452 THE HUMAN BODY 



down it suffers, we ought to be able to estimate how much energy- 

 yielding food, and how much tissue-repair food is required daily; 

 assuming, of course, that we know the amount of energy yielded 

 by definite weights of food-stuffs. By the use of devices called 

 calorimeters the total energy liberation of the Body per day has 

 been determined under various conditions, and the energy content 

 of the various foods has also been found. It is usual to express 

 these energy values in terms of the heat energy they represent, 

 since oxidation processes lie at the basis of the Body's activities. 

 The unit of heat energy commonly used in physiology is the 

 Calory; the amount of heat required to raise 1,000 grams of water 

 through 1 centigrade. In terms of this unit the energy output 

 of man in 24 hours averages from about 2,400 Calories for men of 

 sedentary occupation to 5,000 Calories for those doing heavy 

 manual labor. The energy yield of the various foods is as follows : 



Carbohydrates 4 . 1 C. per gram. 



Proteins 4 . 1 C. " " 



Fats 9.3C. " " 



It is therefore a matter of simple calculation to determine how 

 much of any one food-stuff is needed to supply the required en- 

 ergy, or to arrange suitable mixtures of the three. By reference 

 to the table of food compositions (Chap. XXV), the amounts of 

 actual food materials needed can be found. 



The Protein Requirement of the Body. We have next to con- 

 sider the tissue-maintenance requirement, which as we have seen, 

 is wholly a protein need. We do not know just how much protein 

 is used every twenty-four hours for repair of the tissues. It is 

 thought, however, that it is not large, probably less than twenty 

 grams per day. The reason why the exact requirement cannot be 

 determined is that the Body seems to demand that some of its 

 fuel be furnished as protein, and unless enough protein is eaten 

 to supply this fuel demand in addition to the tissue-repair demand 

 the Body breaks down its own tissues until it gets enough protein 

 for the purpose. The evidence that there is this fuel demand 

 which can only be satisfied by protein will be presented in a future 

 paragraph (p. 458). The reason for its existence is quite inex- 

 plicable, since it seems almost certain that the protein substance 



