350 THE BIOCHEMISTRY OF B VITAMINS 



(d) Pantothenic acid: 48 /tg per cent on first day to 245 tig per cent 

 on the fourth day and rising to 304 tig per cent on the tenth day, with 

 mature milk containing about 250 tig per cent. 



(e) Biotin: Very low first five days, rising to about 0.38 tig per cent 

 on ninth day and 0.80 tig per cent in mature milk. 



Pearson's data on cows and ewes show, however, that these trends are 

 not necessarily general. Thus: 50 



(a) Cow colostrum contains 62 tig per cent thiamine and 610 /xg per 

 cent riboflavin, while cow milk contains 38 tig per cent thiamine and 

 177 tig per cent riboflavin. 



(b) Ewe colostrum contains 180 tig per cent thiamine and 2008 tig per 

 cent riboflavin, while ewe milk contains 60 tig per cent thiamine and 

 436 tig per cent riboflavin. 



(c) Nicotinic acid content of cow colostrum and milk are about the 

 same, but ewe milk is twice as rich as ewe colostrum. 



(d) Pantothenic acid is higher in the milk of both species. 



All workers in this field seem generally agreed that variations between 

 individuals are great in regard to milk vitamin content. Since the level 

 in the milks of individuals receiving similar diets must involve the intes- 

 tinal-circulatory absorption threshold, the excretory thresholds, and the 

 circulatory-acinus threshold, there is ample reason to anticipate such 

 marked individual differences. In consideration of the low content of 

 human and cows' milk in relationship to infant requirements, particularly 

 in regard to thiamine and niacin, at a time when there is only limited 

 intestinal synthesis in the young, and of the broad individual differences 

 in human milk, the concept of milk as a perfect nutrient for the young 

 should not be accepted uncritically. 



Distribution and Storage in Tissues and Body Fluids. The B vitamins 

 are circulated and pass readily from the blood into the various tissues and 

 fluids of the body, in each case establishing an equilibrium between the 

 content of the tissue and the environment. A discussion of tissue profiles 

 and various interrelationships that exist in this regard are to be found 

 elsewhere in this volume. Ultimately, studies of distribution constants 

 may make possible the calculation of tissue contents from vitamin intakes, 

 but this is not now generally possible. In the tissues, the vitamins are 

 frequently fixed into firmly bound coenzyme-enzyme complexes or vita- 

 min-protein storage forms. It has been shown that practically all the 

 pantothenic acid of the body tissues is in coenzyme form, 53, 54 and it 

 seems apparent that in the cases of most other B vitamins it will eventu- 

 ally be shown that the coenzyme form accounts for the majority of the 

 vitamin present. It has previously been mentioned that nicotinamide is 

 the predominating form of that vitamin in animal tissues (p. 330), while 



