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chapter 5
Thermodynamics, Chemical Kinetics, and Energy Metabolism
Since the energy yield of 1 g of fat is 9 kcal, 256 g of fat
yields 256 x 9, or 2304 kcal. The energy yield per liter
of oxygen consumed equals (256 x 9)/513.7 = 4.5 kcal
(18.8 kJ).
In the body, the energy derived from food is released
as body heat and also used in the synthesis of ATP.
The energy captured in ATP is then transformed into
other forms, i.e., chemical (synthesis of new compounds),
mechanical (muscle contraction), electrical (nerve activ-
ity), electrochemical (various ion pumps), thermal (main-
tenance of body temperature), and informational (base
sequences in nucleic acids, amino acids in proteins). In
general, the energy of food provides for the specific dy-
namic action of food, the maintenance of the body’s basal
metabolism, and the energy expenditure associated with
various types of activity.
The term
specific dynamic action
(SDA) describes
the thermogenic effect of food, which consists of the rise in
the metabolic rate during the assimilation of food above the
metabolic rate during fasting. The mechanism of the ther-
mogenic effect is not understood, but it may be due to
the work performed in the assimilation of food and in its
preparation for energy storage or energy yield. These pro-
cesses require energy, which is derived from the body’s
energy stores. The values for SDA vary with the type of
food substance ingested. The SDA value for protein is
about 30% of its energy value because protein must un-
dergo many changes before the carbon skeletons of its
constituent amino acids can be used appropriately. SDA
values for carbohydrates and lipids are, respectively, 5%
and 4% of their energy value. The energy utilized as a re-
sult of SDA is wasted and appears as heat. It is maximal
with protein intake. Thermodynamically, all energy de-
rived from the metabolism of substrates to CO
2
and H
2
O
must of course eventually appear as heat.
Basal metabolism
(determined by the
basal metabolic
rate
[BMR]) is an expression of the body’s vital energy
needs during
physical, emotional, and digestive rests.
It
represents the energy required for the maintenance of
body temperature, muscle tone, the circulation of blood,
the movement of respiration, and the glandular and cel-
lular activities of a person who is awake and not in-
volved in physical, digestive, or emotional activities. The
BMR is measured in an individual at rest after a 12-hour
fast and in comfortably warm, pleasant, quiet surround-
ings. Although the BMR could be obtained directly by
measurement of the amount of heat produced, an indirect
method that measures the volume of oxygen consumed
and carbon dioxide evolved per unit time is less cum-
bersome and provides acceptable values. Oxygen con-
sumption is measured in a respirometer. For every liter
of oxygen consumed, the energy equivalent is 4.83 kcal
(20.2 kJ). This value is based on the average value for the
oxidation of carbohydrates and lipids (see above). BMR
values are standardized and usually expressed as kilo-
calories per square meter of body surface per hour. The
metabolic rate that occurs during sleep is approximately
equal to the BMR. Less stringent conditions often used for
hospitalized patients provide the resting metabolic rate,
which is about 3% less than the BMR. The normal val-
ues for the resting rate of energy expenditure are shown
in Table 5-3. It is clear that the resting metabolic en-
ergy requirements are closely related to lean body mass
(as the body fat increases, the energy requirement de-
creases). Muscle, endocrine glands, and organs such as the
liver are metabolically more active (i.e., consume a larger
amount of oxygen per unit weight) than adipose tissue
and bone.
The BMR for women (who have a greater proportion
of fat per total body weight than men) is lower than that
for men. In both sexes, the development of muscle tissue
as a result of increased exercise increases the metabolic
rate and the basal metabolic needs. Body fat is rapidly de-
termined with calipers by skinfold measurement at several
different sites. In normal young men and women, the aver-
age body fat constitutes about
1 2
% and 26% of total body
weight, respectively; if these values are exceeded by more
than 20% and 30% of the average values respectively, they
indicate the presence of obesity. The BMR changes with
age. It is high at birth, increases up to 2 years of age, then
gradually decreases except for a rise at puberty. This gen-
eral decline in the BMR (approximately 2% per decade
after age
2 1
years) is proportional to the reduction in mus-
cle tissue (or lean body mass) and the accompanying in-
crease in body fat. BMR is also affected by menstruation,
pregnancy, and lactation.
The BMR is altered in a number of pathological states.
The BMR is increased in
hyperthyroidism,
fever (approx-
imately 12% elevation for each degree Celsius above nor-
mal body temperature),
Cushing’s syndrome,
tumors of the
adrenal gland, anemia, leukemia, polycythemia, cardiac
insufficiency, and injury. BMR is decreased in
hypothy-
roidism,
starvation, malnutrition, hypopituitarism, hypoa-
drenalism (e.g.,
Addison’s disease),
and anorexia nervosa.
To calculate energy requirements in an individual, it
is necessary to take into account BMR, physical activ-
ity (muscular work), age, sex, height, and weight (Ap-
pendix I). The energy requirements of muscular work can
be measured. Table 5-4 lists the energy expenditure for
various physical activities.
Maintenance of desirable body weight at any age de-
pends on the balance between energy intake and energy
output or requirement. During growth, pregnancy, or re-
covery from illness, energy demands are greater; hence,
