The respiratory quotients (see Fig. 15) fall during the hours of carbon retention to below that of protein itself (which is 0.80), because the uncoridized carbohydrate is retained in the organism as glycogen. If the carbon were retained in the organism as fat, the respiratory quotient would rise. If one considers the period between the hours of 6.45 to 9.45 p. m., one obtains the following picture of what occurs:

Fig. 15. - Showing the R. Q., the total metabolism determined by indirect (heavy black line) and direct (broken line) calorimetry, as well as the nitrogen elimination (dotted line), during hourly periods after the ingestion of 1200 grams of meat.

1 Schreuer: "Pfluger's Archiv," 1905, cx, 227.

Metabolism Of A Dog During A Three-Hour Period Of Maximal Protein Carbon Retention After 1200 Grams Meat

It is obvious from these figures that the oxygen absorption and the heat production prove the retention of carbon either in the form of glucose or glycogen in the organism. During the fourteen hours of carbon retention following the ingestion of 1200 grams of meat, the actual oxygen absorption was 186.2 grams against a value of 184.5, calculated on the assumption that carbon was stored as glycogen or a difference of 0.9 per cent. If the carbon had been retained as fat, 169.7 grams of oxygen would have been required, or 10 per cent. less.

During these fourteen hours 34.5 grams of glucose were stored as glycogen in the organism and 28.3 grams of N were eliminated in the urine. This yields a D : N ratio of 1.2 :1. Since 3.6 is the maximum yield of glucose per gram of N in diabetic urine, it is evident that one-third of the glucose derivable from protein in metabolism was retained in the organism and deposited in the liver and other glycogen reservoirs. This represents 20 per cent, of the total energy contained in the protein metabolized.

The production of glucose from protein is not an emergency process as some writers maintain, but it is a normal function.

A question which has aroused great interest is that concerning the production of fat from protein. Pettenkofer and Voit1 found that after ingesting considerable quantities of protein, although the nitrogen of the protein was eliminated in the urine, a part of the carbon was retained in the body and not excreted by the usual channels. They estimated that meat protein contained 3.68 grams of carbon to each gram of nitrogen. If less than 3.68 grams of carbon appeared in the total excreta when 1 gram of nitrogen was eliminated, then some protein carbon must have been stored in the body. This carbon might have been retained in two forms - as glycogen or as fat. Claude Bernard had shown that glycogen increases in the liver after the ingestion of protein. The retained carbon as observed by Pettenkofer and Voit was in such large quantity as to preclude the possibility of its retention entirely as glycogen, and therefore they concluded that fat must have been prepared from protein and stored up in the body. This afforded an experimental basis for the theory of a production of fat from protein in fatty degeneration.

Later Rubner,2 in Voit's laboratory, showed that the relation 3.68 C : 1 N in protein, as used by Pettenkofer and Voit, was inaccurate, and that meat fully extracted with ether contains only 3.28 of carbon to one of nitrogen (see p. 39). The polemical arraignment by Pfluger3 of Voit's older work was based upon these results of Rubner. Instead of there being a great retention of protein carbon, there was none in some experiments and very little in others. The formation of fat from protein was evidently less easy of demonstration than it had seemed.

1 Pettenkofer and Voit: "Annalen der Chemie und Pharm.," 1862, II Supplement, pp. 52 and 361; "Zeitschrift fur Biologie," 1871, vii, 433.

2 Rubner: Ibid., 1885, xxi, 324.

3 Pfluger: "Pfluger's Archiv," 1892, lii, 239.

The subject was investigated anew by Cremer,1 who starved a cat for many days, and then gave the animal all the lean meat it would eat, or about 450 grams a day. The cat was kept in a respiration apparatus and the total excreta were collected. The carbon belonging to the meat ingested was calculated at the low ratio of 3.18 to 1 of nitrogen. The average daily metabolism during the eight days of meat ingestion is indicated in the following table:

There was a daily excretion of 13 grams of nitrogen corresponding to the liberation of 41.6 grams (13 X 3.18) of protein carbon. But only 34.3 grams of carbon were actually eliminated from the body, and a difference of 7.3 grams was retained in the body; 17.5 per cent, of the protein carbon therefore was not eliminated. For eight days the whole carbon retention was 58 grams, which corresponds to a glycogen production of 130 grams. The cat, however, contained only 35 grams of glycogen, determined after killing it at the end of the experiment. The balance of the carbon must have been stored as fat.

Cremer2 notes that a cat fed as above contains 1.47 per cent, of muscle glycogen, which is as much as the maximum (1.37 per cent.) found by E. Voit in geese after the ingestion of starch. One should here recall that Pfluger (see p. 175) found as much as 10 per cent, of glycogen in the liver of a previously fasting dog after it had been fed with codfish. This is as much glycogen as would have been deposited after carbohydrate ingestion.

1 Cremer: "Zeitschrift fur Biologie," 1899, xxxviii, 309.

2 Cremer: Ibid., 1899, xxxviii, 313.

Since it is known that sugar in excess may be converted into body fat and that meat may yield 58 per cent, of sugar in metabolism, there is every reason to believe that if protein be ingested in excess the deaminized residues of many of the amino-acids may be converted into glycogen, and then, if this pathway be closed through saturation of the body-cells with glycogen, fat is formed instead (see p. 304).

It is quite possible that the origin of fat from protein is in its nature the same as the origin of fat from carbohydrates.

In the first edition of this work (1906, p. 123) it was computed from the investigations of Cremer with the cat and from those of Rubner with a dog that 40 per cent, of the protein carbon which was capable of conversion into glucose could be retained in the organism either as glucose or as fat. This is to be compared with 33 per cent, of such glucose retention indirectly measured by Williams, Riche, and Lusk.