Energy Balance. 515 



accurate. While, then, it is possible with accurate determinations of carbon 

 dioxide and oxygen, in addition to the ultimate analyses of the urine, to com- 

 pute the total heat production with an accuracy in all probability well within 

 2 or 3 per cent and with a large number of experiments an average accuracy of 

 less than 1 per cent, accurate heat measurements are essential in any study of 

 the energy transformations in the body. The carbon dioxide determination 

 alone is insufficient, likewise the oxygen determination, although it would 

 appear that the relation between the oxygen consumption and heat production 

 is much more constant on the different days of the fast than that between the 

 carbon dioxide output and heat production. 



From the molecular weights and heats of combustion of fat and carbohydrate 

 the amount of energy liberated when 1 gram of oxygen is used to burn either 

 of these bodies may be computed. The energy resulting from the combustion 

 of 1 gram of carbon in the form of fat or carbohydrate may also be found. 

 From the results of the computations it will be seen that the heat production 

 per gram of oxygen in the combustion of both fat and carbohydrate is nearly 

 the same, while the heat production per gram of carbon dioxide produced is 

 considerably more in the case of the combustion of fat than in the combustion 

 of carbohydrate. Hence, it can be seen that irrespective of whether fat or 

 glycogen is burned in the body, the energy production per gram of oxygen 

 consumed will always be nearly the same, while the energy output per gram of 

 carbon dioxide will vary, depending upon the relative amounts of fat and 

 carbohydrate oxidized. 



Theoretically, there should be no discrepancy between the estimated energy 

 of the material oxidized in the body and the total heat production, when com- 

 puted from factors as accurately determined as were the nitrogen, carbon 

 dioxide, and oxygen of these experiments. The fact remains, however, that 

 experiments of this nature are being made with an extremely intricate and 

 complex organism and not in a test tube or flask. The probable limit of error 

 we have arbitrarily set as not far from 1.5 per cent. About half of these 

 experiments come inside of this estimated limit of error, and yet during fasts 

 the assumptions regarding the intermediary metabolism are greater perhaps 

 than in any other class of experiments. On the whole it is doubtful if the experi- 

 ments could have been conducted so as to secure much closer agreement. Cer- 

 tain known errors which could not be properly corrected unavoidably crept 

 into certain of the experiments. The carbon dioxide and oxygen thermal quo- 

 tients, however, suggest strongly the nature of the error in all experiments 

 with marked discrepancies. Undoubtedly many other unknown errors likewise 

 found their way into the conduct of the experiments and the computations. 

 On the whole, however, there is an agreement between the computed energy 

 and that directly determined that is as satisfactory as physiological experiments 

 could ordinarilv demand. 



