It has been shown above that, in fasting experiments, the amount of stored glycogen and fat in the body exerts a "sparing" influence upon protein metabolism, the amount of protein • catabolized being smaller when the supplies of glycogen and fat are more abundant. Similarly the amounts of carbohydrates and fats in the food influence the rate of protein metabolism as indicated by the nitrogen excretion. The loss of protein which occurs on an insufficient diet may be diminished or even stopped by adding carbohydrates or fat to the food; and if carbohydrate or fat be added to the diet of a man in nitrogen equilibrium, there results a temporary decrease in nitrogen output with a corresponding storage of protein in the body. The former observation could be interpreted as meaning simply that the body draws upon its stored protein for energy so long, and only so long, as the fuel value of the food is insufficient; but the fact that addition of carbohydrate or fat to a diet already sufficient may cause an actual storage of protein indicates that the "protein-sparing action" or "protein-protecting power" of carbohydrates and fats involves something more than merely the question whether the body "needs" to burn its stored protein as fuel. As this is a matter of great importance, it may be well to consider somewhat carefully (1) the experimental evidence, and (2) the theoretical explanations, regarding the protein-sparing action of the carbohydrates and fats. For an account of the earlier experiments on this subject, especially those of Voit and Rubner upon dogs, the reader is referred to Lusk's Elements of the Science of Nutrition. Only some of the more important of the experiments upon men can be described here.

Lusk,1 experimenting upon himself, showed the susceptibility of the protein metabolism to the sudden withdrawal of carbohydrate food. In one experiment a liberal mixed diet containing 20.55 grams of nitrogen was taken until the body was nearly in nitrogen equilibrium, and then, without any other change, 350 grams of carbohydrate were withdrawn from the daily food. On the first day the body protein was largely protected by the carbohydrate previously stored in the body in the form of glycogen, but on the second day the nitrogen metabolism had risen from 19.84 to 27.00 grams per day. In another experiment, upon a diet containing less protein, withdrawal of carbohydrate increased the nitrogen excretion from 11.44 to 17.18 grams per day.

In these cases, as in the fasting experiments, the loss of body protein was less in those subjects having a good store of body fat than in those which were thin.

Kayser compared the efficiency of carbohydrates and fats as sparers of protein by observing the effect upon the nitrogen balance of replacing the carbohydrates of the food by such an amount of fat as would furnish the same number of calories, and then after three days resuming the original diet. This experiment and that of Tallquist which follows are given somewhat fully, as they illustrate well the methods and results of investigations based mainly upon the question of nitrogen equilibrium. The observer, who served as his own subject, was twenty-three years old, of good physique, with a small store of body fat, and weighed 67 kilograms. In the first and third periods he ate meat, rice, butter, cakes, sugar, oil, vinegar, and salad. In the second period the diet was changed so as to consist of meat, eggs, oil, vinegar, and salad, so that practically all the carbohydrate was withdrawn and replaced by fat. The two diets had practically the same fuel value and protein content. The results of this experiment are shown in the following table:

1 Zeitschrift fur Biologic, Vol. 27, page 459 (1890).

Nitrogen Balance When Feeding Isodynamic Quantities Of Carbohydrate And Fat (Kayser)

It is evident from the nitrogen balance of the first period that the amount of protein in the food was here greater than necessary, but that equilibrium was fully established in four days. On substituting fat for carbohydrate there is a marked increase of protein catabolism with corresponding loss of nitrogen from the body, and what is especially noteworthy, there was no evidence of any tendency to regain equilibrium during this period, but on the contrary the loss of nitrogen became greater each day the fat diet was continued; whereas, upon returning to the mixed diet, not only was the loss of protein stopped, but the body almost at once began replacing the protein it had lost, although the nitrogen and calories of the food were practically unchanged.

Tallquist1 compared the protein-protecting powers of isodynamic amounts (amounts having equal energy value) of carbohydrates and fats when only a part of either was replaced by the other. The subject was Tallquist himself, a man twenty-eight years old, in good health, and weighing about 80 kilograms. The experiment was performed in Rubner's laboratory, and the diet contained such an amount of total food as was estimated by Rubner to be just about sufficient to supply the energy requirements of the body, viz., 36 Calories per kilogram per day. The experiment covered 8 days divided into two equal periods. In the first four-day period the diet was rich in carbohydrates, in the second period it was rich in fats. An excellent feature of this experiment is that there was no change in the nature of the protein fed. All foods furnishing any significant amount of nitrogen were the same in the two periods of the experiment.

1 Archiv fur Hygiene, Vol. 41, page 177.