248 THE BIOCHEMISTRY OF B VITAMINS 



sists of depleting the nutrition of a factor and then observing the exact 

 amount of the vitamin required to prevent active clinical manifestations 

 of deficiency. While this was possibly the earliest technique, only in 

 recent years has it been directly applied to humans. Even so, such a 

 direct approach to the problem of vitamin requirements is not entirely 

 satisfactory for a number of reasons. Primarily, the results obtained will 

 depend completely upon the symptoms to be prevented. Careful perusal 

 of the tabulated requirements in Chapter IIIC will demonstrate this 

 forcibly as it applies to many species, since requirements have been 

 variously adjudged over wide ranges based on this fact alone. This may 

 perhaps best be illustrated in the case of the folic acid requirements of 

 the chick which have been extensively investigated in recent years. 15 The 

 symptoms which are prevented by different levels of folic acid in the 

 chick diet are indicated in Table 1. 



Table 1. Functions Supported in the Chick by Various Levels of 



Reference 



(15) 

 (16 

 (15) 

 (15) 

 (15) 

 (17) 



Thus for any vitamin and species a number of criteria may be taken 

 (and frequently have been taken) as indicating a deficiency or lack of 

 it. Obviously, the criterion suited to the purpose is that which insures the 

 "well-being" and "general normal character" of the animal in question, 

 such terms being about as indefinite as the requirement itself. Since the 

 full consequences of avitaminoses are seldom realized simultaneously 

 with the first recognition of the etiology of the disease, the optimal type 

 of criteria mentioned is not always definable, and realization of this has 

 done much to foster the study of subclinical deficiencies. 



Another difficulty inherent in this approach is that concerned with the 

 association of specific symptoms with a deficiency. Thus when the list of 

 vitamins consisted largely of "A," "B," "C," and "D," certain symptoms 

 were associated with a deficiency of vitamin "B," and only in more recent 

 times has it been possible to associate certain of these with the precise 

 chemical factor or factors whose absence was responsible for these symp- 

 toms. In many cases the exact correlation is still unclear. Green and Brun- 

 schwig, 18 for instance, in assessing the physiological activity of choline, 

 have only recently come to the conclusion that the factors responsible for 

 hepatic fatty infiltration and for parenchymal necrosis may not be identi- 



