452 METABOLISM, NUTRITION, AND DIET 



The fluid should consist of about 2,500 to 2,800 grams, and might be 

 given as water, with or without tea, coffee, or cacao, which are chiefly 

 stimulants. 



The Energy Requirements of the Body. The food must not only 

 make up for the substances eliminated from the body but must also supply 

 the potential energy of heat and motion set free in the living body. The 

 amount of heat is measured in terms of calories, or more often in large 

 Calories. The work energy may be expressed in gram-centimeters, or in 

 kilogrammeters. Since one calorie of heat is the equivalent of 42,670 gram- 

 centimeters of work, the two units may be computed interchangeably. 



The source of the heat and work energy which is produced in the body is 

 from the metabolic changes of the tissues, the chief part of which is in the 

 nature of oxidation, since it may be supposed that the oxygen of the atmos- 

 phere taken into the system is ultimately combined with carbon and hydro- 

 gen. Any change, indeed, which occurs in the protoplasm of the tissues, 

 resulting in an exhibition of protoplasmic function, is attended by the evolu- 

 tion of heat and the formation of carbon dioxide and water. The more act- 

 ive the changes the greater is the amount of heat produced. In order that 

 the protoplasm may perform its functions, the waste of its own destructive 

 metabolism must be repaired by the due supply of food material to be built 

 up in some way into the protoplasmic molecule. In the tissues, as we have 

 several times remarked, two processes are continually going on: the building 

 up of the protoplasm from the food, anabolism, which is not accompanied 

 by the evolution of heat; and the oxidation of the protoplasmic materials, 

 katabolism, resulting in the production of energy, by which heat is set free. 

 Food is therefore necessary for the production of heat. It is not neccessary 

 to assume that the combustion processes, indeed, are as simple as the bare 

 statement of the fact might seem to indicate. But complicated as the vari- 

 ous stages may be, the ultimate result is as simple as in ordinary combustion 

 outside the body, and the products are the same. 



This view, that the maintenance of the temperature of the living body 

 depends on continual chemical change, chiefly by oxidation of combustible 

 materials in the tissues or by the tissues, has long been established. The 

 quantity of carbon and hydrogen supplied as food, which, in a given time, 

 unites in the body with oxygen, is sufficient to account for the amount of heat 

 generated in the animal within the same period, page 454; an amount capable 

 of maintaining the temperature of the body at from 36.8 to 38.7 C., not- 

 withstanding a large loss by radiation and evaporation. This estimation 

 depends upon the chemical axiom that when a body undergoes a chemical 

 change the amount of energy set free is the same, supposing the resulting 

 products are the same, whether the change takes place suddenly or gradually. 

 If a certain number of grams of different substances are introduced as food, 

 and if they undergo complete oxidation, the amount of kinetic energy, as 



