694 PHYSIOLOGY 



expressing the total potential energy of a food-stuff, we generally 

 express it in calories, and speak of the heat-value of a food-stuff. 

 The heat-value of any given food is the amount of large calories * 

 which it evolves on complete combustion with oxygen, and is deter- 

 mined by burning a weighed quantity of the dried food-stuff in 

 oxygen in the bomb calorimeter. The following heat-values have 

 been obtained for different food-stuffs : 



Substance Heat value 



Lean meat 5-656 (or 5-345 Rubner) 



Lard 9-423 



Butter . 9-231 



Grape sugar ..... 3-692 



Cane sugar. ..... 4-116 



Starch . . . 4-191 



In the case of some food -stuffs 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 5 i.e. 

 the values of these food-stuffs to the organism, are identical with 

 their absolute heat-values. Proteins, however, do not undergo com- 

 plete 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 completed 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 deter- 

 mined 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 



* A calorie is the amount of heat necessary to raise a gramme of water from 

 C. to 1 C. A large calorie is the heat required to raise a kilogramme of water 

 from C. to 1 C., and is therefore equal to 1000 small calories. 



