ENERGY RELATIONSHIPS. 343 



cost of digestion of 16 per cent or more. The average cost of digestion 

 for all of the beefsteak experiments is 13 per cent, more than twice the 

 value observed with the carbohydrates. In the experiment with whole 

 milk, a cost of digestion of 3 per cent was found, while the two experi- 

 ments with a mixed diet gave values of 6 and 4 per cent, respectively, 

 these being not far from the values obtained for the calorimeter 

 experiments. 



Emphasizing again the fact that in drawing conclusions from the 

 results given in these two tables it should be remembered that the 

 figures given are low rather than maximum values, since in the majority 

 of instances the basal value was not reached before the conclusion of 

 the experiment, we may conclude that the average cost of digestion for 

 the ingestion of pure carbohydrates or a predominatingly carbohydrate 

 meal will be not far from 6 per cent of the fuel value of the food ingested. 

 With fat it is approximately 2 per cent and with a protein-rich diet it 

 averages 12 per cent. With a mixed diet, which more property corre- 

 sponds to every-day usage, 6 per cent is doubtless near the correct 

 value. 



SPECIAL RELATIONS OF PROTEIN DIETS TO ENERGY 

 TRANSFORMATIONS. 



With diets consisting primarily of carbohydrates and fat no special 

 indices are available as to the proportion of fat and carbohydrate 

 burned in the body other than the relationship between the carbon- 

 dioxide production and the oxygen consumption; but when protein 

 enters into the katabolism, especially in excessive amounts, the nitro- 

 gen in the urine has commonly been taken as an index of the amount of 

 protein katabolized. The intimate relationship between protein katab- 

 olism and heat production has been so pronounced as to lead writers 

 to calculate quantitative relationships between heat production and the 

 nitrogen excretion of the urine. 



In the computation of the total energy transformation by means of 

 the respiratory exchange, emphasis is laid for the most part upon the 

 measurement of carbon-dioxide excretion and oxygen consumption, and 

 heat production is computed from the calorific value of the carbon- 

 dioxide or oxygen at the respiratory quotient actually measured. 

 There are two methods for computing heat production from the calo- 

 rific values for carbon dioxide and oxygen. In one no special attention 

 is paid to the protein disintegration, on the general ground that usually 

 about 15 per cent of the total energy is derived from protein metabo- 

 lism. When a high degree of accuracy is desired, however, it is custom- 

 ary to compute from the respiratory exchange and the nitrogen in the 

 urine the non-protein respiratory quotient, then compute the energy 

 production due to the katabolism of the protein by multiplying the 

 number of grams of nitrogen in the urine by a standard factor (26.51 



