FACTORS INFLUENCING B VITAMIN REQUIREMENTS 275 



of vitamins. In the study of Denko et alP on the B vitamin excretion of a 

 number of individuals, individual differences are readily apparent, and 

 the similarities in excretion levels between a pair of brothers are most 

 noteworthy in suggesting the genetic basis which must exist to explain 

 individual variations (p. 369). 



Pett has considered this aspect of the vitamin requirements at great 

 length and has emphasized the great danger of taking the average, mean, 

 or modal value for a dietary standard. 53 He points out that when such a 

 standard is taken, a distribution curve for this figure should be available. 

 In line with this suggestion he has presented such a curve for thiamine 

 prepared from data in the literature. 54 These data are indicated in Fig- 

 ure 1. Unfortunately, however, no one has apparently as yet endeavored 

 to prepare more accurate data of this kind for thiamine or for the other 

 members of the B vitamin group. 



Factors Involving the Nature of the B Vitamin Nutritional Supply 



For a given individual the amount of a B vitamin which may be re- 

 quired nutritionally varies considerably according to the efficiency with 

 which the vitamin actually present in the food may be utilized. Thus, 

 other components of the diet or circumstances attendant upon dietary 

 habit may influence the requirement. It has also been found that when 

 more than one form of a given vitamin exists in nature, the relative 

 potencies of these forms for various species may not be the same, and 

 some forms may be completely unavailable due to their occurrence in 

 "bound" states which cannot be utilized by the animal in question. Fur- 

 ther specific inhibitions and inactivations exist which in some cases pre- 

 vent utilization of the vitamin. All these things effectively influence the 

 amount of the vitamin which must be present in the diet to meet the needs 

 of the organism, and they must therefore be considered in some detail. 

 Still other factors conditioning the requirement, such as the effect of the 

 state of nutrition on the proportion of vitamin assimilated, are at present 

 too poorly understood to provide more than passing recognition of their 

 existence. 



Effect of Other Nutritional Components. The ways in which other 

 components of the diet may influence the B vitamin requirement are 

 varied and only a few interrelationships are well understood. In some 

 cases it is quite clear that a given B vitamin is involved in the metabolism 

 of some particular nutritional component and that the amount of that 

 component consumed will directly influence the vitamin requirement. In 

 other cases, certain species are able to utilize biosynthetic precursors of 

 the vitamin so as to increase their supply. In many instances it is not 

 clear whether the animal itself or symbiotic organisms bring about the 



