Page 59 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 59

Box 5-4. Calorimetry.
VetBooks.ir Calorimetry is the measurement of heat. Calorimetry has been of oxygen is exactly known if only glucose or a single fat is oxi-

used to understand how the body metabolizes food energy for hun-
of oxygen are used to estimate HP from oxygen consumption.
dreds of years.The discovery by Lavoiser and LaPlace in 1783 that dized, as well as for mixtures of the two.These thermal equivalents
heat produced by animals was related to oxygen consumption and Protein is incompletely oxidized because the body cannot use
carbon dioxide formation and was analogous to burning of a can- nitrogen. Animals typically do not obtain energy exclusively from
dle was revolutionary and signaled the beginning of the study of carbohydrate, fat or protein; rather, they oxidize mixtures. Because
energy use by calorimetry. Because animals do not store heat, the the ratio of the volume of carbon dioxide produced for each volume
quantity of heat lost from the animal is equal to the quantity pro- of oxygen used is different for carbohydrate, fat and protein, this
duced. Calorimetry allows measurement of the heat lost (heat pro- ratio, known as the respiratory quotient (RQ) can be used to deter-
duction, [HP]). mine the proportions of each nutrient oxidized. The RQ is 1
HP is one of the terms in the energy balance equation: HP = ME (6CO /6O ) for carbohydrate, 0.7 (51CO /72.5O ) for fat, and
2
2
2
2
– RE. Because HP has been equated with energy expenditure and 0.8 for protein. Some food energy is metabolized to hydrogen and
energy requirements, the measurement of heat really means methane by gut microflora.
measurement of energy requirements. HP can be measured The apparatus used to measure the respiratory exchange is
directly (direct calorimetry) or estimated from respiratory exchange called a respiratory chamber. As with direct calorimetry, there are
(indirect or respiratory calorimetry). several different methods for constructing chambers and measur-
ing gas flow (oxygen and carbon dioxide) into and out of the cham-
DIRECT CALORIMETRY ber. Indirect calorimetry chambers typically are less complex and
less costly to construct and maintain compared with direct
With direct calorimetry, an animal is placed in an airtight, insulat- calorimetry chambers. Energy expenditure calculated from indirect
ed chamber. The heat lost from the body includes that lost by calorimetry measurements can be just as reliable and accurate as
radiation, conduction and convection and by evaporation of water direct measures.
from skin and respiratory surfaces (e.g., lungs). The heat pro- Oxygen and carbon dioxide exchange can be measured with a
duced by the animal is measured as the difference in tempera- simple hood, canopy or expiratory collection device instead of a
ture between inside and outside of the chamber over time.
chamber. These systems are portable and are easier to use in clin-
There are several different designs for chambers and ways to ical situations and in animals and people as they perform their
measure heat that give rise to various methods of determining HP daily activities. These more portable systems may not be as accu-
directly. Most direct calorimetry systems are relatively expensive to rate as the chamber systems, but are less costly and highly flexi-
construct and operate, somewhat complex to operate, require con- ble. Energy expenditure can be measured in very sick patients
finement of the subjects, but are very accurate and reliable. using a hood system when other methods are unsuitable.
Direct calorimetry is suited to energy expenditure measure- Knowledge of energy use is important to make accurate esti-
ments for research purposes and, in the clinical setting, for well mates of energy requirements to optimize the health of animals.
patients and moderately sick patients. Direct calorimetry with an Energy requirements vary with nutritional, genetic and environ-
enclosed chamber is not feasible for very sick patients that are mental influences; interactions among the factors are complex.
attached to ventilators or those requiring constant supervision and Therefore, it is easiest to isolate and measure the specific factors
intervention. that alter energy expenditure (e.g., resting energy expenditure,
thermic effect of food, breed, age, gender, energy expenditure due
INDIRECT CALORIMETRY to growth, pregnancy, lactation and work) and then develop predic-
tion equations for total energy requirements of the animal, taking
Indirect calorimetry involves calculation of HP by measuring respi- into account all the relevant factors. Although both methods of
ratory exchange of oxygen and carbon dioxide. Food carbohydrates calorimetry, direct and indirect, have technical challenges, each
and fats are oxidized by the body to yield heat, water and carbon technique is useful in research and clinical practice. Calorimetry is
dioxide as shown in these example equations.
important to build an understanding of factors that influence ener-
Glucose (C H O ) + 6O → 6CO + 6H O + heat (2.82MJ) gy requirements.
2
2
2
6 12 6
Triglyceride tripalmitin C H O (C H ) + 74O →
3 5 3 16 31 3
2
51CO + 49H O + heat (32.02MJ) The Bibliography for Box 5-4 can be found can be found at
2
2
www.markmorris.org.
The amount of heat generated from the consumption of one liter
tion. BER is determined by measuring the energy expenditure should be standardized between animals and experiments.
under the stated conditions. Thus, the terms BER and BEE RER also differs from BER because it includes energy expend-
are synonymous. ed for recovery from physical activity. Depending on the level
RER represents the energy requirement for a normal animal of activity and time between cessation of activity and the ener-
at rest under thermoneutral but not fasted conditions (Blaxter, gy expenditure determination, RER may range from almost the
1989).The amount of time between a meal and when measure- same value as BER to as much as 25% higher (Kleiber, 1961).
ments are made can affect the estimate of RER; therefore, they Therefore, the differences between BER and RER include