GENERAL METHODS HISTORY OF PROTEIN FOOD. 901 



consciously maintained is a striking fact. Among the rich as well 

 as the poor, and in races very differently placed as regards quantity 

 of available food, substantially the same amount of protein (80 to 

 100 gms.) is consumed daily by each individual. The element of 

 the diet which varies most widely, as Cohnheim points out in an 

 interesting discussion of this question, is the non-protein, particu- 

 larly the carbohydrate material. Those who are obliged to do 

 much muscular work to earn a living or for the sake of pleasure 

 (sports, athletics) add to their daily quota of protein an excess of 

 carbohydrate food to furnish the requisite energy. On the con- 

 trary, those whose daily life requires but little muscular exertion 

 cut down the carbohydrates and fats, and make their diet relatively 

 but not absolutely richer in protein. That mankind has made a 

 mistake in adopting instinctively the higher protein level can hardly 

 be claimed on the basis of our present knowledge. 



Nutritive Value of Different Proteins. If we consider all the 

 different kinds of animal and vegetable foods it is evident that 

 a great variety of proteins must be utilized in nutrition. For- 

 merly, it was the belief that all these different proteins (with the 

 exception perhaps of gelatin) have an equal nutritive value. But 

 the knowledge that the composition of these proteins varies in 

 regard to the number and character of their constituent amino- 

 bodies, and the fact that each animal out of the complex offered to 

 it in its food selects certain amino-acids in certain proportions from 

 which to reconstruct its own peculiar body-proteins, suggest natu- 

 rally the thought that the different proteins may have different 

 values in nutrition. Experiments have demonstrated, in fact, that 

 this is the case. From the standpoint of supplying the energy 

 needs of the body the proteins are, so far as we know, pretty much 

 of the same value, but from the standpoint of supplying material 

 for tissue construction they may differ a great deal. Some pro- 

 teins, when fed together with an adequate supply of non-protein 

 material and water and salts, furnish all the nitrogenous com- 

 pounds necessary for maintenance and growth; others under the 

 same conditions fail to support growth or maintenance, or both. 

 The former group may be spoken of as complete or adequate pro- 

 teins, the latter as incomplete or inadequate proteins. The differ- 

 ence between the two kinds seems to lie in the character of the 

 amino-acids of which they are composed. Some of the amino- 

 acids which the body tissues need for repair or growth may be made 

 in the body from other amino-acids, glycin, for example, but others 

 apparently must be furnished in the protein of the food, and if they 

 are lacking, tissue construction is not possible. 



It has long been known that gelatin is an inadequate protein 

 in this sense. It is digested easily and absorbed and eventu- 



