246 THE BIOCHEMISTRY OF B VITAMINS 



significant in view of wide individual variations in nutritional require- 

 ments (p. 273). Unquestionably the most generally accepted recommenda- 

 tions or "allowances" are those of the Food and Nutrition Board of the 

 National Research Council (p. 324). Based on broad considerations 

 and a variety of studies, these are nevertheless frequently challenged, 

 sometimes because of their interpretation as "requirements" rather than 

 recommended intakes, but more often on a seemingly valid basis. Indeed, 

 it is quite apparent that no single recommended level will suffice for all 

 purposes, 6-10 since recommendations for the allowances necessary to ward 

 off deficiency diseases will obviously differ from those used in planning 

 a dietary regimen for therapeutic purposes, or those used in planning 

 broad agricultural or public health programs. These factors will be con- 

 sidered in greater detail somewhat later. 



Methods of Assessing B Vitamin Requirements 



Man's first interest has always been with man; but because of the 

 limitations which have been placed on experimentation involving humans, 

 great ingenuity has been required in assessing human vitamin require- 

 ments. A variety of approaches has been employed, all more or less 

 indirect, and therefore subject to interpretation. It has generally been 

 possible to determine the nutritional requirements of other species with 

 far greater precision as a result of the experimental freedom which it is 

 possible to achieve. This fact led to one of the earlier approaches to 

 the assessment of human requirements, Cowgill's study of comparative 

 requirements which is discussed first. 



Clear-cut experimental data regarding requirements are rare, and this 

 is due among other reasons to the difficulty in obtaining animals free from 

 symbiotic organisms. Some conclusions can be drawn with considerable 

 certainty in fowls, inasmuch as the interiors of eggs are generally germ- 

 free. Thus, since the riboflavin content of hens' eggs does not increase 

 during incubation, we know with certainty that the foetal chicken, at 

 least, does not synthesize riboflavin. 11 Similarly, when hens' eggs are 

 injected with tryptophan, and then incubated and analyzed, they show 

 a higher niacin content than uninjected eggs; lla this is among the more 

 convincing facts that indicate that animal tissues do convert tryptophan 

 to niacin without the aid of bacterial symbiants. A more adequately 

 controlled repetition of this work would seem to be desirable. Unfortu- 

 nately, however, extensive data of this nature are lacking. 



Comparative Studies of Requirements. It has long been known that 

 the food consumption per unit of body weight and the basal metabolic 

 rate per unit of body weight are much greater for small animals than 

 for larger ones. It is thus not strange that it was readily apparent from 



