Page 241 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

In direct calorimetry (! B), the amount of heat pro-
Energy Metabolism and Calorimetry duced is measured directly. The test subject, usually
The chemical energy of foodstuffs is first con- an experimental animal, is placed in a small chamber
verted into energy-rich substances such as immersed in a known volume of ice. The amount of
creatine phosphate and adenosine tri- heat produced is equivalent to the amount of heat
absorbed by the surrounding water or ice. This is re-
phosphate (ATP). Energy, work and amount of spectively calculated as the rise in water temperature
heat are expressed in joules (J) or calories (cal) or the amount of ice that melts.
(! p. 374). The energy produced by hydrolysis
of ATP (! p. 41) is then used to fuel muscle ac- In indirect calorimetry, the amount of heat
tivity, to synthesize many substances, and to produced is determined indirectly by measur-
.
create concentration gradients (e.g., Na or ing the amount of O 2 consumed (V O 2 ;
+
! p. 120). This method is used in humans. To
Ca 2+ gradients; ! p. 26ff.). During all these determine the total metabolic rate (or TEE;
Nutrition and Digestion (ATP etc.) and heat. When a foodstuff is of a foodstuff oxidized in the subject’s metabo-
.
energy conversion processes, part of the
! p. 226) from V O 2 , the caloric equivalent (CE)
energy is always converted to heat (! p. 38ff.).
In oxidative (aerobic) metabolism (! p.
lism during the measurement must be known.
39 C), carbohydrates and fat combine with O 2
The CE is calculated from the PFV and the
to yield CO 2, water, high-energy compounds
amount of O 2 needed to oxidize the food. The
completely oxidized, its biologically useable
PFV of glucose is 15.7 kJ/g and 6 mol of O 2
(6 ! 22.4 L) are required to oxidize 1 mol
physical caloric value (CV).
180 g of glucose therefore generates 2827 kJ of
10 energy content is therefore equivalent to its (= 180 g) of glucose (! C). The oxidation of
The bomb calorimeter (! A), a device consisting of
heat and consumes 134.4 L of O 2 resulting in a
an insulated combustion chamber in a tank of water, CE of 21 kJ/L. This value represents the CE for
is used to measure the CV of foodstuffs. A known glucose under standard conditions (O"C; ! C).
quantity of a foodstuff is placed in the combustion The mean CE of the basic nutrients at 37"C is
chamber of the device and incinerated in pure O 2.
The surrounding water heats up as it absorbs the 18.8 kJ/L O 2 (carbohydrates), 17.6 kJ/L O 2 (fats)
heat of combustion. The degree of warming is equal and 16.8 kJ/L O 2 (proteins).
to the caloric value of the foodstuff. The oxidized nutrients must be known in
order to calculate the metabolic rate from the
Fats and carbohydrates are completely oxi- CE. The respiratory quotient (RQ) is a rough
.
dized to CO 2 and H 2O in the body. Thus, their measure of the nutrients oxidized. RQ = V CO 2 /
.
physiological fuel value (PFV) is identical to V O 2 (! p. 120). For pure carbohydrates oxi-
their CV. The mean PFV is 38.9 kJ/g (= 9.3 kcal/ dized, RQ = 1.0. This can be illustrated for glu-
g) for fats and 17.2 kJ/g (= 4.1 kcal/g) for cose as follows:
digestible carbohydrates (! p. 227 A). In con- 6 CO 2 + 6 H 2O [10.1]
C 6H 12O 6 + 6 O 2
trast, proteins are not completely broken down The oxidation of the fat tripalmitin yields:
to CO 2 and water in the human body but yield
2 C 51H 98O 6 + 145 O 2 102 CO 2 + 98 H 2O
urea, which provides additional energy when [10.2]
it is oxidized in the bomb calorimeter. The CV The RQ of tripalmitin is therefore 102/145 = 0.7.
of proteins (ca. 23 kJ/g) is therefore greater Since the protein fraction of the diet stays rela-
than their PFV, which is a mean of about tively constant, each RQ between 1 and 0.7 can
17.2 kJ/g or 4.1 kcal/g (! p. 227 A). be assigned a CE (! D). Using the known CE,
.
At rest, most of the energy supplied by the the TEE can be calculated as CE · V O 2 .
diet is converted to heat, since hardly any ex-
ternal mechanical work is being performed. Food increases the TEE (diet–induced thermogene-
The heat produced is equivalent to the internal sis, DIT) because energy must be consumed to ab-
energy turnover (e.g., the work performed by sorb and store the nutrients. The DIT of protein is
the heart and respiratory muscles or expended higher than that of other substances, e.g., glucose.
for active transport or synthesis of sub-
stances).
228
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