584
Chapter 16
During metabolism, about 60 percent of the energy
released from organic molecules appears immediately as heat,
and the rest is used for work. The energy used for work must
ﬁ rst be incorporated into molecules of ATP. The subsequent
breakdown of ATP serves as the immediate energy source for
the work. The body is incapable of converting heat to work,
but the heat released in its chemical reactions helps to main-
tain body temperature.
Biological work can be divided into two general catego-
ries: (1)
external work
—the movement of external objects by
contracting skeletal muscles; and (2)
internal work
—all other
forms of work, including skeletal muscle activity not used in
moving external objects. As just stated, much of the energy lib-
erated from nutrient catabolism appears immediately as heat.
What may not be obvious is that internal work, too, is ulti-
mately transformed to heat except during periods of growth.
For example, internal work is performed during cardiac con-
traction, but this energy appears ultimately as heat generated
by the friction of blood ﬂ ow through the blood vessels.
Thus, the total energy liberated when cells catabolize
organic nutrients may be transformed into body heat, can be
used to do external work, or can be stored in the body in the
form of organic molecules. The
total energy expenditure
of
the body is therefore given by the equation
Total energy expenditure
=
Internal heat produced
+
External work performed
+
Energy stored
Metabolic Rate
The basic metric unit of energy is the joule. When quantifying
the energy of metabolism, however, another unit is used, called
the
calorie
(equal to 4.184 joules). One calorie is the amount
of heat required to raise the temperature of one gram of water
from 14.5°C to 15.5°C. Because the amount of energy stored
in food is quite high relative to a calorie, a more convenient
expression of energy in this context is the
kilocalorie (kcal),
which is equal to 1000 calories. (In the ﬁ
eld of nutrition, the
terms “Calorie” with a capital C and “kilocalorie” are syn-
onyms; they are 1000 “calories,” with a small c.) Total energy
expenditure per unit time is called the
metabolic rate.
Because many factors cause the metabolic rate to vary
(
Table 16–5
), the most common method for evaluating it
speciﬁ
es certain standardized conditions and measures what
is known as the
basal metabolic rate (BMR).
In the basal
condition, the subject is at mental and physical rest in a room
at a comfortable temperature and has not eaten for at least 12
hours (i.e., he or she is in a postabsorptive state). These condi-
tions are arbitrarily designated “basal,” even though the meta-
bolic rate during sleep may be lower than the BMR. The BMR
is often called the “metabolic cost of living,” and most of the
energy involved is expended by the heart, muscle, liver, kid-
neys, and brain. For the following discussion, the term
BMR
can be applied to metabolic rate only when the speciﬁ ed con-
ditions are met. Thus, a person who has recently eaten or is
exercising has a metabolic rate, but not a basal metabolic rate.
The next sections describe several of the important determi-
nants of BMR and metabolic rate.
Thyroid Hormones
The thyroid hormones are the single most important determi-
nant of BMR regardless of size, age, or gender. TH increases
the oxygen consumption and heat production of most body
tissues, a notable exception being the brain. This ability to
increase BMR is termed a
calorigenic effect.
Long-term excessive TH, as in people with hyperthy-
roidism, induces a host of effects secondary to the calorigenic
effect. For example, the increased metabolic demands mark-
Table 16–5
Some Factors Affecting the Metabolic Rate
Sleep (
during sleep)
Age (
with
age)
Gender (women less than men at any given size)
Fasting (BMR decreases, which conserves energy stores)
Height, weight, and body surface area
Growth
Pregnancy, menstruation, lactation
Infection or other disease
Body temperature
Recent ingestion of food
Muscular activity
Emotional stress
Environmental temperature
Circulating levels of various hormones, especially
epinephrine, thyroid hormone, and leptin
(The presence of, or an increase in any of these factors causes an increase in
metabolic rate.)