An interesting contribution to the subject of the possible formation of fat from protein has been made by Weinland,1 who found in the case of the blow-fly (calliphora), which lays its eggs in meat, that both the larvae and a pulp made by crushing them had the power, in the absence of oxygen, to split peptone into amino-acids, deaminize these with evolution of ammonia, and then with evolution of carbon dioxid to produce higher fatty acids, presumably through synthetic union of fragments of the acids which had been freed of their amino groups. Such a procedure reasonably explains the formation of fat from protein in the sense of the older theories (see p. 171).

The question of a "fatty degeneration" of protein under pathologic conditions is another matter and will be considered in another place. (See Chapter XVI (Metabolism In Diabetes And In Phosphorus-Poisoning)).

The experiments already described bring to light a very striking change in the metabolism after the ingestion of protein in excess. The total heat production is markedly increased. To what may this be due?

1 Weinland: "Zeltschrift fur Biologie," 1908, li, 197.

Von Mering and Zuntz1 believed that such increased metabolism was due to the activity of the intestinal tract after the ingestion of food.

Voit2 criticised this view, and said that a rise in the carbon dioxid excretion from 366 grams in starvation to 783 grams after ingestion of 2500 grams of meat by a dog (seep. 155) was too great to be due to intestinal activity, and, indeed, corresponded to the rise noted only after the hardest exercise. Furthermore, Voit had shown that after giving a medium quantity of fat, the carbon dioxid excretion and oxygen absorption were almost the same as in hunger, notwithstanding the activity of the filled intestine.3

This question has received very painstaking and elaborate investigation at the hands of Rubner, who has published his results in a book entitled "Die Gesetze des Energieverbrauchs bei der Ernahrung." This volume is an extension of a work of which a preliminary communication was published by Rubner4 from Voit's Munich laboratory in 1885.

Rubner shows that bones given to a dog will not increase his metabolism in spite of the intestinal irritation, so the increase after meat ingestion is not due to a nerve reflex of mechanical nature. Further, the metabolism is not raised after the ingestion of meat extract, so the chemical stimulus of flavors which start activity in the glands does not affect total metabolism. Again, the ingestion of water in the quantity contained in meat, while it may cause a rise in nitrogen in the urine followed by a fall - the rise being due to a rapid washing out of nitrogenous decomposition products - does not alter the total metabolism in any way.

Lusk5 has shown that urea when given in the quantity which would be liberated from considerable amounts of meat, and sodium chlorid, the ingestion of which might induce osmotic exchanges in the cells, have no effect upon the heat production.

1 von Mering and Zuntz: "Pfluger's Archiv," 1877, xv, 634.

2 Voit: "Physiologie des Stoffwechsels," 1881, p. 209.

3 Compare also Benedict, F. G., and Pratt: "Journal of Biological Chemistry," 1913, xv, 1.

4 Rubner: "Sitzungsberichte d. kgl. bayr. Acad. d. Wissenschaft," 1885, Heft 4.

5 Lusk: "Journal of Biological Chemistry," 1912, xiii, 27.

Benedict and Emmes1 have demonstrated that cathartics and agar-agar when given to man have no effect upon total heat production in spite of the intestinal activity which they produce.

The absence of true "intestinal work" or "Darmarbeit" in the sense of Zuntz is further shown by the fact that Johansson2 has given a fasting man 75 grams of glucose without the slightest increase in the output of carbon dioxid. If glucose had been consumed the carbon dioxid excretion would have risen (see p. 289), therefore glucose was retained as glycogen. Since all these processes were without effect on the carbon dioxid output, it follows that the intestinal activities involved did not cause an increase in the total metabolism. Of similar import are the results by the same writer after administering 50 grams of glucose to a diabetic. The sugar was absorbed and eliminated in the urine without affecting the carbon dioxid output.

The increase in metabolism is greater in the case of protein than with any other food-stuff. Rubner calls this action of abundant protein food in raising the metabolism the specific dynamic action of protein. Rubner found that when dogs were fed with meat their bodies metabolized in largely increased measure without doing any external work. A more rapid respiration alone betokened the increased oxidation and the effort of the body to rid itself of excess of heat through physical regulation. The temperature of the dogs scarcely changed, so perfect is the regulatory mechanism for the discharge of heat. Thus in one dog the temperature was 38.160 before the meal, 38.740 during the digestion, and 38.170 at the end of digestion.

If a large quantity of protein be ingested day after day, then the usual specific dynamic action occurs and also a continued "secondary" rise in total day-to-day metabolism, which increases with the continual increase in protein metabolism. When nitrogen equilibrium is established the heat production remains constant at a higher level.

1 Benedict, F. G., and Emmes: "American Journal of Physiology," 1912, xxx, 197. 2 Johansson: "Skan. Archiv fur Physiologie," 1909, xxi, 1.

Rubner1 illustrates this important fact in the following experiment on a dog, the food of which contained 17 grams of nitrogen:

This experiment of Rubner shows that the amount of protein carbon retained in the body for the production of carbohydrate or fat has nothing to do with the intensity of the specific dynamic action. Protein retention is much more readily brought about on a mixed diet containing large quantities of carbohydrates, as will be seen in a subsequent chapter.