INDIVIDUALS AND MEASUREMENTS CONSIDERED. 27 



Before proceeding to technical details a few words on the general 

 principles involved may be useful to the reader who approaches this 

 subject for the first time. 



The calorie is the unit of measurement of energy transformation. 

 Theoretically the measurement of heat-production by the calorimeter 

 is the only correct method of measuring the amount of the katabolism. 

 Practically the technical difficulties of the actual measurement of the 

 quantity of heat produced by a living organism are so great that for 

 many purposes direct may be replaced by indirect calorimetry that is, 

 by the calculation of heat-production from the amount of the respira- 

 tory exchange and the ratio of the volume of carbon dioxide exhaled 

 to the volume of oxygen absorbed. 



The application of this method depends upon the fact that the heat 

 set free in the combustion of a given substance may be determined 

 with precision in the laboratory. Thus to make possible the calculation 

 of the total heat-production from the measurements of the two gases 

 in the respiration chamber, or when possible from measures of the two 

 gases and of nitrogen excretion, it is necessary to ascertain only the 

 calorific values of unit volumes of oxygen and carbon dioxide for the 

 combustion of the substances which are oxidized in the human body. 



The consideration of the CO 2 /0 2 ratio, or the respiratory quotient as 

 it is commonly designated, as well as the actual volumes of the two 

 gases, is necessary because of the fact that the calorific value of either 

 of these gases is determined by the nature of the substances oxidized. 

 Thus a liter of C0 2 derived from the combustion of carbohydrates 

 (starch) corresponds to 5.043 calories, 6 a liter of C0 2 derived from fat 

 corresponds to 6.680 calories, and a liter of C0 2 derived from protein 

 has an equivalent of 5.690 calories. The calorific equivalents for a 

 liter of oxygen are 5.043 calories for carbohydrates, 4.755 calories for 

 fat, and 4.600 calories for protein. 



Thus the ratio of the carbon dioxide set free to the oxygen used in 

 the combustion of carbohydrates, fats, and protein is, within limits, 

 constant and specific. For the combustion of all carbohydrates, the 

 C0 2 /0 2 ratio must be unity. Since the composition of the several fats 

 and proteins varies, the C0 2 /0 2 ratio must also vary slightly. 



There are other difficulties to be considered in the indirect deter- 

 mination of heat-production. The synthesis of fats from carbohydrates 

 greatly disturbs the C0 2 /O 2 ratio. 



The use of indirect calorimetry for work in man has, however, been 

 fully justified by the experimentation of Atwater and his associates 7 

 and shown to be applicable to short periods by Gephart and Du Bois. 8 



8 Benedict and Tompkins, Boston Med. and Surg. Journ., 1916, 174, p. 858; average values 

 obtained from table 1. 



7 Atwater and Benedict, U. S. Dept. Agr., Office Expt. Sta., 1899, Bui. 69; 1902, Bui. 10Q; 



1903, Bui. 136. Benedict and Milner, U. S. Dept. Agr., Office Expt. Sta., 1907. Bui. 175. 



8 Gephart and Du Bois, Arch. Intern. Med., 1915, 15, p. 850 and p. 854. 



