PHYSIOLOGY, PHARMACOLOGY, AND TOXICOLOGY 379 



used here as reference points rather than to designate functional boun- 

 daries. 



Metabolic Interrelationships. Many of the relationships between vita- 

 mins and metabolism primarily involve endocrine function and are dis- 

 cussed in a following section, while other relationships that involve bio- 

 chemically specific metabolic reactions are discussed at other appropriate 

 places. Drawing the many facets of metabolism together results in the 

 concept of "total metabolism" or the overall metabolic rate. The met- 

 abolic rate is a function of all the factors that go into a myriad of 

 metabolic processes, and may be limited by any factor that is deficient, 

 so that when any B vitamin is present in inadequate amounts, the metab- 

 olism may be slowed down, and addition of that B vitamin may increase 

 the metabolic rate to the point where some other factor becomes limiting. 

 Beyond this, the B vitamins have little effect upon the metabolic rate in 

 the sense that many other physiologically active substances do. When 

 for some reason the metabolic rate is increased, the heightened cellular 

 activity of the body results in an increased vitamin attrition and there- 

 fore an increased requirement for the B vitamins, as it does other nutri- 

 tional elements. 



As far as is now known, these same considerations apply to the inter- 

 relationships between the B vitamins and growth. Supplementation of 

 the diet with increased amounts of B vitamins will undoubtedly increase 

 the growth rate in many cases, 117 but this effect is primarily due to the 

 fact that in these cases there are originally present insufficient supplies of 

 vitamin to meet the requirements for the -maximum rate of growth per- 

 mitted by other metabolic factors. Whereas there is some evidence to 

 indicate that certain B vitamins may function specifically to increase 

 the mitotic rate of certain plant tissues, it seems unlikely that such an 

 effect occurs in the higher animals. When B vitamins are added to the 

 diet to a point that they no longer stimulate growth, it is apparent that 

 some other factor then becomes limiting in the process. This factor may 

 well be one whose limiting nature is unimportant, but it may also be 

 one that will result in serious consequences. Thus in an overly rapid rate 

 of growth various other nutritional substances associated with protein 

 metabolism or bone formation may become limiting; and as previously 

 stated, there is no assurance that the fastest rate of growth is the best. 

 There would seem to be some justification, therefore, in permitting growth 

 to proceed at what experience has taught is a reasonable rate, allowing 

 the limiting catalytic activity of the B vitamin, when it exists, to remain 

 the controlling factor. Because of the high rate of turnover of raw mate- 

 rials in the growing animal, and the consequent high metabolic rate, 

 there is an increased B vitamin requirement on a weight basis. In well 



