Rubner determined the amount of heat produced from 1 gram of ash-free feces after meat ingestion and found it to be 6.127 calories, while 1 gram of ash-free feces after protein (washed meat) ingestion yielded 6.852 calories. The total calorific value of 1 gram of beef muscle when Rubner burned it in the calorimeter was 5.345 calories. He had now the principal data required to determine its heat value in the body. If from 100 grams of meat 2.7 grams appear as feces having a calorific value of 6.127 calories per gram, then there is here a loss of 6.127 X 2.7 = 16.83 calories. If from every 100 grams of meat containing 15.4 grams of nitrogen 15.16 grams of the latter appear in the urine and such urine produced by ingesting meat has a calorific value of 7.46 calories for every gram of nitrogen present, then the energy loss in the urine would be 7.46 X 15.16 = 112.94 calories. For dry muscle substance we find therefore:

1 Reale: "Biochemische Zeitschrift," 1913, lvii, 143.

From this value there must be a slight deduction for the heat present in the protein in its colloidal state but lost on drying, and for the heat of solution necessary to dissolve urea and other urinary constituents. Rubner estimates these as:

Subtracting 4.67 from 404.73 leaves 400.06 calories as the maximum of energy obtainable from 100 grams of the dried solids of meat. The calorimeter shows a heat value of 534.5 calories for the same protein. Of this, 400.06 calories, or 74.9 per cent., are available in the organism, while the remainder, or 25 per cent., goes to waste.

A further calculation shows that every gram of nitrogen in the urine and feces represents an elimination of heat from protein metabolism equal to 25.98 calories. The heat value of protein under the different physiologic conditions was estimated by Rubner after the above fashion, and may thus be tabulated:

Calorific Value Of Protein In Nutrition

If we know the amount of nitrogen in the excreta we can calculate from these standard figures of Rubner the heat value of the protein metabolism to the body. Rubner found that the heat value of 1 gram of pig's fat (lard) was 9.423 calories. Since fat contains 76.5 per cent, of carbon, it could be calculated that for every gram of carbon eliminated in the respiration, which was the result of fat metabolism, 12.3 calories must have been liberated in the body. These figures enabled Rubner to calculate the amount of heat liberated by the fasting man of Pettenkofer and Voit, whose metabolism we have already discussed. The N excreted was multiplied by 24.98 and the fat carbon by 12.3 which gave the total heat value of the period:

Rubner applied such calculations as these to the material at hand in the literature of the time, and discovered that the heat value of the metabolism of the resting individual is proportional to the area of the surface of his body. For example, a man in starvation, or on a medium diet, an infant at the breast, and a starving dog were shown to give off similar quantities of heat per square meter of surface. To these Rubner subsequently added the results of his researches upon a dwarf. The following tables illustrate this point:

This law, that an animal in starvation or on a medium diet and at an environmental temperature of 18° gives off the same quantity of heat per square meter of surface, can be extended so that it applies to all warm-blooded animals. Thus E. Voit1 has collected data for the following table:

1 E. Voit: "Zeitschrift fur Biologie," 1901, xli, 120.

Recent work has confirmed the validity of this "law of surface area," but has somewhat modified the idea of the conditions under which it finds expression (see Chapter IV (The Regulation Of Temperature)).

Rubner from his work on protein considered that the heat value of 1 gram in an average mixed diet might well be placed at 4.1 calories. Of course, such a mixed diet would contain casein (4.4 cal.), the organic substance of meat (4.233 cal.), and vegetable proteins (3.96 cal.). The daily food allowance for animal protein was put at 60 per cent., for vegetable protein at 40 per cent., of the total protein in the mixed dietary. For the value of neutral fats Stohmann's figures for olive oil, fat of animal tissue, and butter fat were averaged as follows:

For the heat value of 1 gram of fat in a mixed diet Rubner therefore adopted the value 9.3.

The following heat values have been found for carbohydrates:

The variations in heat value are principally due to variations in the water content of the different molecules. Considering the predominating importance of starch in the average diet, Rubner gave the value of 4.1 to the group of carbohydrates in the foods.

Rubner's "standard values" have been widely used throughout the world in determining the average fuel value of a mixed diet. They are:

Their accuracy has been verified by Rubner1 in the most careful manner.

Rubner,2 still working in the Munich laboratory, showed that if the diet were increased from a medium to an abundant amount, the metabolism as indicated by the heat production rose. This dynamic action resulting from the excessive ingestion of a food-stuff was greatest with protein.

Finally Rubner, in his own laboratory at Marburg, evolved an animal calorimeter which could accurately measure the amount of heat a dog produced in twenty-four hours. The dog was placed within the chamber of the calorimeter, and this chamber was attached to a respiration apparatus, so that the metabolism could be calculated according to the method of Pettenkofer and Voit. From the metabolism the heat production could be estimated. The results were a triumphant demonstration of the truth of the law of the conservation of energy. The amount of heat calculated by Rubner3 as the quantity that should have been derived from the metabolism of the dog during the day spent in the calorimeter was the amount actually given off by the dog to the calorimeter. The metabolism, the cause of the motions of life, was the source of the heat-loss of the body. The results achieved constitute a final verification of the methods of calculating the total metabolism originated by Pettenkofer and Voit.

An epitome of Rubner's experiments is here presented:

Comparison Of Estimated Heat From Metabolism With Heat Actually Produced

1 Rubner: "Zeitschrift fur Biologie," Festschrift zu Voit, 1901, xlii, 261.

2 Rubner: "Sitzungsberichte der bayer. Akademie," 1885, p. 454. 3Rubner: "Zeitschrift fur Biologie," 1894, xxx, 73.