PRINCIPLES OP GENERAL PHYSIOLOGY 



oxidised than carbohydrate. It is not, of course, a necessary article of diet, 

 since it can be formed in the organism from carbohydrates. A certain amount of 

 carbohydrate appears to be a matter of necessity, as we shall see later. And, 

 although the whole of the energy requirements could be supplied by protein, it 

 would be very wasteful, since only a comparatively small amount of nitrogen is 

 actually required. The point is that a minimum total energy value, usually 

 expressed in heat units, must be supplied to an animal, in order to prevent loss 

 of body substance. 



SPECIAL REQUIREMENTS 



As organisms increase in complexity in the course of evolution, it appears that 

 their capacity of synthesising the innumerable compounds of which they consist 



FIG. 72. FAILURE OF RATS TO GROW ON A PURE SYNTHETIC DIET. 



Ordinates average weight of animal in grains. 



Abscissa time in days. 



Lower curve (as far as the 18th day) eight male rats on pure diet, free from 



"accessory factor." 



Upper curve eight similar rats taking, in addition, 3 c.c. of milk per day. 

 On the 18th day, marked by vertical dotted line, the addition of milk was 



changed from one set to the other. 



(Hopkins, 1912, p. 433.) 



is diminished. There are certain differences as regards requirements for growth, 

 maintenance, or energy, so that a diet which is adequate as a supply of energy, 

 that is, one which has a sufficient calorie value, may be inadequate for replacing 

 wear and tear, while a diet which is adequate for this purpose may be unable to 

 allow growth to take place. Hopkins (1912) showed, for example, that voung 

 rats if fed on mixtures of pure caseinogen, fat, carbohydrate, and salts rapidly 

 ceased to grow, although the energy value of the diet was amply sufficient for the 

 purpose. On the other hand, if a minute quantity of fresh milk (3 c.c. per day) 

 was added to the diet, growth recommenced and went on rapidly. Fig. 72 is a 

 representation of one of these experiments. It is to be noted that these amounts 

 of milk merely added some 4 per cent, or less to the total solid eaten and are 

 altogether inadequate to account for the increased growth, which amounted to 

 about half a gram per rat per day, whereas the total solid content of the milk 



