It has been thought that protein is a food which is in itself sufficient for all the requirements of the body. Pfluger1 was able to keep a very thin dog in good condition and doing active exercise during a period of seven months, the sole diet being meat cut as free from fat as possible. Pfluger says that the fat and glycogen content of the meat ingested could not have yielded sufficient energy to provide for the action of the heart alone. It must be remembered, however, that meat is not pure protein, but is mixed with salts and water. The simplest diet capable of maintaining the body in condition is, therefore, a mixture of materials or food-stuffs. Such a mixture of food-stuffs is called a food. A food-stuff is a material capable of being added to the body's substance, or one which when absorbed into the blood-stream will prevent or reduce the wasting of a necessary constituent of the organism.

The food-stuffs are:

Proteins (including albuminoids).

Carbohydrates.

Fats.

Salts.

Water.

A food is a palatable mixture of food-stuffs which is capable of maintaining the body in an equilibrium of substance, or capable of bringing it to a desired condition of substance.

The ideal food is a palatable mixture of food-stuffs arranged together in such proportion as to burden the organism with a minimum of labor. These definitions are Voit's.1

1 Pfliiger: "Pfluger's Archiv," 1891,1, 98.

When protein alone is ingested by a normal adult it is very readily oxidized, and is only with the greatest difficulty deposited so as to form new tissue in the organism.

In the early experiments of Bischoff and Voit the fact is recorded that a dog weighing 35 kilograms may excrete 12 grams of urea in twenty-four hours, and the same dog after receiving 2500 grams of meat may excrete 184 grams, fifteen times as much.

Voit2 has shown that if that quantity of meat be administered which corresponds to what is oxidized in starvation, nitrogen equilibrium will not be established, but some of the body's flesh will also be metabolized. This latter quantity grows steadily less if the amount of meat ingested be gradually increased until finally the point of nitrogen equilibrium is reached, at which the amount of meat ingested is equal to that destroyed in the body. To illustrate this Voit gives the following table, the results of work done on a dog:

Nitrogen equilibrium was not reached until 1200 grams of meat were given, or about five times the amount of the fasting protein metabolism.

The above experiments were made in 1858. It is no longer customary to calculate the protein metabolism in terms of flesh destroyed, but in terms of nitrogen. The old-fashioned term "flesh" meant meat with a nitrogen content of 3.4 per cent. It served to illuminate the significance of metabolism at a time when few were instructed in this field of work.

E. Voit and Korkunof3 have published a research of similar character. They fed a dog with meat which had been treated with lukewarm water to remove the extractives, and which was then squeezed in a press. This process removes most of the nitrogen-containing substances other than protein. A dog will readily eat this washed meat or "protein." The idea was to determine the minimum quantity of protein which it was possible to ingest and still maintain nitrogen equilibrium. The different quantities of meat tabulated below were given continuously for two or three days at a time. Only the results of the last day of each of these periods are quoted:

1 Voit: Hermann's Handbuch, "Stoffwechsel," 1881, pp. 330, 344.

2 Voit: Ibid., 1881, p. 106.

3 E. Voit and Korkunoff: "Zeitschrift fur Biologie," 1895, xxxii, 58.

The figures show that nitrogen equilibrium was reached only after supplying three and a half times the amount of protein metabolized in starvation. The authors calculate that at this time of nitrogen equilibrium the dog was still losing 28 grams of body fat, and that not much more than 50 per cent, of the total energy liberated in the organism was furnished by the protein metabolism of the time. One may thus have nitrogen equilibrium without having carbon equilibrium.

Systems of diet for fat people are based on this knowledge. A loss of protein is highly undesirable, while a gradual loss of adipose tissue may be a great relief to the obese.

Bornstein1 finds that during a period of thirteen days he can add 8.3 grams of protein to his body and oxidize 90 grams of body fat daily when ingesting a mixed diet containing 1600 calories with 118 grams of protein. Such a diet contains a fuel value less than the requirement of his organism (p. 279).

1 Bornstein: "Berliner klinische Wochenschrift," 1904, xli, 1192.

This cannot be accomplished without carbohydrate in the diet, for Thomas1 finds that when a man is given protein alone, administered in fractional portions every two hours even to the extent of double the quantity of protein destroyed in fasting, nitrogen equilibrium cannot be obtained. This experiment is given below; 500 grams of meat contained 18.4 grams of nitrogen, corresponding to about 115 grams of protein:

If the quantity of meat ingested be steadily increased after nitrogenous equilibrium has been reached, the protein metabolism will gradually increase, nitrogenous equilibrium will be established at higher and higher levels, and there will be a corresponding diminution in the amount of fat burned. This was shown in 1862 in the following experiment of Voit,2 who gave different quantities of meat to a large dog weighing 30 kilograms:

Influence Of Ingesting Increasing Quantities Of Meat

Weights are in Grams.

Nitrogen equilibrium existed after the ingestion of 1500 grams of meat and there was also no loss of body fat (carbon equilibrium). When 2000 grams and even 2500 grams of meat were supplied it was all destroyed, as was indicated by the amount of nitrogen in the urine, but a certain quantity of carbon belonging to the ingested protein was not eliminated in the respiration, but was retained in the body. This carbon Pettenkofer and Voit believed to have been laid up in the body in the form of fat.

1 Thomas: "Archiv fur Physiologie," 1010 Suppl., p. 249.

2 Voit: "Stoffwechsel," 1881, p. 117.