THE PLACE OF PROTEIN IN NUTRITION. 



By John Carl Ross, B.A., M.S., Ph.D. 



Only fifty or sixty years back the farmer had no means 

 whatever to guide him in choosing^ rations for his stock. He 

 fed his animals as he had seen others do before him, Ijut had 

 no notion of what it was that aiourished the animals, why one 

 feed was better than another, and so forth. The middle of last 

 century may be said to mark the beginning- of animal nutrition 

 as a science, and since then its development has been remark- 

 ably rapid, especially during the last few years. 



But as our knowledge of animal nutrition has advanced, 

 apparently innumerable new j^roblems have presented them- 

 selves. Thus, we have been accustomed to judging the value of 

 a feed largely on its " nutritive ratio," which expresses the rela- 

 tion 'between digestible carboliydrates + digestible fat, expressed 

 in terms oif carbohydrate and digestible i)rotein. Different 

 digestible carbohydrates and fats ijrobably do not differ much 

 in nutritive value. But it is not true, as was thought until quite 

 recently, that one protein is as valuable as another for the main- 

 tenance and growth of farm animals. It is now we'll kiiown 

 that proteins differ, and may differ markedly, as regards their 

 nutritive value. For example, if we feed to an animal the pro- 

 tein casein, which is obtained from milk, supplemented with the 

 other necessary non-'uitrogenous nutrients, we obtain good 

 growth and fattening. But if we substitute g'liadin, one of the 

 proteins of wheat, for the casein, no growth is produced, though 

 the ration suffices for maintenance. If we use zein, one of the 

 proteins of maize, neither growth nor maintenance will be 

 obtained, and the animal will decline in weight more or less 

 rapidly. 



Researches during comparatively recent years have shown 

 that proteins are built up essentially of large numbers of sinij^ler 

 compounds, called amino-acids. At least 17 amino-acids are 

 known to have a rather wide distribution among the different 

 ])roteins, and a few others ha\e been identified in isolated in- 

 stances. It has been definitelv established that the differences 

 in nutritive value of proteins are due to differences k\ their 

 amino-acid content. It is obvious, then, that the nutritive value 

 of the proteins of ifeeding-stuff's for farm animals can be deter- 

 mined only by thorough and extensive studies of the amino- 

 acids which constitute the proteins. 



This Oldens a new and almost unlimited field of study. The 

 investigator in animal nutrition to-day is being confronted with 

 the problem of determining the ])recise nutritive value of each 

 of the many amino-acids occurring naturally in feeding-stuffs. 

 Already significant results have been obtained. For example, we 

 are now reasonably sure that the amino-acid " tryptophane " is 

 essential for maintenance, and the amino-acid " lysine '" essen- 



