In 1860 Voit3 took a Thomson calorimeter with him from London to Munich. After Frankland's determination of the heat value of the various food-stuffs and urea Voit4 prepared a table in 1866 for use in his lectures showing that the metabolism of the fasting man experimented on by Pettenkofer and Voit indicated the production of 2.25 million small calories, while the metabolism on a medium diet was 2.40 million calories.

In 1873 Pettenkofer and Voit5 calculated that 100 grams of fat were the physiologic equivalent of 175 grams of starch. Liebig at that time had suggested that the amount of these substances which could be burned by a man was proportional to the oxygen supply.

Voit, not content with his results, suggested to Schurmann in 1878-79 that he carry on experiments to see in what way carbohydrates and fat were interchangeable in nutrition. Schurmann died before the work was completed and the investigation was continued by Rubner. The isodynamic law, which showed that the food-stuffs may under given conditions replace each other in accordance with their heat-producing value, was the result.

1 Bischoff and Voit: "Die Gesetze der Ernahrung des Fleischfressers," 1860, p. 43.

2 Bidder and Schmidt: "Verdauungssafte und Stoffwechsel," 1852, p. 353. 3 Voit: "Munchener medizinische Wochenschrift," 1902, xlix, 233. 4 Voit: Ibid. 5 Pettenkofer and Voit: "Zeitschrift fur Biologie," 1873, ix, 534.

Rubner gives the following as the quantities of the different food-stuffs which are isodynamic: 100 gm. fat. 232 gm. starch. 234 gm. cane-sugar. 243 gm. dried meat.

After Stohmann1 published his research on the calorific value of foods, urea, etc., Voit commenced the construction of a calorimeter for the measurement of the heat eliminated from the body of a man whose metabolism was simultaneously determined. The results obtained by the use of this machine were never published.

Rubner2 in Voit's laboratory during this same period was making a series of valuable calorimetric determinations. The heat value to the body of burning starch and fat were obviously the same as that determined in the calorimeter, since in both cases the same end-products, carbon dioxid and water, resulted. The heat value of protein in the calorimeter was different from its fuel value to the body, since the end-products were different in the two cases. When protein is oxidized in the body the products of its metabolism are lost in three different ways - through the respiration, urine, and feces. The two last contain latent heat lost to the body, which must be deducted from the heat value of protein determined calorimetrically.

The custom of Stohmann and previous authorities had been to deduct the heat value of urea from the heat value of protein in order to obtain the actual physiologic or fuel value of protein for the organism. But in the earliest experiments of Pettenkofer and Voit3 it was recognized that in starvation and after the ingestion of meat there was a much larger output of carbon in the urine than corresponded to the quantity of urea present. The ratio of nitrogen to carbon was nearly constant in the urine when the conditions of feeding were similar. If urea alone were present, Rubner estimated there would be 0.429 gram of C to 1 of N or an N : C = 1 :0.429. In starvation the urine contains extractive nitrogen (creatinin, uric acid, etc., having relatively more carbon than urea) which has been derived from the breaking down of tissue protein, and the ratio is N:C = 1:0.728. When meat was ingested the fact that the food contained these extractives made the C : N ratio 0.610. And even after six days' ingestion of meat washed free from extractives the urine of the seventh and eighth days still showed an elimination of carbon other than that due to urea, as was indicated by the ratio 0.532. Therefore, from the metabolism following the ingestion of the proteins of washed meat small amounts of carbon compounds other than urea are eliminated in the urine.

1 Stohmann: "Journal fur praktische Chemie," 1885, xxxi, 273, and earlier papers.

2 Rubner: "Zeitschrift fur Biologie," 1885, xxi, 250, 337. 3 Pettenkofer and Voit: Ibid., 1866, ii, 471.

Rubner saw that it was the heat value of the urinary constituents themselves which had to be subtracted from the heat value of protein if the fuel value of protein to the body was to be determined.

The following table shows Rubner's results after burning the dry urine:

Calorific Value Of Urine

Benedict and Milner1 report that the average C : N ratio in man when he partakes of a mixed diet is 0.75 and the calorific value of a gram of urinary nitrogen is 8.09. When a diet which is high in carbohydrate is ingested the value of a gram of urinary nitrogen may be from 11 to 13 calories,2 an increase which is due to the appearance of products of the intermediary metabolism of glucose (see p. 208), although no glucose itself is present.1

1 Benedict and Milner: United States Dept. of Agriculture, Office of Experiment Stations, 1907, Bulletin 175, p. 144.

2 Tangl. "Archiv fur Physiologie," 1809, Supplement Bd., p. 251.

It was not alone necessary to know the heat value of the urine excreted, but also that of the feces. Rubner found that after giving 100 parts of dry muscle containing 5.5 grams of ash there was an elimination of 38.2 grams of the organic part in the urine and 2.7 grams in the feces. The following table represents this division of material in the excreta: