Chapter IC 



METHODS OF ASSESSING B VITAMIN REQUIREMENTS 



General Considerations 



After the foregoing consideration of the general nature of the B vitamins 

 and their role in the chemistry of cells and aggregates of cells, it would 

 be desirable to proceed by a logical sequence of study to the part played 

 by the B vitamins in entire organisms. Unfortunately, however, the 

 present understanding of the facts does not permit so smooth a transition. 

 In the study of vitamin deficiencies in particular, wherein lie man's most 

 urgent interests, there is seldom more than a vague similarity between 

 the biochemical functions of the vitamins as we have considered them 

 and the clinical characteristics of the avitaminoses. This section of the 

 monograph is developed largely along a new pathway; it therefore draws 

 on the more basic biochemical factors previously considered only on those 

 rare occasions when the circumstances will permit. 



Implicit in the study of the B vitamins as a group is the fact that in 

 nature they always occur together and are essential in the economy of 

 all living cells. Since the problems involving the requirement of any one 

 B vitamin are common to all members of the B group, an attempt is made 

 to present the discussion of these requirements in a general and integrated 

 way, rather than to stress the consideration of each vitamin individually. 



When the supply, whether intracellular or extracellular, of any one of 

 the B vitamins is cut off, the entire metabolic process rapidly comes to 

 a standstill. From the standpoint of a single cell, the time required for 

 this to occur is largely dependent upon how rapidly the various chemi- 

 cal events progressing within the cell bring about attrition of vitamin- 

 containing catalyst molecules. In the absence of a renewed supply of 

 coenzyme, this generally ensues quite rapidly, and the cell becomes func- 

 tionless in a normal sense when the critical vitamin reaches an inoperably 

 low level. The quantitative requirement of that cell for any B vitamin 

 is that amount which it must supply, or which must be supplied to it, 

 to continue in normal operation. This concept is basically true for cell 

 aggregates, whether tissues or entire organisms. Cell aggregates for archi- 

 tectural reasons, however, are able to buffer themselves against deficiency, 

 and therefore do not respond as rapidly to vitamin privation. 



243 



