The respiratory quotient for protein is, for the most part, the resultant of the oxidation of the various amino-acids of which protein is composed (see p. 77). This quotient, as calculated by Zuntz, is based upon the careful analytic data prepared by Rubner, already described. Zuntz, however, substracted the fat in the feces from the material attributable to protein metabolism. A recent computation by Loewy1 is as follows:

1 Lehmann, Muller, Munk, Senator, and Zuntz: "Virchow's Archiv," 1893, cxxxi, Suppl. Bd., p. 131.

2 Zuntz: "Pfluger's Archiv," 1897, lxviii, 201.

3 Zuntz: "Zuntz und Loewy's Lehrbuch der Physiologie des Menschen," 2d edition, Leipzig, 1913, p. 644.

These quantities of carbon and hydrogen would require 138.18 grams of O2 and produce 152.17 grams of CO2. Since 1 gram of oxygen is the equivalent of 0.699 liter and 1 gram of carbon dioxid amounts to 0.5087 liter, the R. Q. would be 77.39liters CO2/96.63 liters O2 = 0.801. From these data it may be calculated that for every gram of urinary nitrogen derived from protein 8.45 grams of oxygen are required for the oxidative process and 9.35 grams of carbon dioxid are eliminated in virtue of such oxidation.

In consequence of this, one may estimate the substances oxidized in the organism by deducting from the total elimination of carbon dioxid the quantity derived from protein (grams urinary N X 9.35), and from the total oxygen absorbed that required to oxidize protein (grams urinary N X 8.45). From the figures so obtained one determines the non-protein R. Q. From this the part played by fat and carbohydrate in metabolism may be computed. For when fat alone is oxidized the quotient will be 0.707, and when carbohydrate is oxidized it will be 1.00. Quotients which are intermediary between these two indicate that mixtures of the two materials are being destroyed (see p. 61). Knowing the quantities of these gases, their relative volumes (the R. Q.), and also the nitrogen elimination, it is possible to calculate exactly what amounts of protein, carbohydrate, and fat have been oxidized during the period of experimentation.

1 Loewy: "Oppenheimer's Handbuch der Biochemie," 1911, iv, 1, 279.

The significance of the Non.protein Respiratory Quotient as regards the heat value of I liter of oxygen, and the relative quantity in calories of carbohydrate and fat consumed. (Modified from Zuntz and Schumburg).

The R. Q., therefore, ranges from 0.707 for fat to 1.00 for carbohydrate. Exceptions may be noted under conditions involving the conversion of carbohydrate into fat in which case the quotient exceeds unity (see p. 306) and in severe diabetes, when the quotient may be less than 0.707 (see p. 470).

From this analysis of the oxidative process associated with the destruction of carbohydrate, fat, and protein in the organism, it is possible to compute the heat value of the respiratory gases when the various substances are oxidized. This knowledge may be compressed into the following table given by Loewy1:

Based upon the analytic figures given for protein, it may be computed that:

1 gram urinary nitrogen = 26.51 calories.

The calories derived from the oxidation of fat and carbohydrate given by Zuntz and Schumburg2 are reproduced on p. 61.

An example of the calculation of indirect calorimetry may be of value as an illustration. The subject was a dog weighing 12.75 kilograms and the period was one hour in duration. The calories directly determined by the calorimeter are also given:

The same method is employed in the calculations of the metabolism of man.

1 For slightly different values consult Benedict and Talbot: "The Gaseous Metabolism of Infants," Carnegie Institution, Publication 201, 1914, p. 26.

2 Zuntz and Schumburg: "Studien zu einer Physiologie des Marsches," Berlin, 1901, p. 361.

In a series of twenty-two different experiments with a dog Murlin and Lusk1 obtained the following results:

In fourteen of the twenty-two experiments the individual error was less than 2 per cent.

The following is a description of the principles of an Atwater-Rosa respiration calorimeter with the improvements added by Benedict,2 Williams,3 and others, which has been adapted for the use of patients in Bellevue Hospital :4