GENERAL METHODS HISTORY OF PROTEIN FOOD. 811 



this kind a nearly perfect balance may be struck between the in- 

 come and the outgo of the body. Absolute accuracy is not sought 

 for, since the materials eaten vary somewhat in composition and 

 some little of the carbon or nitrogen excreted is found in the secre- 

 tions from the skin, the saliva, etc., which are not usually examined. 



More recent experiments made in this country under the direc- 

 tion of Atwater* have attempted to balance not only the material 

 income and outgo of the body during a given period, but also the 

 income and outgo of energy. For this purpose the individuals ex- 

 perimented upon were placed in a very carefully constructed respi- 

 ration chamber so that their expired air could be analyzed as well 

 as the urine and feces. The chamber, however, was also arranged 

 to act as a calorimeter (see p. 855) by means of which the heat given 

 off by the person could be measured. The heat value of the diet 

 being known, it is possible in this way to ascertain whether or not 

 this theoretical amount of heat is actually given off from the body. 

 Atwater's respiration chamber is described as a respiration calorim- 

 eter; some of the results obtained from its use are referred to later on. 



The Effect of Non-protein Food on Nitrogen Equilibrium. 

 By use of the methods referred to above the general influence of 

 the non-protein foods (fats, carbohydrates, albuminoids) upon the 

 protein consumption of the body has been made evident. An ani- 

 mal may be brought into nitrogen equilibrium on protein food alone, 

 the amount of protein required being relatively large. If now 

 non-protein foodstuffs are added to the diet it is found that the 

 amount of protein necessary to maintain nitrogen equilibrium may 

 be reduced correspondingly. With reference to the consumption 

 of protein in the body the non-protein foods are all protein-spar ers, 

 and herein lies one great peculiarity of their nutritional value. 

 On a mixed diet of protein and non-protein food the proportion of 

 the latter may be increased and that of the former decreased to a 

 marked extent without breaking down nitrogen equilibrium 

 that is, without causing a loss of protein tissue from the body. 

 This fact is explained by the consideration that the protein of our 

 food fulfills two general functions : Its oxidation furnishes energy, 

 especially heat energy to the body, and, moreover, a portion of it 

 is used to reconstruct the living protoplasm which breaks down in 

 the functional activity of the tissues. The non-protein food also 

 furnishes heat energy and work energy, and can replace this part 

 of the function fulfilled by the protein. 



The Nutritive History of the Protein Food. The digestive 

 changes undergone by protein and its subsequent absorption have 

 been described in the section on Digestion. It will be remembered 

 * Atwater, Bulletins 45, 63, 69, United States Department of Agriculture. 



