58 THE INDIVIDUAL ORGANISM 



these three diseases were caused by deficiencies in three specific dietary 

 factors. Since their chemical natures were unknown, these factors were 

 designated respectively A, B, and C. Since 1920, the discovery of addi- 

 tional vitamins has been rapid, and the original vitamins A and B have 

 been found to be mixtures of distinct vitamins with unlike properties. By 

 1949, vitamin A had been divided into A, D, and E, about 12 individual 

 vitamins had been separated from the original B, and of the three origi- 

 nally known vitamins only C remained undivided. 



Today the chemical compositions and structures of many of the 

 vitamins are known, and some of them have been synthesized in the 

 laboratory. Present terminology is a mixture of the old alphabetic designa- 

 tions and of chemical names. The tendency is for the letter names to be 

 dropped, since splitting and synonymy have made for confusion among 

 them. In our account we use the name most frequently encountered. 



Most of the important vitamins can now be obtained either as con- 

 centrates from natural sources or in synthetic form. About sixteen (some 

 of them complexes) are currently recognized. Eight of these are of proven 

 importance in human metabolism, and five others may be important 

 though proof is lacking. The eight vitamins known to be essential are as 

 follows : 



Fat-soluble vitamins: 



Vitamin A) „ . . . . . . 



,,., . ._} from the original A complex 

 Vitamin DJ 



Vitamin K 



Water-soluble vitamins : 



Vitamin Bi (thiamine) 



Riboflavin (vitamin B 2 or G) 



„. . . ,, , _ _ ... . ,, > from the original B complex 



Niacin (vitamin r-B or nicotinic acid)' 



Folic acids 



Vitamin C (ascorbic acid) 



THE FAT-SOLUBLE VITAMINS 



Vitamin A is an alcohol (C20H09OH) formed in man and other animals from an 

 essential precursor or provitamin called carotene. The latter is a yellow or orange 

 pigment common in plants and especially abundant in carrots; it gives the yellow- 

 ish color to cream, butter, egg yolk, etc. Herbivorous animals are able to secure 

 their entire vitamin A requirements by eating carotene; man has to eat twice 

 as much carotene as vitamin A to obtain equivalent amounts of the latter. Excess 

 vitamin A is stored in the liver of vertebrate animals; cod-liver oil and other 

 fish-liver oils are especially rich in this substance. 



Vitamin A is specific for the prevention and cure of night blindness, day blind- 

 ness, and a disease known as xerophthalmia, which causes a drying of the outer 



