CHEMISTRY OF DIGESTION AND NUTRITION. 343 



H. Body-metabolism ; Nutritive Value of the Food-stuffs. 



Determination of Total Metabolism. — We have so far studied the 

 changes that the food-stuffs undergo during digestion, the form in which they 

 are absorbed into the blood, their history in the tissues to some extent, and the 

 final condition in which, after being decomposed in the body, they are eliminated 

 in the excreta. To ascertain the true nutritional value of the food-stuffs it 

 is of the utmost importance that we should have some means of estimating 

 accurately the kind and the amount of body-metabolism during a given period 

 in relation to the character of the diet used. Fortunately, this end may be 

 reached by a careful study of the excreta. The methods employed can readily 

 be understood in principle from a brief description. It has been made suf- 

 ficiently clear before this, perhaps, that by determining the total amount of the 

 nitrogenous excreta we can reckon back to the amount of proteid (or albu- 

 minoid) destroyed in the body. In the case of proteids or albuminoids that 

 undergo physiological oxidation all the nitrogen appears in the forms of urea, 

 uric acid, creatinin, xanthin, etc., which are eliminated mainly through the urine, 

 and may therefore be collected and determined. The following practical facts 

 are, however, to be borne in mind in this connection : The nitrogenous excre- 

 tion of the urine is mainly in the form of urea which can be estimated as such, 

 but it is much more accurate to determine the total nitrogen in the urine during 

 a given period, using some one of the approved methods for nitrogen-deter- 

 mination, and to calculate back from the amount of nitrogen to the amount of 

 proteid. By this means all the nitrogenous excreta which may occur in the 

 urine are allowed for ; and since the various proteids differ but little in the 

 amount of nitrogen which they contain, the average being from 15.5 to 16 per 

 cent., it is only necessary to multiply the total quantity of nitrogen found in the 

 excretions by 6.25 (proteid molecule : N :: 100 : 16) to ascertain the amount of 

 proteid destroyed. In accurate calculations it is necessary to determine the total 

 nitrogen in the feces as well as in the urine, and for two reasons: first, in ordi- 

 nary diets of some vegetable and animal proteid they may escape digestion 

 and this amount must be determined and deducted from the total proteid eaten 

 in order to ascertain what nitrogenous material has actually been taken into the 

 body; second, the secretions of the alimentary canal contain a certain quan- 

 tity of nitrogenous material, which represents a genuine excretion, and should 

 be included in estimates of the total proteid-destruction. Recent work 

 seems to show that in ordinary diets most of the nitrogen of the feces has 

 the latter origin. The nitrogen eliminated as urea, etc., in the sweat, milk, 

 and saliva is neglected under ordinary circumstances because the amount is 

 too small to affect materially any calculations made. To determine the total 

 amount of non-nitrogenous material destroyed in the body during a given 

 period, two data are required: first, the total nitrogen in the excreta of the 

 body; second, the total amount of carbon given oil' from the lungs and in the 

 various excreta. From the total nitrogen one calculates how much proteid was 

 destroyed, and, deducting from the total carbon the amount corresponding to 



