PROTEINS IN THE DIET — MITCHELL 229 



used to cover the protein requirements of the body, at ordinary 

 levels of protein intake, then the meat could be said to contain 19 X 

 .80=15.2 per cent of "net protein." Similarly with corn contain- 

 ing 10 per cent of protein, of which 85 per cent is digestible, 58 

 per cent of the digestible protein being utilizable for structural 

 purposes, the "net protein" content would be 10X.85X-58=4.93 

 per cent. Navy beans with 22 per cent of protein, of which 80 

 per cent is digestible, and of the latter only 38 per cent is available 

 for the repair and growth of the protein tissues, would contain 

 only 6.69 per cent of net protein. Navy beans, therefore, are not 

 as valuable a source of protein as their high content in this nutrient 

 would lead one to expect. 



Unfortunately, in the balancing of dietaries, the protein factor 

 can not be so simply assessed, because of the supplementary action 

 of one protein upon another. The value of a protein in the diet 

 depends not only upon its own inherent value, but upon its ability 

 to enhance the value of other proteins. Several proteins have been 

 shown to possess the property of correcting the chemical deficiencies 

 of other proteins, so that a mixture of two proteins may have a 

 greater biological value than the mean value of the proteins them- 

 selves. As an illustration of this supplementing effect of proteins, 

 we may cite an experiment on corn proteins, skim milk proteins, 

 and a mixture of the two in the proportion of 3 of corn protein to 

 1 of milk protein. The average biological value of the corn pro- 

 tein was 61, of the milk proteins 84, and of the mixture, 75, all 

 rations containing 10 per cent of protein. Now the weighted mean 

 of the values for corn and milk in the ratio of 3 to 1 is 67. The 

 difference between the mean value, 67, and the value actually ob- 

 tained, 75, represents the supplementing action of the milk proteins. 

 A more striking instance yet is afforded by a mixture of corn pro- 

 teins and tankage proteins. Alone, these proteins were found to 

 have values of 61 and 33, respectively. Fed in the proportion of 

 3 to 1, the mixture was found to have a value of 65, higher than 

 that for corn proteins alone. 



Kecent work indicates that while vegetable proteins do not in 

 general supplement each other effectively, the proteins of milk, 

 meat, and eggs do exhibit marked supplementary properties, thus 

 giving to animal foods a twofold importance in dietetics. When 

 it is considered in this connection that on the average some 43 per 

 cent of the protein of the American diet is derived from milled 

 cereals, and almost 9 per cent from legumes, making 52 per cent 

 of proteins of exceptionally low biological value for growth, the 

 importance of animal proteins in the diet is obvious, particularly 



