786 NUTRITION AND HEAT REGULATION. 



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. 823) 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-proteid Food on Nitrogen Equilibrium. 

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

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

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

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

 the amount of proteid required being relatively large. If now the 

 proteid food is mixed with any of the non-proteid foodstuffs it is 

 found that the amount of proteid necessary to maintain nitrogen 

 equilibrium may be reduced correspondingly. With reference to 

 the consumption of proteid in the body the non-proteid foods are 

 all proteid-sparers, and herein lies one great peculiarity of their nu- 

 tritional value. On a mixed diet of proteid and non-proteid 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 proteid tissue from 

 the body. This fact is explained by the consideration that the pro- 

 teid of our food fulfills two general functions: Its metabolism or 

 its oxidation furnishes energy, especially heat energy to the body, 

 and, moreover, a portion of it is used to reconstruct the living proto- 

 plasm that breaks down in the functional activity of the tissues. 

 The non-proteid food also furnishes heat energy and work energy, 

 and to a large extent at least can replace this part of the function 

 fulfilled by the proteid. 



The Nutritive History of the Proteid Food. The digestive 

 changes undergone by proteid 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. 



