FOODS — HUMAN NUTRITION. 559 



" The facts here established make it clear that, at least iu so far as nutri- 

 tion in growth is concerned, the normal construction of new tissues is limitetl by 

 the factor of the supply of lysin. In the light of this, little is gained by em- 

 phasizing the quantitative asiiects of the protein needs in growth . . . , unless 

 the qualitatiAe character of the protein available is kept clearly in mind. No 

 amount of energy or protein, however abundant, has induced growth of our 

 animals in the absence of lysin. The animal organism apparently can not 

 sjTithesize lysin, which is evidently not essential for maintenance in the sense 

 of preservation of body weight, though it is, of course, impossible to say that 

 when this amino acid is missing all functions are normally carred out. That 

 the tissues either form a typical protoplasmic product, or none at all, now seems 

 to be axiomatic in physiology. We may therefore reasonably assume that the 

 growth of rats on our gliadin+lysin food represents the construction of typical 

 tissue substance. It is obvious, furthermore, that the possibility of growth 

 must be limited, among other things, by the amount of lysin available." 



The following data are brought together regarding the percentage of lysin 

 in proteins of different sort.s, both animal and vegetable : Lactalbumin 8.10, 

 halibut muscle 7.45, ox muscle 7.59. casein (cow's milk) 7.61, vitellin (egg 

 yolk) 4.81, ciystallized albumin (hen's egg) 3.76, legumin (pea) 4.98, phaseolin 

 (kidney bean) 4.58, glutelin (maize) 2.93, glutenin (wheat) 1.92, edestin (hemp 

 seed) 1.65, amandin (almond) 0.72, gliadin (wheat) 0.16, and hordein (barley) 

 and zein (maize) none. 



" It is a teleologically interesting fact brought out by the foregoing figures 

 that tho.se px'oteins, like casein, lactalbumin, and egg vitellin, which are in 

 nature concerned with the growth of animals, all show a relatively high content 

 of lysin." 



In further proof of their contention that lysin is indispensable, the authors 

 cite experiments with zein, which, like gliadin, is devoid of lysin as well as 

 gylcocoU and tiyptophan. Their experiments have shown that when zein is the 

 sole protein fed to both adult and growing rats, a decline is always noted. 

 When a trytophan or a tryptophan-yielding protein is supplied, maintenance 

 is possible for a long i^eriod. 



" The relative efficiency of different proteins in preventing the failure with 

 zein apparently depends to a dominant degree, in so far as maintenance is con- 

 cerned, on their comparative jield of tryptophan. 



" Where growth is involved in addition to maintenance, the lysin factor as 

 well as others not yet more accurately ascertained must also be taken into 

 account. Here, then, is evidence of the relative economy of different proteins 

 in maintenance, based upon the content of one or more of the amino acids 

 essential for the proper functioning of the organism (in so-called maintenance) 

 or for new tissue consti'uction (in growth). Obviously the relative values of 

 the different jiroteins in nutrition are based upon their content of those special 

 amino acids which can not be synthesized in the animal body and which are 

 indispensable for certain distinct, as yet not clearly defined processes, which 

 we express as maintenance or repair. . . . 



" With the indispeusability of tryptophan for maintenance, and of lysin for 

 growth, thus emphasized, we may expect that the addition of both of these 

 amino acids to zein food will result in growth. This expectation has been 

 fulfilled by the experiments planned. . . . The respective parts played by 

 the two amino acids are here clearly brought out. These are, we believe, the 

 first successful attempts to grow animals on a diet in which zein forms the 

 sole protein." When comparison is made with results of the experiments 

 showing the relative effect of replacing one-fourth of the zein with lactalbumin, 



