METHODS OF ASSESSING B VITAMIN REQUIREMENTS 245 



decision. While nutritional requirements of ruminants for B vitamins are 

 virtually nonexistent, this is only by virtue of an extensive system of 

 bacterial symbiosis; and ruminants do actually require a supply of these 

 substances, even though it may be derived from within the confines of the 

 gastrointestinal tract. Moreover, certain B vitamins may substitute for 

 others. Thus, as will be shown later, substitutions similar to that of 

 alanine for vitamin B 6 in the nutrition of S. fecaelis R x may occur in the 

 animal kingdom. 



The nature of quantitative B vitamin requirements is by comparison 

 far more complex and will require extended discussion. It should be 

 pointed out here, however, that the conditions under which any given 

 quantitative B vitamin requirement exists are so limited that the fixing 

 of a practical value for any particular species is possible only within very 

 broad limits. 



Finally, fundamental to the nature of any requirement is the problem 

 of what criterion shall be taken in judging whether a substance is required 

 and how much is required. 2 Thus, the amount of thiamine necessary to 

 protect a rat from convulsive seizures might be defined as the require- 

 ment, even though the animal suffered from other pathological manifesta- 

 tions. The requirement might also be defined as the amount necessary to 

 maintain the animal in a state of health, or as the amount required to 

 promote growth, or optimum growth, or as the amount necessary to pro- 

 mote longevity. Each of these criteria has found use, and a variety of 

 similar cases makes it necessary to reach an agreement on, or an under- 

 standing of, this issue. Prejudicial interpretation of this factor in prison 

 camps during the recent war has resulted in extensive discussion of defi- 

 ciency criteria. A more extended discussion of this factor, as it refers to 

 the prevention of specific pathology, occurs in the following section. 

 Criteria based on benefits exceeding growth and maintenance are dis- 

 cussed more fully in later chapters. 



Associated with this problem is the one of poor usage and ill-defined 

 terminology. Expressions such as "health," "physical fitness," "optimal 

 requirements," and "maximal requirements," are common to the field and 

 portray clearly the uncertainty and the lack of development of satisfac- 

 tory scientific criteria and nomenclature. 3 One definition of good nutrition, 

 for instance, is "that condition which permits the development and main- 

 tenance of the highest state of fitness." 4 "Fitness," however, is a term of 

 little advantage, since, as Keys points out, 5 "everyone knows what it 

 means but not how to measure it." 



In addition to the problem of precise requirements, there is the further 

 question of recommended intake, i.e., the safety margin believed to be 

 advisable above the absolute requirement. This consideration is largely 



