PROTEINS IN THE DIET — MITCHELL 227 



When fed at a level of 8 to 10 per cent, thus permitting a more 

 or less rapid growth, the proteins of the feeds examined arrange 

 themselves in the following order : 



The biological value of proteins fed at a level of 8 to 10 per cent 



Veal 84 



Milk 85 



Beef: 



8 per cent 81 



10 per cent 68 



Rice bran 67 



Yeast 67 



Cottonseed 6G 



Oat 65 



Soy bean 64 



Potato 67 



Alfalfa 62 



Corn 60 



Coconut 5S 



Navy bean (cooked) 38 



Tankage 31 



Even at this level the differences in nutritive value of proteins do 

 not seem to be extreme, if the proteins of the navy bean and of the 

 packing house by-product known as tankage are disregarded. The 

 differences among the biological values of these 13 protein mix- 

 tures are no wider than differences in their digestibility. The 

 rather distinct drop in the value of beef proteins between the 8 and 

 10 per cent level, we can not explain. Our work with veal seems 

 to indicate a slight superiority of its proteins over those of beef, 

 both for maintenance and growth. We are at present extending 

 this study to the proteins of pork and mutton. Results on two rats 

 with rice proteins at a level of 8 per cent, for two seven-day periods 

 each, consistently indicated a utilization of 85, but until further work 

 has been done on this cereal, we do not feel sufficient confidence in 

 this figure to include it with the others in the table. Unfortunately 

 no tests have been made upon the proteins of wheat as yet, though 

 experiments are now under way to supply this deficiency. 



The experimental work just reviewed indicates rather definitely 

 the superiority of meat and milk proteins over vegetable proteins. 

 However, the differences between the two class of proteins are not so 

 great as to constitute a weighty argument favoring animal foods, 

 nor, with few exceptions, are the differences between the vegetable 

 proteins studied of any great moment. The tendency of this in- 

 vestigation, and of others of recent date, is to minimize the dif- 

 ferences existing among the protein values of the staple foods of the 

 American diet. However, the biological value of the proteins of 

 the milled flours in promoting growth is undoubtedly distinctly 

 lower than that of the entire grains. 



An extensive investigation of the biological values of the proteins 

 of foods has recently been published by McCollum and associates 

 (11), using a method worked out at Johns Hopkins University. In 

 (his investigation the food studied was fed in a ration satisfactory 

 in respect to all food factors known, except for a possible deficiency 

 of protein, which was entirely supplied by the food in question. 



