ENERGY 499 



TABLE 21-1. Approximate Heats of Combustion of Classes of Nutrients 



Heats of Combustion, kcal./g. 



Direct Animal Corrected for Bomb 



Nutrient Calorimetry Digestion Losses Calorimetry 



Carbohydrates 4 4.1 4.1 



Fats 9 9.5 9.5 



Proteins 4 4.4 5.6 



factor, taking 98, 95, and 92 per cent as the respective digestive effi- 

 ciencies of carbohydrates, fats, and proteins. These percentages cer- 

 tainly vary with conditions but are taken as common values for the 

 dietary materials that are available to the animal in question. Cellu- 

 lose, for example, is not considered in studies with man. 



When the corrected data for animals are compared with results 

 from the bomb, it is apparent that carbohydrates and fats are com- 

 pletely oxidized. Proteins, on the other hand, are not, as shown by 

 the larger value obtained in the bomb. The difference is attributed 

 to the necessity for excreting nitrogen in reduced forms. The rela- 

 tively high heats of combustion of fats are correlated with their low 

 level of oxidation. Fats are the lowest in oxygen of the common 

 dietary materials, hence are the least oxidized to start with and yield 

 the most energy on oxidation. This high energy content especially 

 suits fats for their role in energy storage. All the values tabulated are 

 averages since the individual compounds differ somewhat. For exam- 

 ple, 



C6H12O6 + 6O.2 ^6C02"+16H20 A// => 3.75 kcal./g. 



glucose 



(CeHioOe)^ + 6A'02 -^ 6ZCO2 + SZHjO ^H = -4.2 kcal./g. 



starch 



where X is the number of hexose units per starch molecule. The 

 difference in the AH values in these cases is attributed to the removal 

 of water when hexose units are joined to form starch. This process 

 requires energy which is partly recovered as heat during the burning 

 or metabolism of starch. Therefore, the heats of combustion reported 

 can be taken only as representative of the classes of materials. The 

 observed range seems to be within a few tenths kilocalorie of the 

 figiues selected. 



Because direct calorimetry with a large animal severely restricts the 

 animal and requires complex and expensive calorimeters, indirect 



