538 



SCIENCE 



[N. S. Vol. XXXVIII. No. 981 



useful. Trials with few animals, whilst 

 they can not measure accurately the feed- 

 ing value of a new food, are quite good 

 enough to demonstrate its general proper- 

 ties, and its pi-ice will then gradually settle 

 itself as the food gets known. 



Turning to the more strictly scientific 

 aspects of animal nutrition, entirely new 

 ideas have arisen during the last twenty 

 years. I propose to discuss these shortly, 

 beginning with the proteins. Twenty years 

 ago the generally accepted view of the role 

 of proteins in nutrition was that the pro- 

 teins ingested were transformed in the 

 stomach and gut into peptones, and ab- 

 sorbed as such without further change. 

 Splitting into crystalline products, such as 

 leucin and tyrosin, was thought only to take 

 place when the supply of ingested protein 

 exceeded the demand, and peptones re- 

 mained in the gut for some time unab- 

 sorbed. It is now generally agreed that in- 

 gested protein is normally split into crystal- 

 line products which are separately absorbed 

 from the gut, and built up again into the 

 various proteins required by the animal. If 

 the ingested protein does not yield a mix- 

 ture of crystalline products in the right 

 proportions to build up the proteins re- 

 quired, those crystalline products which 

 are in excess are further changed and ex- 

 creted. If the mixture contains none of one 

 of the products required by the animal, then 

 life can not be maintained. This has been 

 actually demonstrated in the case of zein, 

 one of the proteins of maize, which contains 

 no tryptophane. The addition of a trace 

 of tryptophane to a diet, in which zein was 

 the only protein, markedly increased the 

 survival period of mice. 



The adoption of this view emphasizes the 

 importance of a knowledge of the composi- 

 tion of the proteins, and especially of a 

 quantitative knowledge of their splitting 

 products, and much work is being directed 



to this subject in Germany, in America, and 

 more recently in Cambridge as a result of 

 the creation there of an Institute for Re- 

 search in Animal Nutrition by the Board 

 of Agriculture and the Development Com- 

 mission. This work is expected ultimately 

 to provide a scientific basis for the com- 

 pounding of rations, the idea being to 

 combine foods whose proteins are, so to 

 speak, complementary to each other, one 

 giving on digestion much of the products of 

 which the other gives little. Meantime, it 

 is desirable that information should be col- 

 lected as to mixtures of foods which are 

 particularly successful or the reverse. 



Here the question arises, for what pur- 

 pose does the animal require a peculiarly 

 complicated substance like tryptophane? 

 The natural suggestion seems to be that the 

 tryptophane grouping is required for the 

 building up of animal proteins. It has also 

 been suggested that such substances are 

 required for the formation of hormones, the 

 active principles of the internal secretions 

 whose importance in the animal economy 

 has received such ample demonstration in 

 recent years. The importance of even mere 

 traces of various substances in the animal 

 economy is another quite recent conception. 

 Thus it has been shown, both in Cambridge 

 and in America, that young animals fail to 

 grow on a diet of carefully purified casein, 

 starch, fat and ash, although they will re- 

 main alive for long periods. In animals on 

 such a diet, however, normal growth is at 

 once started by the addition of a few drops 

 of milk or meat juice, or a trace of yeast, 

 or other fresh animal or vegetable matter. 

 The amount added is far too small to affect 

 the actual nutritive value of the diet. Its 

 effect can only be due to the presence of a 

 trace of some substance which acts, so to 

 speak, as the hormone of growth. The 

 search for such a substance is now being 

 actively prosecuted. Its discovery will be 



