METABOLISM OF THE B VITAMINS 347 



tribution, and even less of the states of the remainder of the B vitamins 

 in the circulation. On the basis of thiamine, riboflavin, and nicotinic acid, 

 it would appear that free B vitamins entering the plasma are unchanged 

 there, but largely enter the nucleate blood and tissue cells where they are 

 converted to coenzymes, leaving only a low residual free vitamin content 

 in the blood. How true this is of the other vitamins remains to be deter- 

 mined. 



With regard to the relationship of the blood levels of B vitamins to the 

 nutritional requirement for these vitamins, three general relationships are 

 apparent. Except those vitamins that may be supplied to a large extent 

 by endogenous sources such as niacin and biotin, the blood levels of any 

 given vitamin for a number of species tend to increase as the nutritional 

 requirements on a unit body weight basis increase, i.e., as the sizes tend 

 to decrease (p. 246) . 227 This is in line with Williams' observation that there 

 is a "tendency for the vitamin content to be lower in the tissues of larger 

 animals." 49 With the same exceptions, for any given species, the blood 

 levels of various B vitamins tend to vary with the vitamin requirements. 

 These relationships are shown in Fig. 15. Finally, as indicated in an 

 earlier section, at levels of vitamin intake below the nutritional require- 

 ment, the blood level tends to reflect the intake, whereas at higher dietary 

 levels it does not. The urinary levels, conversely, reflect only the higher 

 intake levels, for obvious reasons. With regard to this last generalization, 

 so many qualifications and apparent exceptions exist, and, as previously 

 mentioned, so many other dietary factors influence the balance, that it 

 must be taken only as a rather self-evident and frequently demonstrable 

 trend, and not relied upon quantitatively. 



Levels in Milk. For a number of readily apparent reasons, there has 

 been extensive study of the levels of the various B vitamins in the milk 

 of a number of species. Much of the existing knowledge as to actual levels 

 is summarized in Table 27. Present limited information suggests that 

 thiamine, biotin, and inositol occur in milk in bound forms (or in more 

 firmly bound forms than the other B vitamins) ; but little is actually 

 known about the precise vitamin forms in milk. Within certain limits, 

 the vitamin content of the milk reflects that of the diet. Among a number 

 of species, it is known that the B vitamin content of the milk generally 

 increases as the requirement increases, and in any one species, the relative 

 amounts of B vitamin in milk tend to vary with the requirements, as 

 shown in Fig. 16. 



The colostrum in ruminants seems to be higher in vitamins (Bi and 

 B 2 ) than is the adult milk, 50 whereas the inverse seems to be true in the 

 human. Macy et al. 51 ' r>2 have shown that the following variations occur 

 in human milk: 



