A significant fact is that when the body changes from a carbohydrate diet to one of fat or protein there is a considerable loss of water. This was first noted by Bischoff and Voit,1 who gave bread to a dog forty-one days and witnessed a loss in body weight of 531 grams, although if the nitrogen elimination of the period had been all attributed to muscle breakdown the loss in body weight should have been over 3700 grams. Then when 1800 grams of meat were given in quantity sufficient to cause protein deposit, the weight of the animal fell 310 grams on the first day of this diet.

The phenomenon also occurs in man, as the following experiment of Benedict and Milner2 demonstrates. The experimental period lasted six days, mechanical work was performed daily, and isodynamic quantities of food, which were somewhat under the needs of the body, were ingested. During the first three days 66 per cent, of the energy in the food was contained in carbohydrates, and during the last three days 67 per cent, of the energy was in the form of fat. The following changes were noted:

1 Bischoff and Voit: "Gesetze der Ernahung des Fleischfressers," 1860, pp. 211 and 214.

2 Benedict and Milner: "U. S. Dept. of Agriculture," Office of Experiment -Stations, 1907, Bull. 175, p. 224.

A loss of body glycogen is, therefore, associated with a loss of body weight.

If carbohydrates be ingested alone, immediately after starvation, the protein metabolism may fall below the starvation amount.

This higher protein-sparing property gives to dogs fed on carbohydrates alone a longer lease of life than is granted to those fed on fat alone, although the ultimate outcome is the same.

The protein metabolism may be reduced to one-third the fasting value, a result also obtained by Landergren1 and by Folin2 in man. Cathcart3 gave a man who had been fasting fourteen days a diet of cream (300 c.c.) and starch (400 grams). The nitrogen excretion in the urine was as follows:

The absence of a fall in protein metabolism on the first day is probably to be explained by assuming a large deposit of glycogen within the body at the expense of the starch ingested (see p. 290). On the third day of the diet the protein metabolism had fallen to one-third that observed in fasting (see p. 280).

The sparing influence of carbohydrate oxidation upon protein metabolism has been beautifully illustrated by Landergren.4 Diets containing carbohydrates and fats, but scarcely any nitrogen (about one gram daily), were given men and the protein metabolism noted. This condition is called that of specific nitrogen hunger. After four days' administration of such a diet the urinary nitrogen may be reduced to less than 4 grams.

1 Landergren: "Skan. Archiv fur Physiologie," 1903, xiv, 112.

2 Folin: "American Journal of Physiology," 1905, xiii, 45.

3 Cathcart: "Biochemische Zeitschrift," 1907, vi, 109.

4 Landergren: Loc. cit.

In one experiment in which this was accomplished carbohydrates were entirely replaced by fat, with the result that protein metabolism rose to the amount found in starvation (about 10 grams). It has already (p. 248) been explained that ingestion of fat alone will not reduce protein metabolism below that of starvation. The experiment is as follows:

* Ordinary diet.

On day 5, the first of the fat diet, it is evident that the protein metabolism was affected by the use of the glycogen supply of the body, an influence which became negligible on the second and third days of the fat diet (p. 72).

Landergren gives the following results in various cases of specific nitrogen hunger, showing the nitrogen in the urine before the diet and after four days thereof:

This reduction of protein metabolism to 4 grams on the fourth day was brought about by the following diets in the different cases:

A diet containing half its calories in carbohydrates and half in fat has therefore the same protein protecting power as one made up of carbohydrates alone. This demonstrates the rationality of a mixture of the non-nitrogenous food-stuffs.

The experiments of Karl Thomas have shown the prolonged influence of a previous high protein diet upon the nitrogen output in the urine of man. A starch-cream diet had reduced the urinary nitrogen elimination to 2.2 grams daily. Then, during four days, 76, 87, 85, and 71 grams of nitrogen were given in the diet. The nitrogen retention in the body for the first four days was +43, +25, +8, and - 10 grams, a total of +66 grams. This stored protein was by no means as rapidly demolished in the body as it was added to it. This appears in the third column of the following table:

The Influence Of Previous Protein Ingestion Upon The Excretion Of Nitrogen In Grams In Man When A Diet Of Fat And Carbohydrate Is Administered

Rubner4 has defined the minimal protein metabolism under conditions of administration of carbohydrate in excess, as the "wear-and-tear" quota of protein metabolism. This minimal level is only achieved after the reduction of the cells from their optimal protein condition through loss of body nitrogen. Rubner estimates that a kilogram of body weight contains 30 grams of nitrogen. Since the individual investigated by Thomas weighed 73 kilograms, he contained 2190 grams of nitrogen. When given 89 grams of protein nitrogen on a single day this represented 4.5 per cent, of his body's supply. The 66 grams of protein nitrogen stored during the days of liberal protein ingestion, which raised the cells to an optimal condition, represented 3 per cent, of the total protein content. When carbohydrates alone were given this stored protein was only gradually eliminated - there was a transition period of constantly diminishing protein waste until a minimum of 2.2 grams of urinary nitrogen (with 0.6 grams in the feces) was found. The urinary nitrogen then represented approximately 1 part in 1000 of protein contained in the organism. This is the lowest" wear-and-tear" quota of protein metabolism.

1 Landergren: Loc. cit.

2 Kinberg: "Skan. Archiv f. Physiologie," 1911, xxv, 291. 3 Thomas: "Archiv f. Physiologie," 1910, Suppl., p. 249. 4 Rubner: "Archiv. fur Hygiene," 1908, lxvi, 45.

It is a point of debate whether the "stored protein" becomes true living tissue protein or whether it represents a special variety of deposit protein, which is retained in the tissue cells very much as glycogen is retained by them (see p. 81). The storage takes place largely in the liver.1 It would be interesting to follow the sulphur excretion during the early days of the transition period from high to low protein metabolism and note whether this sulphur elimination runs parallel to that of nitrogen in the usual ratio (see p. 169). Should this be the case it would indicate that "deposit protein" was the same as tissue protein. Phosphorus retention is not always present during the period of protein deposit (see p. 287).