280 THE CHEMISTRY OF GROWTH AXD REPAIR 



kins^^ and Osborne and Mendel.^" Apparently the presence of some 

 of the simple straight-chain amino-acids can be dispensed with (e. g., 

 glycine), and the animal will grow and thrive if other nutritive supplies 

 are adequate, but certain, at least, of the more complex cj'clic amino- 

 acids must be provided. Furthermore, the requirements for growth 

 (quantitatively speaking at least), seem to be something more than 

 the requirements for mere preservation of health and equilibrium, 

 for it was found that animals could live and preserve nitrogen equili- 

 brium when the protein of the diet furnished at most small quantities 

 of lysine, but young animals were unable to grow with such a restricted 

 supply of this amino-acid. If lysine was added to the defective pro- 

 tein (gliadin from wheat) the animal would then be able to grow at a 

 normal rate. Of particular importance is the fact that animals can 

 be kept in a stunted condition on such a deficient diet until they have 

 reached an age at which normally all growth would have long since 

 ceased, and then when supplied with sufficient lysine they will begin to 

 grow and continue until full size is reached. ^^ This last observation 

 proves that growth is not conditioned by age, and that we do not stop 

 growing because a certain age is reached; the capacity for growth may 

 remain latent and capable of exhibiting itself when proper conditions 

 are furnished. But no amount of any amino-acid will cause a fulh^ 

 grown animal to grow any more, so it would seem that the capacity 

 for growth becomes extinguished when it has been utilized to a certain 

 fixed extent, and remains potent until it has been completelj'^ utilized. 

 If the only protein furnished contains no tryptophane the animal 

 cannot maintain itself and slowly loses weight until it dies, unless 

 tryptophane is supplied. If zein from corn, which yields neither ly- 

 sine nor tryptophane, is the sole protein, then the animal cannot grow 

 unless both lysine and tryptophane are added to the diet. So too. 

 pure casein is not adequate to maintain growth because of its low 

 content in cystine, but if cystine is added the nutritive value is 

 much increased. That the pure isolated amino-acids can meet the 

 deficiencies when added to the imperfect protein ration, demonstrates 

 that proteins serve for food as amino-acids, and not as larger 

 complexes. 



VITAMINES OR FOOD HORMONES AND DEFICIENCY DISEASES '- 



Not only must the proteins present certain essential chemical 

 compounds to the living and growing organism, but also an adequate 



29 Jour. Physiol., 1906 (35), 88. 



^° Series of papers in Jour. Biol. Cheni., 1912, et seg. 



'Mour. Biol. Chem., 1915 (23), 439. 



'2 See "Report on the Present State of Knowledge Concerning Accessory Food 

 Factors (Vitainines), Special Report No. 38 National Health Insurance Act, 

 London, 1919; "The Newer Knowledge of Nutrition," E. V. McCoUum, New 

 York, 1919; Blunt and Wang, Jour. Homo Economics, 1920 (12), 1; also Sympo- 

 sium in Jour. Ainer. Med. Assoc, 1918 (71), 937. 



