78 THE VERTEBRATE ANIMAL: METABOLISM 



56 grams of fat (including lipins), and 120 grams of proteins daily, 

 the so-called " balanced ration." It will be recalled that all car- 

 bohydrates are composed of structural units, the simple sugars, of 

 which glucose is the most universal; that the fats and lipins have 

 relatively simple and constant structural components; but that 

 the proteins are built up of amino acids, of which there are at least 

 twenty, combined in many different ways. It has been deter- 

 mined that certain proteins in our foodstuffs are lacking in certain 

 amino acids that are necessary for the compounding of our proto- 



FiG. 43. — The rations of these two rats had the same composition as shown 

 by chemical analysis. They differed only in the source of the protein which 



they contained. 



The rat on the right grew up on a mixture of proteins from the corn kernel and wheat 

 gluten; that on the left, on a mixture of corn proteins and gelatin. The difference in size, 

 and remarkable difference in appearance is solely the result of the difference in the quality 

 of the proteins in the two diets. Corn proteins and gelatin do not supplement each other's 

 amino-acid deficiencies. These animals were the same age when photographed, and had 

 been confined for the same number of days to the experimental diets. (From McCollum, 

 "Newer Knowledge of Nutrition," copyright, 1922, by the Macmillan Co., reprinted by 

 permission.) 



plasm. Thus, the characteristic protein of corn is lacking in three 

 of the amino acids that are necessary for maintaining human pro- 

 toplasmic proteins (Fig. 43). Therefore, even if one were to eat 

 120 grams of corn protein daily, this food would not fulfill the 

 requirements for upkeep. Also, lean-meat proteins contain from 

 12 to 14 per cent of a particular amino acid, while wheat proteins 

 contain 40 per cent of the same acid. The proteins of our food- 

 stuffs differ widely in their content of particular amino acids. 

 Without going further into this matter, it may be said that 

 kidney, liver, milk, and wheat contain proteins of unusual value 

 which should be used in all diets to insure the presence of the 



