METHODS OF ASSESSING B VITAMIN REQUIREMENTS 259 



over, the riboflavin level of the liver is a function not only of dietary 

 protein, but also of methionine and cystine. 02 



Interesting results concerning the riboflavin requirements of fowls 

 have been obtained from studies employing eggs. Jackson and co- 

 workers 63 have found that the highest level of dietary riboflavin that 

 would affect the riboflavin concentration in hens' eggs was from 1400 to 

 1600 tig per pound of feed. This is about 330 /xg per cent and very close 

 to the requirement of hens as assessed by other means (p. 327) . 



In order to maintain a normal blood level of niacin in humans (0.6-0.65 

 mg per cent) , it has been found that a daily intake of from 12 to 16 mg 

 is required. Since the blood level of nicotinic acid varies with the dietary 

 intake, this value may be taken as an estimate (albeit a high one) of the 

 average human requirement. 04 In white rats the liver and spinal fluid 

 levels of nicotinic acid have also been shown to be a function of niacin 

 intake, 65 - 60 but also to be inversely related to the thiamine intake. Old- 

 ham 39 has concluded that urinary and fecal niacin excretion are inde- 

 pendent of the intake, each being about 1 mg per day in the young women 

 studied. 



Urinary and fecal pantothenic acid, however, have been found by Old- 

 ham et al. 37 to vary with the dietary intake. Pearson et al. found that in 

 the chick, both the blood and muscle tissue reflected the dietary supply 

 of pantothenic acid, although the liver did not. 67 Dietary levels of panto- 

 thenic acid greater than those required for "adequate nutrition," how- 

 ever, do not further increase the muscle pantothenic acid level in the 

 chick. Silber 68 similarly finds that in dogs the blood pantothenic acid 

 level reflects the nutritional supply. 



With regard to vitamin B 6 , studies have shown that the level in rat 

 liver is independent of the level of intake, for dietary levels greater than 

 25 /xg per day, thereby suggesting this as the dietary requirement for the 

 rat. The value so obtained does not vary greatly from other assessments 

 of this requirement. 



Unquestionably, studies of the kind mentioned will be more numerous 

 in the future. Techniques such as that recently developed for the determi- 

 nation of riboflavin in very small amounts of serum 68a may eventually 

 make this approach widespread in nutritional survey work. Moreover, 

 by similar processes it will be possible eventually to study the nutritional 

 requirements of individual animal tissues grown in vitro and thus arrive 

 at a more fundamental understanding of B vitamin requirements. Such 

 studies, already under way, 69, 70 may well be one of the great advances 

 in this field within the immediate future. 



Natural Selection Studies. It has been reported from time to time that 

 among lower animals at least there exists an instinctive tendency to 



