n8 



TEXT-BOOK OF PHYSIOLOGY. 



perhaps the intestinal glands as well. This necessitates on the part of the 

 kidneys, the chief eliminating organs, the expenditure of a certain amount 

 of energy. The wear and tear of these organs will be proportional to the 

 amount of urea and other materials which they are called upon to excrete 

 and if the kidneys fail to excrete them, they become deposited in the tissues 

 and give rise to certain nutritional disorders. Any unnecessary consump- 

 tion of proteins should therefore be avoided. 



It must be remembered, however, as protein yields energy when meta- 

 bolized, that the heat value of the excluded protein must be balanced by an 

 increase in the amount of either starch or fat or both, an increase that will 

 yield on oxidation an equivalent amount of heat. 



In arranging tables showing the relation between the income and the 

 outcome it is generally customary to state merely the amounts by weight 

 of the nitrogen and carbon each contains. This method furnishes ac- 

 curate information regarding the metabolism of the body, for the reason 

 that the nitrogen represents the protein, and the carbon, with the exception 

 of that contained in the protein, the fat and carbohydrates which have under- 

 gone disintegration or metabolism. 



The following balance table, as given by Ranke, shows the relation 

 of the nitrogen to the carbon in the average mixed diet and in the excretions 

 of a man weighing 70 kilograms, in a condition of nutritive equilibrium : 



From the above it will be observed that the daily discharge for each 

 kilogram of body-weight is 0.22 gram of nitrogen and 3.21 grams of carbon; 

 the relation of the two being =1.46. On a diet in which there is an 

 excess of either protein or carbohydrates this ratio necessarily changes. 



CLASSIFICATION OF FOOD PRINCIPLES. 



Though the food principles are grouped as proteins, fats, carbohydrates, 

 etc., the members of each group differ somewhat in chemic composition, 

 digestibility, and nutritive value. These groups are as follows: 



