GENERAL METHODS —HISTORY OF PROTEIN FOOD. 899 



obvious that we must find out the truth in this matter by obser- 

 vation and experiment. There is one consideration that should 

 induce us to be cautious in making a radical change in the diet- 

 ing of collections of individuals, such as armies, institutions, 

 etc., and that is, that our instinctive appetite seems to lead us 

 to seek the higher protein diet. We know very little, indeed, about 

 the mechanism of these appetites, but, as a matter of fact, they 

 and not the results of science control the amount and kind of 

 food that we eat. Science is rapidty gaining information that 

 enables us to guide or control the appetite for food in a conscious 

 and reasonable way, but at present our knowledge is not sufficient 

 to warrant disregarding the bodily sensations when, as in the case 

 in question, they seem to cause a similar reaction in normal men 

 under varying conditions. Studies of dietaries seem to show that 

 mankind, left to the guidance of the natural appetites, has always, 

 when possible, adopted the high protein level of 90 to 100 gm. per 

 day. Indeed, the uniformity with which this level has been un- 

 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 is 

 consumed daily by each individual. The element of the diet 

 which varies most widely, as Cohnheim points out in an interest- 

 ing discussion of this question, is the non-protein, particularly the 

 carbohj^drate material. Those who 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 contrary, those whose daily 

 hfe requires but little muscular exertion cut down the carbo- 

 hydrates and fats, and make their diet relatively but not abso- 

 lutely 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 



