834 METABOLISM. 



that this, if it relates to the force and heat production in the animal body, 

 Is a proportion that corresponds with the figures of the heat value of the 

 same. This is apparent from the following table. In this is found the 

 weight of the various foods equal to 100 grams of fat, a part determined 

 from experiments on animals and a part calculated from figures of the 

 .heat values: 



100 grams fat are equal to or isodynamic with 



From Experiments From the Difference, 



on Animals. Heat Value. per cent. 



Syntonin 225 213 +5.6 



Muscle-flesh (dried) 243 235 +4.3 



Starch 232 229 +1.3 



Cane-sugar 234 235 -0 



Dextrose 256 255 -0 



From the given isodynamic value of the various foods it follows that 

 these substances replace one another in the body almost in exact ratio to 

 the energy contained in them. Thus in round numbers 227 grams of 

 protein and carbohydrate are equal to or isodynamic \vith 100 grams of 

 fat in regard to source of energy, because each yields 930 calories on com- 

 bustion in the body. 



By means of recent very important calorimetric investigations, RCJBNER 1 

 has shown that the heat produced in an animal in several series of experi- 

 ments extending over forty-five days corresponded to within 0.47 per cent 

 of the physiological heat of combustion calculated from the decomposed 

 body and foods. ATWATER and his collaborators 2 have made some 

 very thorough investigations on this subject on men. In their experiments 

 they made use of a large respiration calorimeter, which not only exactly 

 determined the excreta, but also made a calorimetric determination of 

 the heat given out by the person experimented upon, i.e., the work per- 

 formed. From the results of these experiments they found an almost 

 absolutely complete agreement between the calories found directly and 

 those calculated. 



This isodynamic law is of fundamental value in the study of metabo- 

 lism and nutrition. The quantity of energy in the transformed foods 

 or the constituents of the body may be used as a measure for the total 

 consumption of energy, and the knowledge of the quantity of energy 

 in the foods must also be the basis for the calculation of dietaries for 

 human beings under various conditions. 



The heat value of a foodstuff can be directly determined in a calorim- 

 eter, but may also be calculated from its composition. If one subtracts 

 from the gross heat value of the food obtained in one way or another, 



1 Zeitschr. f . Biologic, 30. 



2 Bull, of Dept. of Agnc., Washington, 44, 63, 69, and 109 and Ergebnisse des 

 Physiologic, 3. 



