620 PHYSIOLOGY 



and 138 grm. of fat had been completely oxidised. The amount of protein 

 used up during this time can be obtained by multiplying the nitrogen of 

 the urine plus 1 grm. of nitrogen of the faeces by the factor 6-25, and is found 

 to amount to 271-9 grm. 



THE ENERGY BALANCE-SHEET OF THE BODY 

 The energy income of the body is measured by the potential energy of the 

 food-stuffs, i.e. the amount of energy which can be evolved, either as heat, 

 work, or in any other form, by the oxidation of the food-stuffs to the end- 

 products which occur in the body. Since it is convenient to have a uniform 

 method of 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 determined 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 be- 

 tween 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 Kubner 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 



* 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. 



