METHODS OF ASSESSING B VITAMIN REQUIREMENTS 257 



per day. Although urinary excretion tests suggested a niacin deficiency, 

 only the mildest of clinical symptoms were observed. Najjar and co- 

 workers 49 in similar experiments maintained subjects on diets containing 

 1.5-2.0 mg of nicotinic acid per day and observed neither signs of nico- 

 tinic acid deficiency nor a reduction in N'-methylnicotinamide excretion, 

 which would suggest depletion of body tissues. These authors attribute 

 such survival on obviously low levels of nicotinic acid to intestinal syn- 

 thesis. Cossandi, 50 on the basis of loading tests with infants in which 

 only 1 per cent recovery was obtained as compared with 5-10 per cent 

 in adults, concluded that up to one year of age there exists a relative 

 niacin deficiency, more particularly in breast-fed as compared with bottle- 

 fed infants. 



Table 6. Percentage of the Average Total Riboflavin Intake and of the Increase in Average 

 Intake Excreted at Each Dietary Level.™ 



Normal persons excrete about 2.3 /xg of folic acid per day in urine, and 

 after oral doses of 5-16 mg, excrete about 28 per cent of the dose according 

 to a recent study. In patients hospitalized for various causes, recoveries 

 were much lower, however. There is therefore some reason to believe that 

 an estimate of human folic acid requirements may be derived from such 

 studies. 51 In the case of some other factors, such a possibility seemingly 

 does not exist. Thus, since biotin excretion nearly always exceeds the 

 intake 52 (due to intestinal synthesis) accurate results seem improbable. 

 In other cases where intestinal synthesis is of a relatively low order, the 

 error so introduced is not as serious an objection to the method. A more 

 detailed consideration of many of the factors involved in the validity of 

 these studies occurs in a later section on B vitamin excretion (p. 364). 



Studies of Levels in Various Biological Materials. Closely related to 

 those studies in which the B vitamin requirement has been assessed on 

 the basis of excretory levels are those studies of vitamin tissue concen- 

 trations which contribute to an understanding of B vitamin requirements. 

 This latter type of study, however, has been very little pursued, to some 

 extent because of the uncertainty regarding the variable nature of tissue 

 storage, but in most cases because of the difficulty in obtaining suitable 

 material. 



Studies of fecal vitamin content have proved to be almost totally value- 

 less, because of both the influence of bacteria directly and the influence 



