Fatty Acids and Glycerol
The end products of fat digestion that are not needed
immediately for energy production may be stored as
fat (triglycerides) in adipose tissue. Most adipose tis-
sue is found subcutaneously and is potential energy for
times when food intake decreases. Notice in Table
17–4 that insulin promotes fat synthesis and storage.
One theory of weight gain proposes that a diet high in
sugars and starches stimulates the secretion of so
much insulin that fat can only be stored, not taken out
of storage and used for energy.
Fatty acids and glycerol are also used for the syn-
thesis of phospholipids, which are essential compo-
nents of all cell membranes. Myelin, for example, is a
phospholipid of the membranes of Schwann cells,
which form the myelin sheath of peripheral neurons.
The liver can synthesize most of the fatty acids
needed by the body. Two exceptions are linoleic acid
and linolenic acid, which are essential fatty acidsand
must be obtained from the diet. Linoleic acid is part of
lecithin, which in turn is part of all cell membranes.
Vegetable oils are good sources of these essential fatty
acids.
When fatty acids are broken down in the process of
beta-oxidation, the resulting acetyl groups may also be
used for the synthesis of cholesterol, a steroid. This
takes place primarily in the liver, although all cells are
capable of synthesizing cholesterol for their cell mem-
branes. The liver uses cholesterol to synthesize bile
salts for the emulsification of fats in digestion. The
steroid hormonesare also synthesized from choles-
terol. Cortisol and aldosterone are produced by the
adrenal cortex, estrogen and progesterone by the
ovaries, and testosterone by the testes.
VITAMINS AND MINERALS
Vitaminsare organic molecules needed in very small
amounts for normal body functioning. Some vitamins
are coenzymes; that is, they are necessary for the
functioning of certain enzymes. Others are antioxi-
dant vitamins, including vitamins C, E, and beta-
carotene (a precursor for vitamin A). Antioxidants
prevent damage from free radicals, which are mole-
cules that contain an unpaired electron and are highly
reactive. The reactions of free radicals can damage
DNA, cell membranes, and the cell organelles. Free
radicals are formed during some normal body reac-
tions, but smoking and exposure to pollution will
increase their formation. Antioxidant vitamins com-
bine with free radicals before they can react with cel-
lular components. Plant foods are good sources of
these vitamins. Table 17–5 summarizes some impor-
tant metabolic and nutritional aspects of the vitamins
we need.
Deficiencies of vitamins often result in disease:
vitamin C deficiency and scurvy, for example (see Box
4–2). Other deficiency diseases that have been known
for decades include pellagra (lack of niacin), beri-beri
(riboflavin), pernicious anemia (B 12 ), and rickets (D).
More recently the importance of folic acid (folacin)
for the development of the fetal central nervous sys-
tem has been recognized. Adequate folic acid during
pregnancy can significantly decrease the chance of
spina bifida (open spinal column) and anencephaly
(absence of the cerebrum, always fatal) in a fetus. All
women should be aware of the need for extra (400
micrograms) folic acid during pregnancy.
Mineralsare simple inorganic chemicals and have
a variety of functions, many of which you are already
familiar with. Table 17–6 lists some important aspects
of minerals. We will return to the minerals as part of
our study of fluid–electrolyte balance in Chapter 19.METABOLIC RATE
Although the term metabolismis used to describe all
of the chemical reactions that take place within the
body, metabolic rate is usually expressed as an
amount of heat production. This is because many
body processes that utilize ATP also produce heat.
These processes include the contraction of skeletal
muscle, the pumping of the heart, and the normal
breakdown of cellular components. Therefore, it is
possible to quantify heat production as a measure of
metabolic activity.
As mentioned previously, the energy available from
food is measured in kilocalories (kcal). Kilocalories are
also the units used to measure the energy expended by
the body. During sleep, for example, energy expended
by a 150-pound person is about 60 to 70 kcal per hour.
Getting up and preparing breakfast increases energy
expenditure to 80 to 90 kcal per hour. For mothers
with several small children, this value may be signifi-
cantly higher. Clearly, greater activity results in
greater energy expenditure.
The energy required for merely living (lying qui-
etly in bed) is the basal metabolic rate(BMR). See
Box 17–4: Metabolic Rate for a formula to estimate408 Body Temperature and Metabolism