GROWTH AND NUTRITION 83 



Another element we should mention, iodin, is required only 

 in minute quantities, yet it is known to be of the utmost im- 

 portance. Many years ago Baumann 18 discovered iodin in the 

 thyroid gland, and more recently the compound thyroxin in 

 which it is present has been isolated by a young American chem- 

 ist, Kendall. 19 It has been repeatedly and conclusively demon- 

 strated that iodin, and thyroxin of the gland, play a determin- 

 ing role in growth and in normal metabolism. It is now possible 

 to buy on the market iodized salt that may serve the same pur- 

 pose. Practically all salt deposits contain iodin, but in our mania 

 for consuming "pure" foods, we purify our salt until it is free 

 of iodin, and then we put it back. 



There is still some debate as to the dispensability of other 

 elements, most of which are required only in minute traces, if 

 required at all. Thus boron, silicon, manganese, arsenic, fluorine, 

 zinc, and copper, have all been suggested as being indispen- 

 sable. The difficulty in determining the facts is that even the 

 most highly purified foods it is practicable to use in experi- 

 mental work may contain a trace of the mineral in question. 



The Vitamins. It seems strange that any nutritional factor as 

 important as the vitamins could have eluded detection up to a 

 time within the memory of us all. That these substances were 

 not detected may be explained by the fact that they exist in such 

 small quantities, and there is an enormous disproportion be- 

 tween their importance and the quantities that are necessary to 

 insure nutritional well-being. It is easy to see now that ample 

 evidence has existed for years to prove the existence of some 

 unknown factor or factors in nutrition. This evidence did not 

 become sufficiently conspicuous, however, to force itself upon 

 our attention until comparatively recent years. 



It is impossible to review the earlier history of the vitamin 

 problem, but we may say that physiologists formerly believed 

 that proteins, fats, carbohydrates, and salts were the only neces- 

 sary constituents of an adequate diet. We would now say that, 

 before the belief was justified, an experimental demonstration * 



