FACTORS INFLUENCING B VITAMIN REQUIREMENTS 283 



topic further lessens the expediency of a consideration of it here. It should 

 be emphasized in passing, however, that the topic is of extreme impor- 

 tance in estimating the B vitamin requirements of any population, and 

 must be weighed carefully in evaluating the adequacy of diets and of new 

 processes in food technology. 



Biological Potency and Availability. It frequently occurs among the 

 B vitamins that one vitamin may have a number of structural forms, the 

 biological activities of which differ for a given species, and between dif- 

 ferent species. The requirement for a B vitamin in a given animal cannot 

 be met by a form that is without activity for that animal, regardless of 

 how high its activity may be for another. It is necessary to insure that in 

 the diet there is an adequate amount of B vitamin analogues which are 

 active for the species in question, if the requirement is to be met. For this 

 reason, the nutritional requirement cannot be stated accurately in terms 

 of, say, vitamin B 6 , since vitamin B (i has a number of active forms with 

 different biological potencies. It is therefore necessary to consider, as one 

 aspect of the problem of assessing the nutritional requirements, what the 

 various naturally occurring forms of the B vitamins are, and how they 

 compare with each other as regards their biological activity. 



Beyond this, it has been found in recent years that certain forms of 

 some B vitamins are not available to animals for use; indeed the 

 problem of the availability of vitamins, even when they are present in 

 otherwise adequate amounts, is one worthy of considerable attention. 



Human Bioassay Techniques. While it is generally possible to deter- 

 mine the vitamin requirements of lower animals and the potencies of 

 various diets or vitamin derivatives for any given species by a suitable 

 direct approach, it is seldom possible to employ such direct methods with 

 man himself. Laboratory depletion studies upon members of the human 

 race are relatively rare, 06 and indirect methods must generally be used. 

 These have largely employed studies of vitamin excretion. 



Melnick et al. 97 have applied such studies to a number of problems. 

 Generally the assay involves suitably sized groups of about five individ- 

 uals on diets containing ample vitamins for their requirements. Twenty- 

 four-hour samples of urine are collected and the basal level of vitamins 

 determined. Immediately thereafter a known dose of vitamin is admin- 

 istered, and the percentage of this dose recovered in the urine is observed. 

 When the test dose is then administered, it is assumed that the percentage 

 recovery in the urine is similar, and the potency of the test material can 

 presumably thus be calculated. Despite the fact that there are large 

 individual variations in basal levels, control experiments are reported to 

 indicate an accuracy generally as good as that obtained in other animal 

 assays. In view of the various factors discussed previously in regard to 



