CHAPTER 27Digestion, Absorption, & Nutritional Principles 461produced by their bodies is measured by the change in tem-
perature of the water in the walls of the calorimeter.
The caloric values of the common foodstuffs, as measured
in a bomb calorimeter, are found to be 4.1 kcal/g of carbohy-
drate, 9.3 kcal/g of fat, and 5.3 kcal/g of protein. In the body,
similar values are obtained for carbohydrate and fat, but the
oxidation of protein is incomplete, the end products of pro-
tein catabolism being urea and related nitrogenous com-
pounds in addition to CO 2 and H 2 O (see below). Therefore,
the caloric value of protein in the body is only 4.1 kcal/g.
INDIRECT CALORIMETRY
Energy production can also be calculated by measuring the
products of the energy-producing biologic oxidations; that is,
CO 2 , H 2 O, and the end products of protein catabolism pro-
duced, but this is difficult. However, O 2 is not stored, and ex-
cept when an O 2 debt is being incurred, the amount of O 2
consumption per unit of time is proportionate to the energy
liberated by metabolism. Consequently, measurement of O 2
consumption (indirect calorimetry) is used to determine the
metabolic rate.
RESPIRATORY QUOTIENT (RQ)
The respiratory quotient (RQ) is the ratio in the steady state of
the volume of CO 2 produced to the volume of O 2 consumed per
unit of time. It should be distinguished from the respiratory ex-
change ratio (R), which is the ratio of CO 2 to O 2 at any given
time whether or not equilibrium has been reached. R is affected
by factors other than metabolism. RQ and R can be calculated
for reactions outside the body, for individual organs and tissues,
and for the whole body. The RQ of carbohydrate is 1.00, and
that of fat is about 0.70. This is because H and O are present in
carbohydrate in the same proportions as in water, whereas in
the various fats, extra O 2 is necessary for the formation of H 2 O.
Carbohydrate:
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
(glucose)
RQ = 6/6 = 1.00
Fat:
2C 51 H 98 O 6 + 145O 2 → 102CO 2 + 98H 2 O
(tripalmitin)
RQ = 102/145 = 0.703Determining the RQ of protein in the body is a complex
process, but an average value of 0.82 has been calculated. The
approximate amounts of carbohydrate, protein, and fat being
oxidized in the body at any given time can be calculated from
the RQ and the urinary nitrogen excretion. RQ and R for the
whole body differ in various conditions. For example, during
hyperventilation, R rises because CO 2 is being blown off. Dur-
ing strenuous exercise, R may reach 2.00 because CO 2 is being
blown off and lactic acid from anaerobic glycolysis is being
converted to CO 2 (see below). After exercise, R may fall for a
while to 0.50 or less. In metabolic acidosis, R rises because res-
piratory compensation for the acidosis causes the amount of
CO 2 expired to rise (see Chapter 39). In severe acidosis, R may
be greater than 1.00. In metabolic alkalosis, R falls.
The O 2 consumption and CO 2 production of an organ can
be calculated at equilibrium by multiplying its blood flow per
unit of time by the arteriovenous differences for O 2 and CO 2
across the organ, and the RQ can then be calculated. Data on
the RQ of individual organs are of considerable interest in
drawing inferences about the metabolic processes occurring
in them. For example, the RQ of the brain is regularly 0.97–
0.99, indicating that its principal but not its only fuel is carbo-
hydrate. During secretion of gastric juice, the stomach has a
negative R because it takes up more CO 2 from the arterial
blood than it puts into the venous blood (see Chapter 26).MEASURING THE METABOLIC RATE
In determining the metabolic rate, O 2 consumption is usually
measured with some form of oxygen-filled spirometer and a
CO 2 -absorbing system. Such a device is illustrated in Figure
27–8. The spirometer bell is connected to a pen that writes on a
rotating drum as the bell moves up and down. The slope of a line
joining the ends of each of the spirometer excursions is propor-
tional to the O 2 consumption. The amount of O 2 (in milliliters)
consumed per unit of time is corrected to standard temperature
and pressure (see Chapter 35) and then converted to energy pro-
duction by multiplying by 4.82 kcal/L of O 2 consumed.FACTORS AFFECTING THE
METABOLIC RATEThe metabolic rate is affected by many factors (Table 27–2).
The most important is muscular exertion. O 2 consumption is
elevated not only during exertion but also for as long after-
ward as is necessary to repay the O 2 debt (see Chapter 5). Re-
cently ingested foods also increase the metabolic rate because
of their specific dynamic action (SDA). The SDA of a food is
the obligatory energy expenditure that occurs during its as-
similation into the body. It takes 30 kcal to assimilate the
amount of protein sufficient to raise the metabolic rate 100
kcal; 6 kcal to assimilate a similar amount of carbohydrate;
and 5 kcal to assimilate a similar amount of fat. The cause of
the SDA, which may last up to 6 h, is uncertain.
Another factor that stimulates metabolism is the environ-
mental temperature. The curve relating the metabolic rate to
the environmental temperature is U-shaped. When the envi-
ronmental temperature is lower than body temperature, heat-
producing mechanisms such as shivering are activated and the
metabolic rate rises. When the temperature is high enough to
raise the body temperature, metabolic processes generally
accelerate, and the metabolic rate rises about 14% for each
degree Celsius of elevation.