V IT AMINES 211 



is therefore the rate of growth determined and limited by the 

 growth factor. 



Is there a distinction between the food requirement for 

 maintenance and the food requirement for growth? This 

 question has been answered in the affirmative by Osborne 

 and Mendel. These observers fed young rats on a diet 

 in which the sole protein was gliadin, which is deficient 

 in lysine, and found that they remained in good health 

 but ceased to grow. On the addition of lysine to the diet 

 the stunted animals resumed their growth. Lysine there- 

 fore, while not essential for maintenance, is necessary for 

 growth, while its temporary absence from the diet does not 

 lead to loss of the capacity to grow. Lysine is necessary 

 for the full play of the growth factor. 



ACCESSORY FOOD FACTORS— VITAMINES 



Within the last few years there has been accumulating 

 chnical and experimental evidence to show that something 

 more is required in the diet than carbohydrates, fats, 

 proteins and inorganic salts. There are also necessary 

 certain substances which the animal cannot manufacture, 

 and which must therefore be derived in the first instance 

 from plants. They are termed accessory food factors or 

 vitamines. For our knowledge of these substances we are 

 indebted to Hopkins. 



Their chemical nature is entirely unknown. As to the 

 part they play in the animal economy, it is clear from the 

 minute amounts which are sufficient that they do not 

 contribute energy. They must therefore either form cer- 

 tain components of the cell architecture or play a part, 

 like catalytic agents, in determining or regulating metabohc 

 changes. 



Absence of these substances leads in the young to 

 failure of growth, and in both young and old to signs of 

 malnutrition, decreased fertility, and abnormal proneness 

 to infection, and in extreme cases to the development of 

 certain specific diseases — " deficiency diseases^ 



