In a previous chapter it was shown that the amount of fat in the fasting organism materially affected the amount of protein burned. Where there was much fat present little protein was consumed; where there was little fat, much protein burned; and where there was no fat, protein alone yielded the energy necessary for life.

The ingestion of fat alone will not prevent the death of the organism because there is a continual loss of tissue protein from the body, which finally weakens some vital organ to such an extent that death takes place.

In a fasting animal which still contained fat, Voit1 found that the ingestion of 100, 200, and 300 grams of fat scarcely influenced the protein metabolism. The latter was slightly increased, if anything. Voit's table is as follows:

These results have been confirmed by Bartmann,2 who noted that fat given to the extent of 150 per cent, of the energy requirement was readily absorbed and spared protein to a maximum of 7 per cent. Sometimes when much fat was given there was an increased elimination of nitrogen in the urine, at which time there was also an increased amount of nitrogen in the stools.

1 Voit: "Physiologie des Stoffwechsels und der Ernahrung," 1881, p. 128. 2 Bartmann: "Zeitschrift fur Biologie," 1912, lviii, 375.

To another dog, which in starvation burned 96 grams of fat, Voit gave 100 grams, with the result that it then burned 97 grams. The conditions of the metabolism in these cases were therefore identical. The fat ingested simply burned instead of the body's fat, but the total amount of protein and fat burned remained the same.

One reason why the ingestion of fat up to the requirement does not alter the metabolism may be found in the observation of Schulz1 that in starvation there is an increase in the quantity of fat in the blood, and of Rosenfeld2 that the amount of fat in the liver increases. He rinds that a fasting liver contains 10 per cent, of fat. If carbohydrates or protein (which yields carbohydrate in metabolism) be ingested, the fat content falls to 6.2 per cent. If fat be given to a fasting dog, the liver may contain 25 per cent, of fat; but if carbohydrates are ingested at the same time, the liver does not retain the fat, which must be deposited elsewhere. Thus, in the liver there is an antagonism between glycogen deposit, which follows carbohydrate ingestion, and fat deposition.

Pfluger3 gave a dog fat alone in large quantities for thirty days and found that the fresh substance of the liver at the end of the period contained 45 per cent, of fat and no glycogen.

Miescher found fat globules in the muscle-cells of salmon after their five to fifteen months' fast in fresh water, during which time they had laid their eggs. It is undoubted that the deposits of fat in the adipose tissue of these fishes are drawn on in starvation, and that the blood then carries to the hungry cells all the fat they require for their continued function. Greene4 states that large quantities of fat are present in the fibers of the great lateral muscle of the Columbia River salmon at the beginning of its travels up the river, and this fat remains there in strikingly uniform quantity during the whole of the migration journey. It seems that the fat supply to the cells is regulated by the quantity of other foods available, and that even in starvation there is at first ample fat to meet the requirement of the organism (see p. 100). These are important principles which will be further discussed when the subject of fatty infiltration is considered. (See chapter on Diabetes).

1 Schulz: "Pfluger's Archiv," 1896, Lxv, 299.

2 Rosenfeld: "Ergebnisse der Physiologie," 1903, ii, I, 86.

3 Pfluger: "Pfluger's Archiv," 1907, cxix, 123.

4 Greene: "Journal of Biological Chemistry," 1912, xi, p. xviii.

The method of the oxidation of fat has already been described (see p. 182), and one would expect to find β-oxybutyric acid as an end-product of this metabolism. In fact, the blood of normal human subjects, as well as the blood of dogs, pigs, and cattle, contains usually a little less than 1.5 mg. of β-oxybutyric acid in 100 c.c.1 Sassa2 reports between 1 and 2 mg. to be widely distributed in the blood and organs of man and various mammals. In normal conditions this end-product is, therefore, present in only minimal amounts.

When fat is oxidized in excess, as in fasting, /3-oxybutyric acid appears in the urine (see p. 182). So also when fat forms the main portion of the diet the same phenomenon occurs. Forssner3 gave a man a diet which contained 3380 calories, of which only 160 were in carbohydrate. The last meal was taken at 4.00 p. m., and then olive oil was given at 9.00 p. m. The urines between 11.00 p. m. and 10.00 a. m. contained the following amounts of acetone bodies:

These results indicate the formation of β-oxybutyric acid in large amounts.

The work of Bloor4 has shown that after giving fat to a dog there is a gradual rise in the fat content of the blood, the maximum being attained in the sixth hour, after which there is a fall. The following shows an example:

1 Marriott: "Journal of Biological Chemistry," 1914, xviii, 507.

2 Sassa: "Biochemische Zeitschrift," 1914, lix, 362.

3 Forssner: "Skan. Archiv fur Physiologie," 1910, xxiii, 305.

4 Bloor: "Journal of Biological Chemistry," 1914, xix, 1.

Furthermore, when fat was injected intravenously in such quantity that the fat content of the blood was doubled, the excess disappeared within five minutes after the cessation of the injection.

Work of fundamental character by Magnus-Levy1 showed the influence of the ingestion of very fat bacon upon the metabolism of the dog. Respiration experiments lasting about thirty minutes each, using the Zuntz method, were made upon a dog breathing through a tracheal cannula. These showed that after giving 140 grams of fat bacon the metabolism increased from the end of the third hour through the eighth to a height which was about 10 per cent, above the original basal level as measured twenty-four hours after the last ingestion of food. After 320 grams of fat bacon had been taken the metabolism showed a maximal increase of 19 per cent, from the end of the third hour through the sixth. The increased metabolism extended from the fourth to the thirteenth hours after food ingestion, and then subsided to the original basal level. The total increase in heat production could be estimated as 2.5 per cent, of the energy content of the fat ingested. The environmental temperature of the dog varied between 160 and 19°, and all extraneous movements were avoided.

1 Magnus-Levy, A.: "Arch. f. d. ges. Physiol.," 1894, lv, 1.