1060 ABSORPTION 



food as an adult, and between the ages of fourteen to nineteen even 

 more than that. In the females there is a similar absolute increase 

 to about the eleventh year, when it becomes more constant and equals 

 about that of a woman of thirty. These brief data show very clearly 

 that the total energy and food requirements of the young animal are 

 higher than those of the adult. In the second place, it has been made 

 evident by the work of Mendel^ and others that growing tissues de- 

 mand not only an abundance of protein, but proteins of the proper 

 kind. 



This statement leads us to infer that a diet may be well balanced, 

 as far as the ingo and outgo of the proteins are concerned, and yet fail 

 absolutely in supplying those substances which are absolutely essential 

 to growth. Thus, it has been shown by Osborne, McCoUum and 

 others that such proteins as legume.lin (soy bean), gliadin (wheat and 

 rye), legumin (pea), hordein (barley), zein (maize), and phaseolin 

 (kidney bean) may maintain life, but prove quite insufficient for 

 growth. Other proteins which are capable of sustaining growth are 

 glycinin (soy bean), glutein (wheat), glutelin (maize), globulin (squash 

 seed), edestin (hemp seed), and casein. In the case of casein it is of 

 interest to note that it does not contain glycocoll, one of the simplest 

 of the amino-acids, but this deficiency does not prove disturbing, 

 because the body is in a position to synthetize this substance from 

 other sources. Just the opposite result follows the withdrawal of 

 cystine, which the body cannot build up and must, therefore, obtain in 

 an available form. Quite similarly, the tissues may be maintained 

 in their present condition without lysine, although they cannot grow 

 in its absence. This substance seems to be a requirement of all 

 growing tissues, because it is present in large amounts in casein, 

 lactalbumin and egg vitellin. It will be seen, therefore, that the body 

 demands a mixture of protein foodstuffs from which it may then select 

 those amino-acids which are most essential for its growth. An ex- 

 clusive vegetable diet might easily prove insufficient, because it lacks 

 the aromatic amino-acids, tyrosine and tryptophane, the diamino- 

 acid, lysine, and the sulphur amino-acid, cystine. But this is also 

 true of certain proteins of animal origin; for example, gelatin, which 

 for this reason cannot be regarded as an adequate food. 



Since milk is practically the sole food of the growing mammal, we 

 should expect to find its content in proteins to correspond closely to 

 the above principles. In support of this contention it might be men- 

 tioned that the analyses of milk from different animals have shown 

 that the protein content of this secretion varies with the speed with 

 which their young grow. For example, since the infant doubles its 

 weight in about 180 days and the kitten in 7 days, human milk 

 contains only 1.6 per cent, of protein and that of the cat 9.5 per cent. 

 Furthermore, the infant receives a relatively much greater proportion 

 of protein than the adult and, besides, an excess of fat in order to be 

 1 Harvey Lectures, Lippincott and Co., New York, 1915. 



