336 



LECTURE XV. 



As the values show, it is evident that the law of the conservation of 

 energy holds with surprising exactness for the whole animal organism. It 

 was found possible to obtain such a close agreement between the sum of 

 the amounts of energy introduced into the body and that produced by 

 combustion within the organism, only by extending the experiment 

 through quite a number of days. 



This brings us to the important question as to whether the chemical 

 energies introduced into the body by the individual nutrients are all 

 equivalent, or, in other words, whether it is immaterial in what form the 

 animal organism receives its chemical energy. After considerable investi- 

 gation Max Rubner l was able to show that the different organic nutrients 

 could replace one another in amounts corresponding approximately to 

 their relative heat values. This principle comprises the Law of Isody- 

 namics. According to this law we can represent each substance used as 

 food in a common unit and give it a calorific value. Thus, for example, 

 100 grams of fat are isodynamic with the following weights of 



Strictly speaking, the Law of Isodynamics only applies to fats and 

 carbohydrates. It fails with the albumins. These, to a certain extent, 

 are absolutely necessary for the animal organism. It is indeed possible 

 to keep a dog alive for a long time on albumin alone; i.e., the albumin 

 itself may be looked upon as isodynamic with the fats and carbohydrates. 

 It is, however, as we shall see later on, impossible to nourish a dog on 

 fats and carbohydrates exclusively, even when more than sufficient calo- 

 rific units are provided. Starvation metabolism begins in the absence 

 of albuminous material; i.e., the animal draws on its own body albumin, 

 for which it has no substitute. 



Studies on metabolism have shown how much nourishment is required 

 for the maintenance of a definite organism, and how to express this require- 

 ment in terms of calories. We shall consider these relations more in detail 

 later. Here we shall only state, that the exact formulation of the total 

 metabolism obtained by considering the foods solely as combustible 

 material, although of great importance for the entire conception of meta- 



1 Cf. Max Rubner: Die Gesetze des Energieverbrauches bei der Ernahrung. Franz 

 Deuticke, Leipsic and Vienna, 1902. 



