ISODYNAMIC VALUES OF THE FOODSTUFFS 577 



insufficient utilization of the carbohydrates of the diet for the produc- 

 tion of heat and work. We have seen (Chapter XX), however, that 

 certain essential Substrates of Growth, or raw materials for the synthesis 

 of protoplasm are contained in the animal fats and, so far as we are 

 yet aware, in no other abundant constituents of the diet. The total 

 replacement of fats by carbohydrates, therefore, is likely to result in 

 unbalanced tissue-waste through the lack of non-synthesizable atom- 

 complexes which do not necessarily contribute any appreciable share to 

 the energy-output. The total replacement of animal fats by Vegetable 

 Oils is for a like reason impracticable. The proportion of animal fat 

 which is requisite for maintenance is, however, very small, and pro- 

 vided this small residuum is retained, the fats of the dietary may be 

 replaced by carbohydrates in equicalorific proportions without affect- 

 ing the balance of energy-input and -output. 



In the case of the Proteins a number of complications arise which 

 limit in a variety of directions the application of the principle of 

 isodynamic values. In the first place the proteins are the medium 

 through which the body acquires its nitrogen. Their complete replace- 

 ment by fats or carbohydrates is therefore obviously impossible. Then, 

 again, different types of protein are not even isodynamic with each 

 other, for those which lack or are deficient in certain amino-acids, such 

 as Gelatin, Zein or Gliadin will not replace the protein in a normal 

 mixed diet however great an excess of the incomplete protein may 

 .be employed (Chapter XX). No nitrogen balance is possible unless 

 the missing amino-acids are supplied, and upon a diet containing an 

 abundance of nitrogen the output will continuously exceed the intake. 

 If, however, the missing amino-acids are added to these proteins, as, 

 for example, tyrosine, cystine and tryptophane to gelatin, then the 

 attainment of nitrogenous equilibrium becomes possible because all of 

 the constituent parts of tissue-protein are then present in the diet. 



Although gelatin cannot replace other proteins in the diet, yet it is 

 possible to attain nitrogenous equilibrium on a smaller amount of 

 normal dietary protein if gelatin be also present. If the total heat- 

 requirement of the normally fed animal be supplied solely in the form of 

 carbohydrates and fats a certain daily loss of nitrogen will occur which 

 is due to the consumption of tissue-proteins. If 7.5 per cent, of the 

 heat- value be now supplied in the form of gelatin the excess of loss over 

 intake is diminished by 23 per cent. If, however, 60 per cent, of the 

 heat-value of the food is supplied by gelatin the saving of tissue-protein 

 is only 35 per cent., and if the whole of the heat-value be supplied 

 in gelatin only 37.5 per cent, of the tissue-wastage is spared. The 

 principle of isodynamic values is therefore manifestly inapplicable to 

 the quantitative relationship between gelatin and the other dietary 

 constituents unless a sufficiency of other protein be at the same 

 time supplied to furnish the full requirement of tyrosine, cystine, and 

 tryptophane. 



A further limitation upon the application of the principle of isody- 

 37 



