FACTORS INFLUENCING B VITAMIN REQUIREMENTS 291 



preventing perosis in chicks, for instance, but the mono- derivative will 

 not promote growth, and the dimethyl compound only does so to the 

 extent that methionine is present. 151 These relationships are discussed at 

 greater length elsewhere (p. 353). 



(9) p-Aminobe?izoic Acid 



p-Aminobenzoic acid occurs in both free and combined forms in nature 

 and as part of the folic acid group of compounds. Little is known beyond 

 this concerning the variety of its occurrence, or the relative potency of 

 its forms (p. Ill D). 



(10) Inositol 



Inositol occurs in nature in the free form, as the hexaphosphoric ester 

 and its salts, and in certain "cephalins." 152 In addition it occurs in bound 

 forms that have been as yet unelucidated. 153 The majority of these forms 

 are apparently available to most higher animals, except those cases where 

 the insoluble calcium or magnesium salts of the phosphate make solution 

 and liberation of the inositol impossible. 



Availability of Thiamine from Yeast. Mention has been made of the 

 existence of bound and/or unavailable forms of a number of the B vita- 

 mins. In no case, however, has this fact been so vitally important and so 

 vividly demonstrated as in the case of the thiamine of yeast. Particularly 

 is this so because of the extensive use of yeast as a thiamine source. 



It has been known for some time that dried yeast is superior to live 

 yeast as a thiamine source for rats, 154 but it was not until recent years 

 that this was shown to be true for man." In some cases indeed, as little 

 as 17 per cent of the total thiamine was found to be available for nutri- 

 tional purposes in humans. 97, 9S Recently, it has been shown that live 

 yeast, when added to a diet containing adequate thiamine, 155 decreases 

 the available thiamine in the diet, and the technique has even been em- 

 ployed to produce thiamine deficiencies in man. In one experiment, for 

 instance, five women on a diet containing 1.6 mg per day of thiamine were 

 fed 15 gm of live baker's yeast, which decreased the average urinary 

 thiamine level from 374 to 101 fig per day. As little as 150 gm of yeast 

 was found to depress the excretion to 40 ^g per day. 



A variety of studies on this phenomenon have resulted in the conclusion 

 that the effect is due to the inability of the intestine to absorb the thi- 

 amine, for mechanical reasons. An inverse relationship exists in such 

 cases between the fecal and urinary thiamine, and it appears that a large 

 proportion of the living yeast cells ingested pass through the intestinal 

 tract without rupturing. These cells apparently have an active thiamine 

 uptake, and the yeast thiamine, whether free or phosphorylated, does not 



