ISODYNAMICS OF FOOD-STUFFS. 891 



posed body and foods. ATWATER and his collaborators 1 have made some 

 very thorough investigations on this subject on men. In their experi- 

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

 exactly determined the excreta, but also made a calorimetric determina- 

 tion of the heat given out by the person experimented upon, i.e., the work 

 performed. 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 isodynamic theory has been accepted by a large number of inves- 

 tigators, but not by all. Certain of them, especially the French, accept 

 an isoglucosic instead of the isodynamic. According to this theory the 

 organism for its physiological functions can use glucose only, and as a 

 formation of glucose is possible from proteins as well as fats, those quan- 

 tities of food-stuffs are to be considered as equivalent which yield an 

 equal amount of glucose. 



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 

 the combustion heat of the feces and urine with the same diet, there is 

 obtained the net calorific value of the diet. This value, calculated in 

 percentage of the total energy content of the food, is called the physio- 

 logical availability by RuBNER. 2 In order to elucidate this we will give 

 a few of RUBNER'S values. The loss in calories, as well as the physio- 

 logical availability, is calculated in percentages of the total energy 

 content of the food. 



In order to simplify the calculation of the energy exchange there exist other 

 standard factors, besides the above-mentioned standard figures for the physiological 



1 Bull, of Dept. of Agric., Washington, 44, 63, 69, and 109, and Ergebnisse der 

 Physiologic, 3. 



2 Zeitschr. f . Biologic, 42. 



