When vegetable protein was administered in large quantity there was about the same percentage retention as when it was given in smaller amount. Hence, McCollum concludes that the limitation of growth when vegetable proteins are taken alone is due to the chemical make-up of these proteins and not to any diminution in the animal's power to grow.

The work of Osborne and Mendel upon the subject of the behavior of gliadin, one of the principal proteins derived from wheat, has been of very great interest. Gliadin is a protein which yields 44 per cent, of glutamic acid and 13 per cent, of prolin, these being present in exceptionally large quantities. On the other hand, it contains only 0.92 per cent, of lysin and very little arginin and histidin. When gliadin is the only protein in the diet grown rats may be maintained over long periods (546 days), but ungrown rats fail to grow,1 although the gliadin administered is completely digested and absorbed. The animals remain stunted and resume growth only when an adequate protein in the diet is offered to them. Osborne and Mendel2 have stunted albino rats until they were 550 days old, and then by a change of diet observed a resumption and completion of growth, although ordinarily such completion of growth is accomplished before the age of 300 days. It appears that if in these animals the function of growth has not been fulfilled at the usual period of life the capacity to grow is never lost.

If a diet be made up which contains gliadin as the dominant protein, and lysin be added so that the protein quota contains 2 or 3 per cent, of lysin, normal growth is resumed by a rat which had been stunted through the influence of the diet poor in lysin.8

The principal proteins existing in wheat are gliadin and wheat glutenin, there being equal amounts of each. Since the latter form of protein completely suffices for the growth of rats, it is evident that the value of wheat protein is greatly enhanced by the presence of this constituent.

A notable contribution to the knowledge of the relative value of lactalbumin and casein has been presented by Osborne and Mendel.1 It will be remembered that Thomas found that casein was inferior to milk protein for the maintenance of nitrogen equilibrium in man. The cause of the inferiority of casein is largely due to the fact that it contains only 0.6 per cent, of cystin (Fig. 23).

1 Osborne and Mendel: "Journal of Biological Chemistry," 1912, xii, 473.

2 Osborne and Mendel: Ibid., 1915, xxiii, 439. 3 Osborne and Mendel: Ibid., 1916, xxv, 1.

Fig. 23. - Comparison of growth on diets containing approximately the same percentage (4.5 per cent.) of different proteins, namely, lactalbumin, edestin, casein, globulin (squash-seed), and glycinin (soy bean).

1 Osborne and Mendel: "Journal of Biological Chemistry," 1915, xx, 351; 1916, xxvi, 1.

When 3 per cent, of this latter amino-acid is added to the casein content of a diet, Mendel and Osborne found that growth in the rat was accomplished with a much smaller quantity of protein than when casein alone was given. These results are presented in the following table:

Influence Of The Amount Of Different Varieties Of Milk Protein Upon The Growth Of Rats

It is evident that a diet containing 15 per cent, of casein may be given to rats and produce normal growth, whereas when 12 per cent, is present normal growth does not take place. The addition of 3 per cent, of cystein to casein so that this mixture forms 9 per cent, of the diet yields a food capable of supporting normal growth. There is greater value in lactalbumin in promoting growth than in casein because the amino-acids are arranged in more suitable proportions. The protein of whey appears to be as perfect a material for use in the service of growth as any protein known.

The following table, which is arranged from data given by Mendel,1 presents the proteins with a supply of which an organism may grow, and also those which, if fed, do not produce growth of the organism:

1 Mendel: Harvey Society Lecture, "Journal of the American Medical Association," 1915, lxiv, 1539.

Value Of Proteins In The Function Of Growth

Allow Growth.

Casein..................milk.

Lactalbumin............milk.

Ovalbumin..............hen's egg.

Ovovitellin..............hen's egg.

Edestin.................hemp-seed.

Globulin................squash-seed.

Excelsin................Brazil-nut.

Glutelin................maize.

Globulin................cotton-seed.

Glutenin................wheat.

Glycinin................soy bean.

Cannabin...............hemp-seed.

Failure to Grow.

Legumelin........soy bean.

Vignin...........vetch.

Gliadin..........wheat or rye.

Legumin.........pea.

Legumin..........vetch.

Hordein..........barley.

Conglutin........lupin.

Gelatin..........horn.

Zein.............maize.

Phaseolin.........white kidney bean.

It is evident from the material presented in this chapter that the science of nutrition includes something more than the production of energy from fat, carbohydrate, and protein. There must be certain salts and certain qualities of protein in the diet, and there must be minute amounts of "vitamins." The chemical composition of the latter will some day be known,1 even as the chemical composition of epinephrin is known. Epinephrin, an essential of life, is present in the blood to the extent of 1 part in 100,000,000. In like manner, vitamins which are present in meat, milk, fresh green vegetables, and grains are essential to the harmonious correlation of the nutritive functions of animals. Lafayette Mendel first suggested the use of the word "hormone" in connection with the vitamins. Gowland Hopkins adopts the term "exogenous hormones." The expression "food hormones" would also be exactly descriptive of the nature of these substances.

1 Williams and Seidell ("Journal of Biological Chemistry," 1916, xxvi, 431) have separated from the filtrate of autolyzed yeast a crystalline antineuritic substance which, on recrystallization, lost its antineuritic properties and was converted into adenin. The authors suggest that an isomer of adenin is the chemical entity responsible for the physiologic properties of the vitamin of yeast. They are continuing this inquiry.