FAT AS A SOURCE OF ENERGY 303 



was deposited when fat was added to a ration already more than 

 sufficient for maintenance. 



On the other hand, in the case of peanut oil added to a mainten- 

 ance ration Kellner found only 35 to 61 per cent, of the fat stored. 

 The loss of energy here implied suggests that the fat underwent 

 molecular changes after assimilation and was converted in part into 

 animal fat. 



To sum up, then, experiments show that food fat is stored up 

 in the depots as body fat, and that excess of one kind of food fat 

 modifies the characters of the depot fat to that of the food fat. 

 How far the organism strives to adjust the food fat to a normal 

 standard peculiar to the individual is undetermined. 



FAT AS A SOURCE OF ENERGY 



The well-being of an adult animal is not conditional upon the 

 maintenance of any fixed relation between the fats, carbohydrates, 

 and proteids in the food supply, apart from the minimum proteid 

 ration which is absolutely necessary to cover body loss. The 

 organism has the power to use all three classes indifferently for 

 the manifestation of its energy. A fasting animal uses up its 

 body proteid and fat. Proteid fed in excess of the amount 

 metabolised during starvation serves as a source of biotic energy 

 and takes the place of body fat. The portion of the proteid 

 supply which thus serves as a source of energy may be replaced 

 indifferently by fat or carbohydrate. Thus in a dog brought into 

 equilibrium on a ration of 1500 grm. meat, the feeding of only 

 100-200 grm. fat or carbohydrate makes it possible to dispense 

 with no less than two-thirds of the proteid. 



The taking of proteid stimulates the metabolic activity of 

 the cells, so that cleavage of the fed proteid rapidly takes place, 

 and the excretion of nitrogen keeps pace with the intake. The 

 non-nitrogenous foods flatten out the curve of nitrogen excretion 

 which follows the intake of proteid, and by their simultaneous 

 absorption lessen the rate of its cleavage. 



The minimum amount of nitrogen on a diet of proteid and fat 

 lies between 130 per cent, and 160 per cent, of the amount of nitro- 

 gen excreted during fasting. With a very large supply of carbo- 

 hydrate, on the other hand, the N intake may be reduced even 

 below the output during fasting. Thus Siven maintained himself 



