FUNCTIONS OF THE NUTRIENTS 191 



275. A single amino acid a limiting factor. — Evidence 

 on this question is seen in experimental work carried on 

 by Osborne and Mendell. These authors brought rats to 

 full size and kept them in health when the diet contained 

 18 per cent of casein. When the casein was reduced to 12 

 per cent, growth fell below the normal. When reduced to 

 9 per cent, growth was promptly limited by the protein 

 factor. If, however, cystine was added to the 9 per cent 

 of casein, the ration was rendered much more efficient 

 for growth, showing that the presence of an insufficient 

 quantity of this one building-stone was the limiting 

 factor. Similar results were secured in experiments with 

 edestin. When 15 per cent of the ration consisted of 

 edestin, normal growth was secured, but not with 9 per 

 cent. The addition of lysine to the 9 per cent of edestin 

 caused an improvement. Lact-albumin was efficient 

 because all the building-stones, including lysine and trypto- 

 phane, are relatively abundant in this protein. 



McCoUum has determined, through a series of experi- 

 ments in which he fed single foods to pigs, the proportion 

 of protein used by the animal for buildmg protein tissue. 

 His conclusions are as follows: Percent 



deposited 



Oil meal proteins • 16-17 



Wheat proteins ... 20 



Com proteins . ... 24 



Oat proteins . • ■ 25 



Wheat germ .... 40 



Casein ... ... .45 



Skimmed milk proteins . 63 



The author also gives figures showing that the proteins 

 of one food supplement those of another in producing 

 more growth when the two foods are combined than when 

 fed singly: 



