244 THE BIOCHEMISTRY OF B VITAMINS 



The present discussion will not be primarily concerned with that portion 

 of the requirement with which living matter is able to supply itself. This 

 fraction of the requirement has been assessed from a practical standpoint 

 in the consideration of the sources of vitamins for nutritional purposes, 

 and from an academic standpoint in the discussion of biosynthetic 

 processes. Far more expedient is the evaluation of that portion of the 

 requirement which must be supplied to the cell, tissue, and complete 

 organism ; and this qualitative and quantitative fraction of the total will 

 hereafter be referred to as the "requirement," as is the custom. The term 

 "nutritional requirement" differs critically from this, in that it is that part 

 of the requirement which must be supplied in the diet, and does not in- 

 clude that portion which may be supplied by symbiants such as intestinal 

 bacteria. 



The natures of the B vitamin requirements of living organisms are as 

 diverse as are the forms of life themselves. To a considerable extent the 

 members of the plant kingdom are able to meet their own B vitamin 

 requirements, demonstrating thereby a higher degree of synthetic ability 

 than is found generally in the animal kingdom. This fact is most funda- 

 mental to the overall economy of life. Green plants have, therefore, been 

 considered in this discussion primarily as a food source ; and, though they 

 will be referred to again when considerations arise that seem to warrant 

 their separate discussion, a more extended consideration of the B vitamins 

 in green plants does not at present seem practical. Similarly, the role of 

 the B vitamins in the lower plant forms is of importance from a number 

 of diverse aspects, which seems to dictate the advisability of their dis- 

 cussion as the occasion arises rather than by separate treatment. The 

 B vitamins are therefore presented here in their relationship to the animal 

 as a whole, while the plant kingdom is considered only as the pattern of 

 the treatment and as the availability of data permit. 



From a purely qualitative standpoint, organisms exist which require 

 none of the B vitamins, and others exist which require all those now 

 known and probably still other substances of chemical natures which are 

 at present unknown. Although many bacteria are able to synthesize all 

 the B vitamins in sufficient amounts to meet their needs, this cannot be 

 said at present of any member of the animal kingdom. Indeed, no higher 

 animal has as yet been found which is able to survive even when all the 

 presently known B vitamins are supplied; and it must be concluded that 

 as yet unidentified factors, whether they turn out to be B vitamins or 

 not, are necessary in animal nutrition. A summary of some of these fac- 

 tors is given on pp. 12-16. 



Qualitative requirements may not, however, be dismissed as a matter 

 of "required" or "not required," since a variety of factors influence the 



