain the
tissues and promote growth.
i. Incomplete proteins lack adequate amounts of one
or more essential amino acids.
2. Nitrogen balance
a. In healthy adults, the gain of protein equals the
loss of protein, and a nitrogen balance exists.
b. A starving person has a negative nitrogen balance;
a growing child, a pregnant woman, or an athlete
in training usually has a positive nitrogen balance.
3. Protein requirements
a. Proteins and amino acids are needed to supply
essential amino acids and nitrogen for the
synthesis of nitrogen-containing molecules.
b. The consequences of protein de±
particularly severe among growing children.
INTRODUCTION (PAGE 699)
Nutrients include carbohydrates, lipids, proteins,
vitamins, and minerals. The ways nutrients are used to
support life processes constitute metabolism. Essential
nutrients are required for health, and body cells cannot
synthesize them. Hormones communicate from the
gastrointestinal tract to the hypothalamus to control
appetite, and monitor fat stores.
CARBOHYDRATES (PAGE 700)
Carbohydrates are organic compounds primarily used to
supply cellular energy.
1. Carbohydrate sources
a. Carbohydrates are ingested in a variety of forms.
b. Polysaccharides, disaccharides, and
monosaccharides are carbohydrates.
c. Cellulose is a polysaccharide that human enzymes
cannot digest, but it provides bulk that facilitates
movement of intestinal contents.
2. Carbohydrate use
a. Carbohydrates are absorbed as monosaccharides.
b. Enzymes in the liver catalyze reactions that
convert fructose and galactose into glucose.
c. Oxidation releases energy from glucose.
d. Excess glucose is stored as glycogen or combined
to produce fat.
3. Carbohydrate requirements
a. Most carbohydrates supply energy; some are used
to produce sugars.
b. Some cells require a continuous supply of glucose
c. If glucose is scarce, amino acids may react to
d. Humans survive with a wide range of carbohydrate
e. Poor nutritional status is usually related to low
intake of nutrients other than carbohydrates.
LIPIDS (PAGE 702)
Lipids are organic compounds that supply energy and
are used to build cell structures. They include fats,
phospholipids, and cholesterol.
1. Lipid sources
a. Triglycerides are obtained from foods of plant and
b. Cholesterol is mostly obtained in foods of animal
2. Lipid use
a. Before fats can be used as an energy source, they
must be broken down into glycerol and fatty acids.
b. Beta oxidation decomposes fatty acids.
(1) Beta oxidation activates fatty acids and breaks
them down into segments of two carbon atoms
(2) Fatty acid segments are converted into acetyl
coenzyme A, which can then be oxidized in
the citric acid cycle.