The respiratory quotient in the foregoing series gradually rises, as would be expected from the increasing prominence of the protein in the metabolism (p. 60). Meat alone will therefore support a dog. Rubner1 says that a man cannot live on meat alone, not because the intestinal canal cannot digest it, but because of the physical limitations of the apparatus of mastication. A dog weighing 10 kilograms may ingest 1000 grams of chopped meat in forty-five seconds, while a man requires between five and ten minutes rapidly to cut and partake of 200 grams of good sirloin steak.

A subject of interest in considering the value of protein in metabolism is that of the value of gelatin. Gelatin is an artificial derivative of collagen, an albuminoid largely found in the skeletal structure of animals. Gelatin contains very nearly the same quantity of nitrogen as protein; it breaks up on chemical treatment into the same amino-acids, except that it does not yield tyrosin, cystin, and tryptophan. In the diabetic, gelatin yields the same amount of sugar as does protein.2 To what extent gelatin may take the place of protein in the body's metabolism has long been the subject of inquiry.

It was shown first by Bischoff and Voit3 that no matter how much gelatin was ingested, it was always completely burned, and some of the body's protein in addition. Therefore gelatin never builds up new tissue, although it may somewhat diminish tissue waste. Gelatin may be formed from protein in the body, but it cannot be reconverted into protein nor act like protein in metabolism. Kirchmann,4 working in the laboratory of Erwin Voit, has shown to what extent gelatin spares protein in metabolism. If one takes the amount of protein metabolism in starvation as one, then the ingestion of about the same quantity of gelatin reduces the body's protein waste 23 per cent., and if eight times this amount of gelatin be given, the tissue waste may be reduced 35 per cent. In other words, the ingestion of 7.5 per cent, of the total heat requirement of the organism in the form of gelatin spares 23 per cent, of the body's protein, while the ingestion of 60 per cent, of the requirement will only cause a decrease of 35 per cent, in protein waste. Krummacher1 showed that the ingestion of the full heat requirement of the animal in the form of gelatin reduced the fasting protein metabolism by only 37.5 per cent. It is evident that no matter how much gelatin be given, tissue protein continues to be destroyed, and it is also evident that a small quantity of gelatin has almost as great an effect as a large quantity.

1 Rubner: von Leyden's "Handbuch der Ernihrungstherapie," 1903, i, 42. 2 Reilly, Nolan, and Lusk: "American Journal of Physiology," 1898, i, 395. 3Voit: Hermann's Handbuch, "Stoffwechsel," 1881, p. 396. 4 Kirchmann: "Zeitschrift fur Biologie," 1900, xl, 54.

An extremely interesting experiment of Kauffmann2 shows that when the lacking tyrosin, cystin, and tryptophan are mixed with gelatin in the proportions in which they occur in true protein, and are given to a dog or to a man, nitrogen equilibrium may be established. Abderhalden3 confirms this in similar experiments.

It is evident, therefore, that the value of the various proteins in nutrition may depend upon their constituent amino-acids, and this will be considered on another occasion (see p.

371).

It appears that protein bodies must be broken up into amino-acids before absorption in the intestine (p. 79). If this be true, then ingestion of the cleavage products of protein should maintain nitrogen equilibrium in the same way as the ingestion of meat. The first experiments in this direction were done by Loewi,4 who gave a dog pancreas which had been self-digested until all the protein had been converted into amino-acids, as was indicated by the almost complete disappearance of the biuret reaction. Fat and carbohydrates were given with the digest, and nitrogen equilibrium was obtained and even nitrogen retention accomplished. Thus, in one experiment covering a period of eleven days, proteolytic digestive products containing an average of 6.08 grams of nitrogen were given daily, of which only 5.19 grams were eliminated in the excreta, while the balance, or 0.89 gram of nitrogen, was retained in the body of the animal. This amounted to 9.79 grams of nitrogen in eleven days. Accompanying this nitrogen retention was one of 0.649 gram of phosphoric acid (P2O5), an amount larger than was necessary for the upbuilding of new tissue from the nitrogen compounds retained. Loewi concluded that he had demonstrated the synthesis of new protein within the organism.

1 Krummacher: "Zeitschrift fur Biologie," 1901, xlii, 242.

2 Kauffmann: "Pfluger's Archiv," 1905, cix, 440.

3 Abderhalden: "Zeitschrift fur physiologische Chemie," 1912, lxxvii, 22. 4 Loewi: "Archiv fur ex. Path, und Pharm.," 1902, xlviii, 303.

Henderson and Dean1 confirmed Loewi by finding that they could obtain nitrogen equilibrium by feeding a dog with the cleavage products of meat produced by treatment with sulphuric acid.

Abderhalden and Rona2 find that mice live on casein split with pancreatin as long as they do on casein alone; whereas they die much earlier if the casein has been submitted to peptic and then pancreatic digestion, or if it has been broken up by acid hydrolysis. Henriques and Hansen8 also find that casein broken up by acid will not maintain nitrogen equilibrium in rats, but that if the pancreas of the ox and a small piece of the intestine of the dog (to furnish erepsin) be digested for two months at 400, and the resulting material given to rats, nitrogen equilibrium will be maintained. The authors further find that the mono-amino-acid fraction (the filtrate after precipitation with phosphowolframic acid) and also the alcoholic extract of the last-named digest maintain rats in nitrogen equilibrium. The residue left after alcoholic extraction will not do so.

1 Henderson and Dean: "American Journal of Physiology," 1903, ix, 386. 2Abderhalden and Rona: "Zeitschnft fur physiologische Chemie," 1904, xlii, 528.

3 Henriques and Hansen: Ibid., 1905, xliii, 417.