CHAPTER IX 

 GROWTH FACTORS 



PRIOR to the last two or three decades it was con- 

 sidered that an adequate supply of protein, fat and 

 carbohydrates, together with some mineral salts, 

 was all that was necessary for the normal growth and 

 development of animals and man. Then knowledge of 

 the more detailed composition of " foodstuff s led to the 

 recognition of the part played in nutrition by minor and 

 formerly unsuspected constituents. The cause of several 

 of what are now called " deficiency diseases " was shown 

 to be a lack of certain essential growth factors in the diets 

 of the afflicted persons. Well-known examples are 

 scurvy, rickets and beri-beri. As the use of diets con- 

 taining, as far as possible, only known constituents for 

 experimental investigation of \atamins became common 

 more and more such substances were discovered. The 

 stages in the development of our knowledge have usually 

 been first, the recognition of a condition as due to a 

 deficiency of some "essential metabolite," secondly, the 

 discovery of some crude preparation which would supply 

 the lacking factor or vitamin and finally the identification 

 of a chemically defined substance which could replace 

 the crude preparation. Micro-organisms are similar to 

 animals in that they, too, require essential growth 

 factors in addition to the normal sources of carbon, 

 nitrogen, mineral salts and other elements, necessary 

 for the supply of energy and raw materials for growth. 

 These growth factors were sometimes called " bacterial 

 vitamins " by analogy with the vitamins concerned in 



