254 THE BIOCHEMISTRY OF B VITAMINS 



These values for a recommended intake obviously represent a much 

 higher level than is arrived at for the requirement by other means. They 

 are, however, not extravagant as an assessment for practical use in view 

 of the great variation which may exist in individual needs (p. 273). 



Table 5. Vitamin B Content of Various Materials (per 2500 Calories). 



Studies of Vitamin Excretion. A fourth approach to the assessment 

 of requirements for B vitamins is the evaluation of vitamin excretion 

 levels in terms of the B vitamin intake. Actually this represents a varia- 

 tion of a previous approach wherein the excretion is taken as the criterion 

 of deficiency or sufficiency. Economizing processes in living tissues pre- 

 sumably work to retain and utilize essential nutrients when the require- 

 ment is not being met by the nutrition or when a so-called "tissue hunger" 

 exists for some factor. When, however, the tissues have absorbed as much 

 of a vitamin as they require, an organism will frequently excrete amounts 

 of essential food constituents proportional to the intake. Melnick and 

 co-workers 2S have utilized this latter principle extensively in biological 

 assay work employing human subjects, and in the study of the biological 

 availability and inactivation of certain B vitamins. More generally, how- 

 ever, test doses of a given vitamin are administered to a subject and, 

 depending upon the portion excreted as compared with known values 

 from individuals on an adequate diet, an estimate is made of whether 

 a depletion exists and therefore whether a diet of known vitamin content 

 meets the requirement. Still simpler but infinitely less satisfactory, one 

 may attempt to determine whether the level of urinary excretion of a 

 B vitamin is within the range of values considered to be normal. 



This general approach, like the others, has its drawbacks. Unquestion- 

 ably the greatest errors have been due to the individual variability in the 

 proportion of an administered test dose which is excreted. Another source 

 of question has been concerned with the validity of the general principle 

 that the absorption capacity of the body is a true function of the require- 

 ment. In general, results obtained by this method agree well with other 

 evaluations of requirements, although they tend to be somewhat higher. 

 A third source of error, particularly in earlier studies, is in the measure- 

 ment of inappropriate excretion products. Increased understanding of 



