Chap. XVI] METABOLISM 311 



Classification of proteins. — Proteins vary in their constituents and 

 vary in their nutritive value. Because of this they are classed as 

 adequate and inadequate proteins. Adequate proteins contain all 

 the constituents for the maintenance and growth of the body. I?i- 

 adequafe proteins furnish material for energy needs, but not for 

 the repair of tissue waste. Gelatine is an example of an inade- 

 quate protein. It is easily digested and absorbed, undergoes 

 oxidation, which results in the liberation of energy and the produc- 

 tion of urea, carbon dioxide, and water, but it does not supply the 

 material needed for the repair of tissue waste. 



Function of proteins. — The main function of proteins is to build 

 up tissue, and they are the one class of foods capable of doing this. 

 In addition they serve the same purpose as carbohydrates and fats 

 and may even be converted into adipose tissue. 



Nitrogen equilibrium, — The protein molecules are charac- 

 terized by containing nitrogen (some say as much as 16 per cent). 

 After the metabolism of protein, nitrogen is eliminated chiefly in 

 the urine, and to a limited extent in the feces and sweat. The 

 body is said to be in nitrogen equilibrium when the amount of 

 protein nitrogen taken into the body is equal to the amount elim- 

 inated in the excreta. If there is a plus balance in favor of the 

 food, it means that protein is being stored in the body, and 

 this is an ideal condition during the period of growth, or con- 

 valescence from wasting illness. If the balance is minus, the 

 body must be losing protein, but under normal conditions this 

 does not occur. 



Heat value of food. — - The supply of heat needed to main- 

 tain the body temperature comes from the processes of oxidation. 

 The heat produced is estimated in terms of calories ^ and is meas- 

 ured with the calorimeter. There are two kinds of calories, i.e. 

 small and large. A small calorie is the quantity of heat necessary 

 to raise one gram of water, one degree centigrade in temperature. 

 A large calorie is the quantity of heat necessary to raise one thou- 

 sand grams of water one degree centigrade. The large calorie is 

 the one referred to in physiology. It has been estimated that each 

 gram of fat yields about 9.3 calories, each gram of carbohydrate 

 yields about 4.1 calories, and each gram of protein yields about 4.1 

 calories, 



1 See page 12, 



