THE ABSORPTION AND USE OF FOODS 501 



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. Before proceeding with 

 a further discussion of the energy relationships of the Body it will 

 be well to consider the tissue-maintenance requirement, which as 

 we have seen, is wholly a protein need. In order to analyze this 

 requirement intelligently we need to know, first of all, what use 

 the Body makes of protein, and second, how much is required. 

 We have already seen that protein can be oxidized in the 

 Body with the liberation of energy, and constitutes, therefore, a 

 good fuel. In this respect, however, it is in no degree superior 

 to the other nutrients, fats and carbohydrates. Our special in- 

 terest in it is for the function which it alone can exercise, that of 

 making good tissue wear and tear. 



Since protein is the only food stuff that contains nitrogen we 

 can tell how much of it is used up in the Body by measuring the 

 amount of nitrogen eliminated (p. 512). All except a very small 

 portion (roughly 2 per cent) is discharged in the urine. Chemical 

 tests of the urine will furnish us, then, with the data we seek. 

 Evidently if a man abstains wholly from food for a while all the 

 nitrogen in his urine must come from tissue break-down. We have 

 a means, thus, of finding out how rapidly this break-down occurs. 

 A moment's thought will show us, however, that the tissue break- 

 down in complete starvation is not necessarily the same as in 

 ordinary life. When no food is eaten the energy requirements of 

 the Body must be met at the expense of its own tissues; particularly 

 in prolonged starvation, when the stored fuels, fat and glycogen 

 have been used up; so that in addition to the usual loss of sub- 

 stance by wear and tear there is a further consumption of ma- 

 terial as fuel. To get at the amount of tissue break-down under 

 ordinary conditions by this method the starvation must not be 

 complete. The subject must be given abundant supplies of fat, 

 carbohydrates, and essential accessories, but no proteins. When 

 this is done the nitrogen eliminated from the Body can be as- 

 sumed to represent the normal tissue break-down. Experiments 

 conducted along this line have shown that in an adult man of 

 ordinary size (70 kilos, 165 Ibs.) the protein lost from the Body 

 daily by tissue wear and tear amounts to about 20-25 grams 



