Since the foodstuffs yield their energy through being oxidized in the body, it is evident that a measure of the energy metabolism can be obtained by finding either the amount of foodstuffs oxidized or the amount of oxygen which is consumed in the process. The apparatus devised and used by Zuntz for this purpose provides a mask, fitting airtight over the mouth and nose and connected by means of valved pipes with apparatus for measuring and analyzing the inspired and expired air. In this way one can determine the volume of oxygen entering, and the volume leaving, the lungs. The difference is the volume consumed in the body.

Benedict has devised an improved form of respiration apparatus in which the subject breathes, either through a mouth-or nose-piece, from a current of air which is purified and kept in circulation in the same manner as that of the respiration calorimeter chamber described below. The carbon dioxide which the man produces is absorbed quantitatively and the oxygen which he consumes is exactly replaced by admitting measured volumes of analyzed oxygen gas from a cylinder of compressed oxygen.

* Including some alcohol (taken in the form of beer), which is estimated as equivalent in fuel value to 1.75 times its weight of carbohydrates.

A given volume of oxygen used in the body may liberate somewhat different amounts of heat, according as it oxidizes fat, carbohydrate, or protein. For accurate estimations of the energy liberated it is therefore necessary to know the kind of material oxidized, as well as the amount of oxygen consumed. This is calculated from the respiratory quotient.

Fig. 7. - Diagram of Benedict respiration apparatus. Courtesy of Dr. F. G. Benedict.

Since the amount of protein broken down in the body can be estimated from the nitrogen excretion, the determination of the respiratory quotient along with the oxygen consumption shows the extent of the combustion in the body and the proportions of fat and carbohydrate burned.* From these data the energy can be calculated.

As a matter of fact it is not necessary to go through the actual calculation of the amounts of fat and carbohydrate burned since the energy derived from a liter of oxygen when used to burn carbohydrate and fat in different proportions can be calculated once for all and expressed in relation to the respiratory quotient as shown in the accompanying table.

Energy Values Of Oxygen And Carbon Dioxide At Different Respiratory Quotients (Zuntz And Schumberg)

* Or, with very little error, it may be assumed that 15 per cent of the oxygen goes to burn protein and the rest is divided between fat and carbohydrate. The values given in the table herewith agree with this assumption. Attention should be called to the fact that estimates of energy metabolism based on carbon dioxide production alone involve larger errors than those based on oxygen consumption alone.

It is then only necessary to determine the respiratory quotient and the volume of oxygen used in order to know the number of Calories of energy metabolized. This is sometimes called the method of indirect calorimetry.

This method of studying the total metabolism permits of experiments being carried out very quickly, and is therefore especially useful for the direct investigation of conditions which affect metabolism promptly, such as muscular work or the eating of food. The periods of observation cannot be very long, but the probable results for the 24 hours' metabolism can be estimated by the data obtained during frequent short periods at different times of the day and night. For a critical comparison of this method with the Pettenkofer and Voit method of studying metabolism by the determination of the carbon balance, the reader is referred to the discussion by Magnus-Levy in Von Noorden's Metabolism and Practical Medicine, Vol. I, pages 186-198.

From the results of many observations by the Zuntz method Magnus-Levy estimates the minimum metabolism of a man of average size kept absolutely motionless and fasting at 1625

Calories per day. Food barely sufficient for maintenance would increase this by 175, and such incidental muscular movements as would ordinarily be made by a man at rest in bed would involve another 200, making a total of 2000 Calories as the estimated food requirement of a man at rest with a maintenance diet. Magnus-Levy further estimates that the man, if doing no work (in the ordinary sense), but allowed to move about the room instead of remaining in bed, would require 2230 Calories per day.