498 ANIMAL BIOCHEMISTRY 



nutrition and most popular literature refer to the "large" calorie, writ- 

 ten as Calorie or Cal., which is the kilocalorie. 



Animal Calorimetry 



Use of the bomb calorimeter provides data on the complete combus- 

 tion of foodstuffs. However, animals may be unable to oxidize the 

 material so completely. As a matter of fact in Chapter 20 we have 

 seen that animals excrete their waste nitrogen in reduced forms like 

 ammonia, urea, luic acid, allantoin, amino acids, and peptides. There- 

 fore, calorimetric methods using animals have been developed to 

 ascertain the heat actually available to the animal. 



Direct calorimetry with animals is similar in principle to work with 

 the bomb calorimeter but with refinements designed to provide an 

 environment that ensures a normal metabolism. For this purpose the 

 temperature must be controlled to a suitable range for the species con- 

 cerned and air must be provided. Then the animal burns the food 

 metabolically in contrast to the purely chemical processes in the bomb 

 calorimeter. 



Since the animal generates heat, the insulated chamber would be- 

 come abnormally hot without provision for cooling. The best ar- 

 rangement uses a system of water pipes to keep the compartment at 

 constant temperature. Measurements of the inflowing and outflowing 

 water temperatures and of the rate of flow allow calculation of the 

 heat loss by the animal by radiation and conduction. The other 

 major loss of heat is in the evaporation of water in the expired air or 

 as perspiration. This factor is determined by trapping and weighing 

 the water vapor in the air pumped out of the calorimeter and calculat- 

 ing the heat needed to evaporate the observed quantity of water. 

 About 0.58 kcal. is necessary per g. of water at 25°C. with the value 

 changing somewhat with temperature. Heat lost by evaporation is 

 about 20 to 25 per cent of the total for common mammals, so this 

 transfer mechanism is important. 



The amount of heat evolved by an animal depends upon the 

 species, size, individual, environmental temperature, and activity. 

 This heat must come from the chemical energy of the diet. Therefore, 

 any change in the output of heat is normally reflected in a change in 

 the diet, either in composition or quantity or both. The interrela- 

 tionships with heat production can all be studied with calorimeters. 

 Such work has shown that different classes of foods yield different 

 quantities of energy as shown in Table 21-1. 



Since digestion is never completely efficient, some of the potentially 

 available energy is lost. One column of the table is corrected for this 



