THE TOTAL EXCHANGES OF THE BODY 667 



In the case of some foodstuffs it is necessary to draw a distinction 

 between the absolute heat value and the physiological heat value. Since 

 carbohydrates and fats undergo complete oxidation in the body to carbonic 

 acid and water; their physiological heat values, i. e. the values of these food- 

 stuffs to the organism, are identical with their absolute heat values. Pro- 

 teins however do not undergo complete oxidation . When they are oxidised 

 in the bomb calorimeter the nitrogen is set free in a gaseous form. In the 

 animal body no nitrogen is eliminated in the gaseous form, the whole of it 

 being excreted as urea and allied substances still -endowed with a considerable 

 store of potential energy, which can be set free when their oxidation is com- 

 pleted in a calorimeter. In order to determine the physiological heat value 

 of protein, we must subtract from its absolute heat value the heat value of 

 the excretory products in the form of which it leaves the body. The 

 physiological heat value of proteins has been determined by Rubner in the 

 following way : A dog was fed with the same protein which had served for 

 the determination of the absolute heat value. While the dog was receiving 

 this food its urine was collected, dried, and its heat value determined by 

 combustion in the calorimeter. 'It was found that for each gramme of 

 protein which had undergone disintegration in the body an amount of urine 

 was passed corresponding to a heat value of 1-0945 Calories. The heat value 

 of the faeces formed under the same diet was 0-1854 Calorie for each gramme 

 of protein. Rubner further reckoned that a certain amount of heat would 

 be required for the solution of the proteins and of the urea, and reckoned 

 this at 0-05' Calorie. The reduced or physiological heat value of protein is 

 therefore equal to 5-345 (1-0945 -f- 0-1854 + 0-05) = 4-015 Calories. 



A determination of the heat values of the various foodstuffs shows 

 minute differences between individual members of the same class. Since it 

 is impossible to reckon out accurately the relative amounts of the different 

 kinds of protein, carbohydrate, etc., contained in each diet, Rubner has 

 calculated the average physiological heat values of the three classes of food- 

 stuffs. These figures have been universally adopted, and are as follows : 



1 grm-. protein =4-1 Calories 



1 grm. fat = 9-3 



1 grm. carbohydrate = 4-1 



These figures are accurate only for a diet containing the normal pro- 

 portion of vegetable to animal foods 60 to 40. The heat value of vegetable 

 protein is, as a rule, less than that of animal protein. It -has been pointed 

 out that these figures are rather too low for the food as ingested and too 

 high if taken to represent food as digested. Taking the normal mixture 

 of foods used in civilised countries, the following figures give more accurately 

 the energy available from any given diet (allowing for the loss in digestion) : 



Carbohydrates Proteins Fats 



4 Calories per gramme 4 Calories per gramme 8-9 Calories per gramme 



Careful experiments have shown that just as there is no loss of matter 

 in the body, so also the sum of the energies put out by the body is equal to 

 the sum of the energy obtained by the oxidation of the tissues and of the 



