130 
necessary accompaniment of undigested material. The available energy 
of the food is the total heat of combustion of the food minus the heat 
of combustion of the unoxidized materials of feces and urine. No 
further correction for the labor of chewing and digesting is included. 
It is assumed that the quantity of carbohydrates in the body is the same 
at the beginning as at the end of the experiment. The gains and losses 
of body protein and body fat are computed from the gains and losses 
of the nitrogen and carbon. 8 Accordingly the figures show the 
average daily amounts of available protein and energy supplied by the 
food and the amounts actually used by the body when the subject had 
a minimum amount of exercise and when he was engaged in decidedly 
active muscular work. 
The materials actually oxidized in the body are the digested nutri- 
ents of the food minus the protein or fat gained or plus the protein or 
fat lost by the body. The data thus show very clearly the demands of 
the body under the different conditions and the increase in the demand 
which accompanied the performance of muscular work. 
These experiments simply show the quantities of material and energy 
metabolized by a small number of men under specific conditions of 
work and rest. Their bearing upon the general subject of dietary 
standards can be more advantageously discussed when it is possible to 
take into account not only these and other experiments with men in 
the respiration calorimeter, but also a large number of experimental 
inquiries and observations of dietary usage of people of different 
classes and occupations in different countries. 
There is no doubt that in many cases the body can be maintained in 
nitrogen and carbon equilibrium with much smaller quantities of nitro- 
gen and energy than those actually used by any of the men in these 
experiments. It is equally certain that in other cases the requirements 
are much larger. The tentative standards for daily diet which have 
been proposed by a number of investigators have served a useful pur- 
pose, but they will doubtless have to be modified as the fundamental 
data become more exact and numerous. 
One principle which thus far has not received adequate recognition 
in dietary standards may perhaps be expressed by saying that the 
standard must vary not only with the conditions of activity and envi- 
ronment, but also with the nutritive plane at which the body is to be 
maintained. A man may live and work and maintain bodily equilib- 
rium on either a higher or lower nitrogen level or energy level. One 
essential question is. What level is most advantageous ( The answer 
to this must be sought not simply in metabolism experiments and diet- 
ary studies, but also in broader observations regarding bodily and 
mental efficiency and general health, strength, and welfare. 
a By the method previously described, U. S. Dept. Agr., Office of Experiment Sta- 
tions Bui. 69, pp. 44, 45. 
