912 NUTRITION AND HEAT REGULATION. 



tryptophan and lysin. Gliadin and hordein are also inadequate, 

 so far as growth is concerned, because they lack lysin. If the miss- 

 ing amino-acids are supplied, either directly or by using other pro- 

 teins, then the functions of maintenance and growth are adequately 

 supported. In an ordinary diet we do not, of course, use single 

 proteins. Our foods contain a variety of proteins, and it is not at all 

 probable that a natural diet would be lacking entirely in any of the 

 essential building-stones. Work of the kind described above helps 

 to bring out important data in regard to the particular role played 

 by the different amino-acids. One evident outcome of the work 

 reported seems to be that some of these amino-acids, glycin, for 

 example, may be formed de novo in the animal body by synthetic 

 processes or by transmutation of other amino-acids, while others, 

 such as tryptophan, lysin, and probably tyrosin, cannot be so man- 

 ufactured, but have to be supplied from without in the protein food. 

 From this point of view a protein may be adequate for nutrition 

 even though it contains no glycin, since this latter amino-acid can 

 be constructed within the body from other amino-acids. A pro- 

 tein that lacks tryptophan, tyrosin, or lysin is inadequate if supplied 

 alone in the diet, since these building stones are essential in the 

 construction of tissue protein and cannot be formed within the 

 body itself. The amino-acids also show differences in nutritive 

 value when they are deaminized and used as sources of energy, 

 rather than as building stones for tissue protein. This conclusion 

 seems justified by experiments which have shown that certain 

 of these amino-acids, for example, glycin, alanin, aspartic acid, 

 glutaminic acid, prolin and arginin, serve as a source of sugar within 

 the body; while others, as leucin, lysin, tyrosin, and tryptophan, 

 do not. Some of these latter, in fact, in their metabolism may yield 

 oxj^butyric acid after the manner of fats. It is evident from these 

 facts that one important result to be expected from future work 

 is the history of each of the known amino-acids furnished by the 

 proteins of the food. Some such knowledge will be necessary in 

 order to make intelligent use of these substances in dietary work. 



The Unknown Accessories (Vitamines). From the state- 

 ments made in the preceding paragraphs of this chapter one might 

 conclude that a perfect food would be one which supplied enough 

 non-protein material (fats and carbohydrates) for the energy needs, 

 all the necessary amino-acids, and the essential amounts of water 

 and inorganic salts. This conclusion has been made doubtful by 

 much recent work, which tends to show that in addition to these 

 fundamental requisites there are certain accessory materials that 

 are necessary, either because they play some essential role in the 

 syntheses of the body or influence in some more indirect way the 

 normal direction and character of the metabolism. This unex- 



