'* INTRODUCTION 



whether they grew normally and remained healthy. Usually growth 

 declined, but by supplementing the purified diet with some natural food 

 material, such as yeast, whole milk, or liver extract, growth was restored. 

 Such experiments showed that the supplement must have contained some 

 essential food factor lacking from the purified experimental (basal) diet. 

 The next step was to isolate this substance, determine its chemical struc- 

 ture, and add it as a pure compound to the basal diet for further feeding 

 trials. Whenever this was done, it usually was found that extra supple- 

 ments were again needed, or in other words, that the supplement first 

 used must have been contributing more than one essential food factor. 



By such methods it has now been shown that a long list of chemical 

 substances is required to fulfill the dietary needs of animals. In the 

 case of rats and chickens most, if not all,, of the essential food factors 

 have been discovered, since rapid growth and apparently normal develop- 

 ment can be obtained on diets composed exclusively of pure chemicals. 

 However, when such a "synthetic diet" is fed to other animals such 

 as guinea pigs, they respond so poorly that other still-unknown food 

 substances are obviously needed. In fact the use of many different 

 species of animals for nutritional studies has been a fruitful source of 

 information, for, although many requirements are similar, many differ- 

 ences have also been found. Not only animals, but plants and micro- 

 organisms have been extensively studied as to their nutritional require- 

 ments, and the latter especially, because of their small size and rapid 

 growth, have served as admirable test subjects. 



Study of Metabolic Reactions. The study of the chemical reactions 

 that take place in living organisms is regarded by many biochemists as 

 the most significant and fundamental aspect of the science. As pointed 

 out above, relatively little progress along this line was made until recently, 

 but since emphasis is now shifting strongly in this direction, the rate of 

 discovery of new information has sharply increased, and extensive addi- 

 tions to our knowledge may be expected in the relatively near future. 

 In studying metabolic reactions one approach has been to investigate 

 the composition of the food consumed and the waste eliminated by an 

 organism in order to attempt to deduce what must have happened inside 

 the organism to convert the one into the other. This method has yielded 

 some information, but obviously suffers from severe limitations. 



A more fruitful approach has been to transfer the reactions being 

 studied from the organism to the test tube. In several instances it has 

 been possible to duplicate cellular reactions in the absence of the cells 

 themselves. For example, many of the intermediates, such as succinic 

 acid, involved in carbohydrate metabolism are oxidized by molecular 

 oxygen to carbon dioxide and water when added to suitable tissue 

 preparations. Finely ground suspensions of liver tissue in an aqueous 

 buffer are suitable for this purpose. Similarly, cell-free yeast prepara- 

 tions can ferment glucose to carbon dioxide and alcohol. Once such a 



