From a comparison of the constituents of the food consumed ("intake") and of the substances eliminated from the body ("output"), the material actually oxidized and the energy liberated in the oxidation may be determined.

The intake is found by weighing and analyzing all food eaten; the output by collecting and determining the end products eliminated through the lungs, the kidneys, the intestines, and sometimes (in very exact experiments) the skin. The time unit in experiments upon the intake and output is almost always 24 hours, the experimental day beginning preferably just before breakfast. The feces belonging to the experimental days are marked, usually by giving a small amount of lampblack with the food as in ordinary digestion experiments, separated and analyzed. The end products given off by the lungs and kidneys during an experimental day are taken as measuring the material broken down in the body during the same period.

Some time is of course required for the elimination of the nitrogenous end products through the kidneys. This unavoidable "lag" in the elimination of nitrogen may introduce an error in determining the nitrogen balance unless the subject has been kept for a few days in advance upon the same diet which is to be used in the experiment.

Assuming that the total nitrogen and carbon of the absorbed food existed in the form of protein, fat, and carbohydrate, and that the amount of carbohydrates in the body is constant from day to day, it is only necessary to determine the carbon dioxide of the expired air and the carbon and nitrogen of the waste products, in order to calculate the amounts of material oxidized and of energy liberated in the body. Experiments of this sort have played a most important part in the development of our knowledge of nutrition. The calculations are usually based on the following average analyses of protein and body fat:

The following data were obtained with a man on ordinary mixed diet:

Calculation Of Energy Metabolism From Carbon And Nitrogen Balance. Man Of 64 Kilograms At Rest In Atwater Respiration Apparatus

A loss of 2.0 grams body nitrogen indicates (2.0 X 6.25 =) 12.5 grams body protein burned. Also there were 89.0 grams absorbed from food, and, therefore, in all 101.5 grams total protein burned.

Since the respiratory quotient showed that the body was in carbohydrate equilibrium at the beginning and end of each experimental day, i.e. at seven o'clock each morning, it may be concluded that the amount of carbohydrate burned was the same as that absorbed from the food, viz. 286.6 grams per day.

From the carbon balance, therefore, we estimate the amount of fat burned as follows:

Since fat contains 76.5 per cent carbon, 1 gram carbon ≈ 1.307 grams fat. \ 18.7 grams carbon = 24.4 grams fat.

The body therefore absorbed 78.8 grams fat stored 24.4 grams fat burned 54.4 grams fat In all the body burned per day.

By means of the carbon and nitrogen balance Sonden and Tigerstedt studied the energy metabolism of eight resting men between nineteen and forty-four years of age, with results which varied for the different subjects from 1853 to 2292 Calories per day. Many other experimenters have used the same method with similar results.

* Here the factors for fuel value are not reduced to allow for loss in digestion, because this loss has already been deducted in computing the amount of each nutrient actually absorbed and rendered available.