Rubner has given further useful definitions. A "repair quota" of protein is required in the food in order to replace that lost in the "wear and-tear" quota. A "growth quota" of protein is necessary in addition to a repair quota under the circumstances of multiplication of cells and of developing protoplasm in the young. Furthermore, an "improvement quota" of protein may be necessary in the adult after wasting disease, or after fasting, in order to bring the cells to an optimum of protein condition, thereby improving the welfare of the living organized protein.1 When protein is given in excess so that it is not used for repair nor for growth nor deposit, its constituent amino-acids are deaminized and the residual oxy- or keto-acids are in part converted into glucose, in part into fatty acid, thus yielding fuel to the cells just as would carbohydrate and fat. This fraction of protein Rubner designates as the "dynamic quota".

1 Tichmeneff: "Biochemische Zeitschrift," 1914, lix, 326; Cahn-Bronner: Ibid., 1914, lxvi, 289.

Thomas calculated that during the period of minimal "wear-and-tear" protein metabolism, 0.4 calories were derived from the metabolism of 1.5 milligrams of protein per kilogram of body weight every hour, while 0.96 calories were derived from the oxidation of 259 milligrams of glucose. In other words, protein furnished only 4 per cent, of the energy required by a man at rest. Since mechanical work scarcely influences the "wear-and-tear" quota of protein metabolism (see p. 311), although it largely increases the oxidation of carbohydrate, it is evident that protein may play a very small role as a producer of energy for the maintenance of the function of life.

When carbohydrates are given in the diet, it is possible to establish nitrogen equilibrium at a much lower level than when protein alone or protein and fat are ingested.

When carbohydrates and protein are ingested together in quantity sufficient for the requirement of the organism, it has been found that, taking the starvation protein metabolism as one, nitrogen equilibrium can be maintained by ingesting one part of protein.2

The work of Siven,3 however, was the first indication that nitrogen equilibrium may be maintained at even a lower level than that ordinarily present in starvation. A somewhat undersized healthy man, weighing 60 kilograms, who normally ate a mixed diet containing 16 grams of nitrogen, was given less and less protein, and an attempt was made to establish nitrogen equilibrium at lower and lower levels. The daily ration was rich in carbohydrates and yielded 2444 calories. The experiment was divided into four periods of about a week each, which may be summarized as follows:

1 Rubner: "Archiv fur Physiologie," 1911, p. 67.

2 E. Voit and Korkunoff: "Zeitschrift fur Biologie," 1895, xxxi 1, 117.

3 Siven: "Skan. Archiv fur Physiologie," 1900, x, 91.

It is apparent that nitrogen equilibrium may be established after ingesting 6.26 grams of nitrogen, although, as has been seen, the elimination during the early days of starvation in man is usually 10 grams. During the first three periods of reduced protein intake, as much as 20.16 grams of protein nitrogen were actually added to the body. In a fifth period nitrogen equilibrium was obtained on the fourth day on a diet containing 4.52 grams of nitrogen.

Thomas1 administered during frequent intervals small quantities of meat washed free from extractives to the man who had partaken of a starch-cream diet and had reduced his protein metabolism to one represented by a minimum of 2.2 grams of urinary nitrogen daily. Nitrogen equilibrium was nearly achieved after administration of that quantity of protein nitrogen which corresponded to the quantity eliminated in the urine and feces. This is shown below in grams per day:

In this experiment the ingestion of the quantity of protein which was the equivalent of the "wear-and-tear" quota was at first nearly sufficient to completely "repair" the tissue. While living upon this low protein diet the mental and muscular power was unchanged.

1 Thomas: "Archiv fur Physiologie," 1910, Suppl., p. 249.

Chittenden1 finds that nitrogen equilibrium may be maintained on a diet containing a very small amount of protein and two-thirds of the body's requirement of energy. The first experiment was on Fletcher and lasted six days. The daily ration contained 7.19 g. nitrogen + 38.0 g. fat + 253 g. carbohydrates = 21.3 calories per kilogram. The excreta contained 6.90 grams of nitrogen daily. On this diet the individual showed "remarkable physical strength and endurance".

Another experiment was performed by Chittenden on himself and lends itself for interesting comparison with the results of the ingestion of a maintenance ration. The food was principally vegetable. The results may be thus tabulated:

A Low Level Of Nitrogen Equilibrium In Normal And Undernutrition

Nitrogen equilibrium may therefore be maintained at a low level, even during the state of undernutrition present when 22.4 calories per kilogram are in the daily diet. On a milk diet . Rubner2 found that the ingestion of 2483 grams of milk containing 84 grams of protein and two-thirds the body's requirement or energy resulted in the addition of 6.7 grams of protein to the body daily for three days (see p. 353).