892 NUTRITION AND HEAT REGULATION. 



obtained by multiplying by 6.25, as explained above. If then 

 the carbon belonging to the amount of protein metabolized was 

 deducted from the total carbon excreta, what was left represented 

 either fat or carbohydrate burnt in the body, and, knowing the 

 amount of these materials taken in the diet, it was possil^le to 

 ascertain whether the corresponding amount of carbon had all 

 been excreted. By experiments of this kind a nearh' perfect 

 balance may be struck between the income 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 secretions from the skin, the 

 saliva, etc., which are not usually examined. 



More recent experiments made in this countr}^ 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. 955) 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) upon the protein 

 consumption of the body has been made evident. An animal 

 may be brought into nitrogen equilibrium on proteua 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-sparers, 

 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 equiUbrium — ■ 

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

 This fact is explained by the consideration that in our body the 

 food fulfils two great functions. First, it furnishes the material 

 for the formation of new living matter or the replacement of 

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



