Grafe5 has announced that urea when given with carbohydrate protects body protein from waste just as ammonium citrate does. This is denied by Abderhalden.6 Henriques and Andersen7 explain Grafe's results as due to the growth of bacteria within the medium of the intestinal tract of herbivora.

5 Grafe and Turban: "Zeitschrift fur physiologische Chemie," 1913, lxxxiii, 25.

6 Abderhalden: Ibid., 1913, lxxxiv, 218.

7 Henriques and Andersen: Ibid., 1914, xcii, 21.

Rats may be maintained when given such bacterial masses as the source of their protein food.

Lower organisms have the power to synthesize protein from sugar and some simple source of nitrogen. Thus, Armand-Delille1 found that the tubercle bacillus grew as well in a culture-medium of sugar, glycerin, glycocoll, and arginin, with addition of appropriate salts, as it did in a solution of 1 per cent, of peptone in bouillon.

Delbruck, in Germany, discovered that yeast cells developed rapidly and formed body protein when they were placed in a solution of sugar and ammonium sulphate. The mass thus developed is stated to have been used on a large scale as a source of protein food for cattle during the time of fodder scarcity introduced by the present war.

Lower organisms may therefore form the various aliphatic, aromatic, and heterocyclic amino-acids from carbohydrate and ammonium salts.

Within the body of the mammal there is evidence that some of the simpler deamination reactions are reversible (see p. 194), but the experiments with gelatin demonstrate that when tryptophan and phenylalanin are lacking these important building-stones of protein cannot be synthesized, for nitrogen equilibrium can only be obtained when they are admixed with the gelatin food. The consideration of other "deficient" proteins will be given elsewhere.

The cause of the great reduction in the fasting quantity of protein metabolism when carbohydrates alone are ingested has been thus stated by Knoop:2 "The animal body may therefore synthesize amino-acids from ammonia. If amino-acids can be produced from oxyacids, such as originate from carbohydrate metabolism, for example, then it is possible to comprehend chemically not only the production of sugar from protein but also reactions in a reverse direction. The minimal nitrogen metabolism of fasting may be reduced either through the ingestion or through the intermediary production of non-nitrogenous acids, which unite with ammonia prior to its synthesis to urea and form amino-acids,"

1 Armand-Delille, Mayer, Schaeffer, and Terroine: "Archive de Physiologie et de Pathologie generale," 1913, xv, 797.

2 Knoop: "Zeitschrift fur physiologische Chemie," 1910, lxvii, 489.

Since amino-acids when ingested tend to reduce protein metabolism this seems a plausible hypothesis. However, one should bear in mind the experiment of McCollum (see p. 188), in which 40 per cent, of the urinary endogenous protein nitrogen could be removed in the form of glycocoll when benzoate of soda was ingested without affecting the amount of protein metabolism. This glycocoll nitrogen when once bound as hippuric acid could not have participated in any interplay of chemical reaction with keto- or oxyacids produced in carbohydrate metabolism.

Rubner has called attention to the extremely soluble character of the monosaccharids, and it may be that a plethora of carbohydrate molecules reduces the demands upon the structural protein of the cells. Furthermore, it has been noted that the production of /3-oxybutyric acid is associated with an increased protein breakdown (see p. 94), so that the action of carbohydrate may perhaps prevent chemical injury to the cellular framework by promoting the normal oxidation of /3-oxybutyric acid.

Since carbohydrates so effectively spare protein from combustion, it would seem logical that their use should render the retention of protein in the body easier than when fat is given with protein.

Luthje1 finds a long-continued nitrogen retention in man when much nitrogen in protein is ingested (up to 50 gm. N daily!) and carbohydrates and fat making a total of 4000 calories or 66 calories per kilo. (See also Bornstein's experiment, p. 154).

In a subsequent paper Luthje2 finds that the P205 retention in convalescence is that which corresponds to the retention of protein for the formation of new tissue, including bone.

1 Luthje: "Zeitschrift fur klinische Medizin," 1902, xliv, 22.

2 Luthje: "Deutsches Archiv fur klinische Medizin," 1904, lxxxi, 278.

Sometimes in a healthy person not enough P205 is retained to build up "flesh," and the protein retained must, therefore, exist in the form of "deposit protein." This protein, he says, is not stored in the blood, for the composition of the blood does not alter, but is perhaps retained in the cellular fluids, just as glycogen is retained by the cells.

Rubner states that the greater the impoverishment of the protein supply in an animal fed with fat, the more powerful is the protective effect of small quantities of ingested protein over the loss of body protein. Also the retention of protein depends on the protein content of the animal as well as on the quantity of protein ingested. This is illustrated in the following table:

Influence Of The Protein Content Of A Dog On The Retention Of Protein Ingested

It is evident from this that of the same diet of protein more will be retained when the nitrogen content of the dog is low than when it is high; and also that a small protein intake may cause the same retention of nitrogen as a large protein intake, if in the first instance there be a relative impoverishment of the protein content of the animal.

According to these laws adult cells which have been depleted of their protein may gradually improve their nutritive condition until they reach an optimum, at which point they lose their power to attach additional protein.

This is also illustrated in an experiment by McCollum,1 who gave to a pig a diet containing 14 grams of nitrogen per day in the form of casein and starch, so that the value of the diet was 100 calories per kilogram during a period of thirty-six days. The animal retained 43 per cent, of the nitrogen ingested. During the first three days it added 9.65 grams of nitrogen to the body daily; during the last three, 3.69 grams. With the increase in active protoplasm the creatinin nitrogen excretion rose from 0.24 to 0.31 grams per day.

1 McCollum: "American Journal of Physiology," 1911-12, xxix, 215.

The conditions of protein metabolism are entirely similar to those of starch metabolism: (1) Digestive hydrolysis; (2) partial combustion of the end-products; and (3) possible regeneration of portions of the end-products into substances akin to the originals but characteristic of the organism - i. e., glycogen and body proteins. In the case of proteins the second or metabolic process involves the production of sugar and of fatty acids from the amino-acids involved. The third or regenerative process is promoted by such a protein as casein, which yields the proper variety of cleavage products.

In conclusion, it may be said that carbohydrates are the most economical of the food-stuffs, both physiologically and financially. They are the greatest sparers of protein. Ingestion of fat has for its object the relieving of the intestine from excessive carbohydrate digestion and absorption. Ingestion of fat in too large quantities leads to digestive disturbances, and if carbohydrates are entirely abandoned, to acetonuria.