POTENTIAL ENERGY OF THE FOODSTUFFS 93 



servation of energy is true for the body, the heat value of these substances 

 as determined by the calorimeter must be their heat value in the body. 



Quite otherwise is it with proteid. The end products of its metabolism 

 are not all completely oxidized, and hence contain not a little potential energy. 

 To obtain the heat value of proteid for the body we must therefore deduct 

 from its calorimetric heat value the heat value of the waste products resulting 

 from its metabolism. This Rubner has done in the following way. 



He fed a small dog with a proteid material whose calorific energy had 

 been previously determined by combustion, and then determined the calorific 

 energy of the corresponding urine and faeces. The former for 1 gram of 

 proteid decomposed was 1.0945 Cal. ; the latter, also for 1 g. of proteid, was 

 0.1854 Cal. Finally, he deducted 0.05 Cal. for the hydrolytic absorption on 

 the part of the proteid while in the body and for the solution of urea. For 

 1 g. of dry proteid we would have therefore a physiological heat value of 

 5.754 -(1.0945 + 0.1854 + 0.05)= 4.424 Cal. 



In an analogous way the net physiological heat value of muscle deprived of 

 its fat only was found to be 4.001 Cal., and that of the proteid of the body 

 destroyed in fasting 3.842 Cal. For every gram of 1ST found in the excreta after 

 feeding the former we should estimate, therefore, 25.98 (6.25 X 4.001) Cal. and 

 in fasting 24.94 (6.25 X 3.84) Cal. 



Human urine yields on the average 8.0 Cal. for every gram of nitrogen con- 

 tained. Human faeces, per 1 g. of N, yield all the way from 66 to 159 Cal., but 

 per gram of organic substance the fairly constant value of 5.2-7.7 (mean 6.5) 

 Cal. (Rubner, Atwater, Loewy). 



In most metabolism experiments one has to deal not with pure lean meat, 

 pure starch, or a definite kind of fat, but with a mixture of various fats, 

 carbohydrates, etc. One must be content with the determination by direct 

 analysis of the quantity of total proteid, total fat and total carbohydrate; 

 for absolutely exact analysis of separate kinds of proteid, etc., would make all 

 metabolism experiments impracticable. From such determinations as those 

 above mentioned, Rubner calculated the mean dynamic value of the chief 

 groups of the organic foodstuffs to be as follows: 



1 g. proteid 4.1 Cal. 



Ig. fat 9.3 



1 g. carbohydrate 4.1 " 



From the standpoint of the conservation of energy, it is to be assumed 

 beforehand that these theoretical calorific values must be correct also for these 

 substances when burned in the living body. In fact we have direct experi- 

 mental proofs of this ; and precisely because these proofs verify the assumption, 

 they are of the greatest importance for the whole subject of physiology. 



As long ago as 1883, Rubner showed by a long series of experiments, the 

 details of which we cannot enter into here, that the different organic foodstuffs 

 can replace one another in isodynamic quantities i. e., in quantities which 

 yield equal amounts of calorific energy. The foregoing assumption was suf- 

 ficiently substantiated by these results alone. But Rubner carried his investi- 

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