It is obvious that the use of the chamber would act as a great handicap if the determination of energy expenditure were con fined to it. To enable measurement of energy expenditure of sub jects engaged in actual work a portable apparatus was first de vised by Zuntz and later much simplified by Douglas (fig. 2). In this last form the subject wearing either a mask or special mouth piece, with nose clip, fitted with two one-way valves breathes into a gas-tight bag carried on his back. The air collected in the bag at the end of the experiment is measured by passage through a meter, a sample of the expired air is analyzed in the Haldane gas analysis apparatus and the amount of carbon dioxide and oxygen present is determined. As the composition of the atmos pheric air is known it is readily calculated how much carbon diox ide the subject excreted and how much oxygen is utilized in a given time and the caloric values can be determined as above. In order to relate the carbon dioxide output and oxygen utilization to the non-protein moiety of the food the protein metabolized during the period of the experiment is determined from the nitro gen output in the urine. For every gram of urinary nitrogen de rived from protein 8.45g. of oxygen are required and 9.35g. of car bon dioxide are given off. Hence to determine the non-protein utilization the appropriate amounts of carbon dioxide and oxygen are deducted from the total amounts. As the amount of nitrogen excreted during the period of examination is minute, in practice it is usually ignored.
When the alterations of the gaseous metabolism are considered they are commonly referred to variations from the so-called Basal or Standard Metabolism. The basal metabolism may be defined as that of a subject lying comfortably at rest in a warm bed and in the post-absorptive condition, i.e., about r 2 to 15 hours
after the last meal. With the subject in such a condition the metabolism reaches its lowest level. As it has been estimated that functional activities of the various organs may account for about 25% of the resting metabolism (thus the activity of the heart for about 3.6%, respiratory movements for about 0% and the kid ney for about 5%) Krogh maintains that it is better to call this resting metabolism standard rather than basal. It, of course, could only be truly basal if all functional activities were in abey ance at the time of the determination.
It is obvious, in view of the fact that humanity is of varying sex and size, that if a universally applicable standard is to be ob tained there must be some fundamental unit on which to base assessments. It has been shown that body-weight is not satisfac tory. The modern unit selected is the surface-area of the body. A very useful formula for the determination of this area based on height X weight X a constant has been worked out by E. F. and D. Du Bois. As the result of their work it is common practice now to take as the fundamental unit the calorie output per square metre surface area.
This basal metabolism is shown to be high in childhood and that as adolescence is reached it falls to a level which is more or less uniformly maintained until about the age of fifty. There after the decline is steady although small. The basal metabolism is also influenced by the sex of the subject, the nature of the food consumed, and environmental conditions like temperature, cli mate, etc. The most potent factor is, however, muscular work.