The discussion of the more important details of the breakdown of amino-acids in the organism reveals the modern beginning of mental penetration into the biochemical reactions in the organism.

The gross results of protein ingestion are to be ascertained by other means, by a study of the respiratory metabolism and by calorimeter observations.

Bidder and Schmidt1 gave meat to the full extent of its appetite to a cat which had previously been starved and reported the following figures for the respiratory exchange:

Many subsequent experiments have brought to light this characteristic increase in metabolism after the ingestion of protein in excess.

In 1862 Pettenkofer and Voit (see p. 155) noted that after giving meat in large quantity a portion of the carbon of the protein metabolized was retained in the body, which they interpreted as indicating a production of fat from protein. Frank and Trommsdorff2 and also Rubner3 gave meat in large amount to dogs, and determined the carbonic acid output of the animals during intervals lasting between three to six hours.

1 Bidder and Schmidt: "Verdauungssafte und Stoffwechsel," 1852, p. 356.

2 Frank and Trommsdorff: "Zeitschrift fur Biologie," 1902, xliii, 266.

3 Rubner: "Gesetze des Energieverbrauchs," 1902, p. 365.

The first named authors noted that although the urinary nitrogen elimination showed a maximum rise of nearly eight times that of fasting and varied greatly, the carbonic acid elimination was not so largely increased and was much more even.

The details of the results following the ingestion of large quantities of meat by a dog are to be found in the calorimetric observations of Williams, Riche, and Lusk.1 These authors made observations in hourly periods upon the nitrogen in the urine, the carbonic acid elimination and oxygen absorption, and the heat production of a dog following the ingestion of 1200 grams of meat. The results are in part presented in the accompanying curve:

1 Williams, Riche, and Lusk: "Journal of Biological Chemistry," 1912, xii, 349.

During the fourth hour the nitrogen in the urine reached a level of 1.80 grams and remained between 1.76 and 2 grams per hour during a period of eleven hours. During this period the heat production was nearly twice the normal basal metabolism, and the increase was proportional to the increase in protein metabolized as calculated from the increased nitrogen elim~ ination above that of the basal metabolism. However, during the second and third hours, in which the increase in heat production almost reached its maximum, the urinary nitrogen was only 0.89 and 1.55 grams respectively. This is due to the fact that urea was accumulating in the blood and the quantity of its elimination in the urine did not at first truly represent the intensity of the metabolism of protein (see p. 173). This fact is made certain by the curve of glucose and nitrogen elimination obtained by Janney1 after giving serum albumin to a phlorhizinized dog (see p. 243). In this curve the glucose elimination reached its maximum during the first hour, the nitrogen elimination during the fifth. It seems also probable that after giving 1200 grams of meat to the normal dog the establishment of a plateau of even nitrogen elimination indicates that during this period the influx of protein nitrogen from the intestine equalled its destruction within the cells and its outgo through the urine. When a fall in the nitrogen output set in, the metabolism also fell as the result of the decrease in protein metabolism.

During an experimental period of twenty-two hours the heat production calculated from the excreta was 738.5 calories, and directly measured by the calorimeter was 718.5 calories, a difference of 20 calories, or 2.7 per cent. During the first two experimental hours there was always a considerable discrepancy between indirect and direct calorimetry. It is now certain that this was due to the fact that the meat was given when cold (see p. 123). Allowing for this error, the indirect and direct methods agree within less than 2 per cent.

An interesting fact revealed in the analysis of the respiratory exchange is that, beginning with the second hour and continuing for fourteen hours after the ingestion of protein, the respiratory carbon dioxid is less than that which one would expect if all parts of the protein complex were oxidized. There is, therefore, carbon retention during this period. Such carbon might have been retained in the form of carbohydrate or of fat. Schreuer1 gave 900 and 1500 grams of meat to a dog and determined the metabolism by the Zuntz method from three to four hours after meat ingestion. He concluded from the respiratory quotient that carbon derived from protein was retained in the form of carbohydrate. New confirmation of the conversion of part of the protein molecule into glucose was afforded by the oxygen absorption of the dog of Williams, Riche, and Lusk during various periods following the ingestion of 1200 grams of meat. These facts are here set forth:

1 Janney: "Journal of Biological Chemistry," 1915, xx, 329.

Table Contrasting The Actual Oxygen Intake With That Required By Theory If The Carbon Retention Had Been In The Form Of Glycogen Or Of Fat. 1200 Grams Meat Ingested At Noon

34.5 grams glucose : 28.3 grams N : : 1.2 : 1

* Small leak in the apparatus during this period determined the day following to amount to about 1 gram O2 per hour.