Thus far in this book the influence of external temperature upon the course of protein metabolism has not been discussed. Rubner has shown that this is a factor of profound significance. It has already been demonstrated how, through chemical regulation, the basal requirement of the body is reflexly increased by increasing cold in the environment. Rubner2 compared the starving metabolism of a dog at different temperatures with that of the same dog when 100, 200, and 320 grams of meat were ingested. The results are presented as follows in terms of calories produced per kilogram of body weight:

1 Rubner: "Die Gesetze des Energieverbrauchs," 1902, p. 246. 2 Rubner: Ibid., p. 109.

Influence Of External Temperature On Metabolism After Protein Ingestion

One hundred grams of meat did not change the metabolism at 200, 250, or 300; 200 grams of meat had no effect at 200 or at 70, but at 250 and at 300 there was an increase, although the food contained fewer calories than the requirement. With 320 grams of meat there was a great increase above the starvation requirement, except at 70, where it is a maintenance diet and the metabolism remains unchanged. In other words, at a temperature of 300 the specific dynamic action of this amount of protein is capable of increasing the heat production above that of starvation by about 53 per cent., while at 70 there is no change whatever. It is also evident that at a high temperature even a small quantity of protein, such as 200 grams of meat, causes a considerable rise of metabolism.

Rubner gives the metabolism in terms of calories per kilogram after the ingestion of 550 grams of meat or 173.8 calories per kilogram of body weight in a dog, as follows:

In certain cases after food ingestion the carbon dioxid excretion may remain constant at different temperatures of environment. This action is seen in the dog mentioned on this page after he had eaten 320 grams of meat at various room temperatures. The increase in body metabolism due to the stimulus of cold (chemical regulation) is not necessary, since heat in excess of the requirement is already available. All that is needed is the arrangement of avenues of escape for the excess of heat produced from the food ingested (physical regulation). This physical regulation is brought about by the evaporation of water and by a change in the distribution of the blood.

How the increased evaporation of water enters as a refrigerating factor is beautifully shown in the experiment on the dog (p. 234) which fasted and then received 100, 200, and 320 grams of meat at various room temperatures. The distribution of the loss of heat by radiation and conduction and by the evaporation of water was as follows:

Distribution Of Heat Loss From A Dog After Meat Ingestion

It is evident from the above that the greater part of the loss of heat at a low temperature was by radiation and conduction, but at a high temperature (300) the loss by the evaporation of water was largely increased. The extra heat production on account of the specific dynamic action of the protein was lost through the increased evaporation of water. Much meat on a hot day would, therefore, seem contraindicated.

While the chemical regulation protects the body from an abnormal fall in temperature, the physical regulation prevents an abnormal rise in temperature. The organism may be at times under the influence of one means of regulation, at times of the other, and without being conscious of any difference. Cold-blooded animals have inadequate chemical regulation, and their temperature falls with that of their surroundings (see p. 114).

A study of the specific dynamic action of protein in its relation to temperature changes gave Rubner1 new points of view. He saw (experiment on p. 234) that by chemical regulation the metabolism in a fasting dog was increased from 54 to 86 calories per kilogram, an increment of 32. And he likewise observed that after the ingestion of 320 grams of meat the heat produced at a room temperature of 300 rose from 56 in starvation to 83, a difference of 27 calories. The source of the increase through chemical regulation is known to be chiefly in the muscles. The increase brought about by protein ingestion had been shown by Rubner to be due not to any such thing as intestinal activity (see p. 231), but rather to some specific heat-raising effect of protein metabolism itself. It was apparent that these two sources of increased heat might enter into a reciprocal arrangement, because on cooling the atmosphere in which the dog lived to 70 C. the metabolism, after the ingestion of 320 grams of meat, remained at 87.9 calories in contrast with 83.0 on feeding at 300. Here the heat due to the specific dynamic action was used in replacement of that induced by chemical regulation. This illustrates Rubner's modified idea of his compensation theory, or a reciprocity between heat produced in the muscles by chemical regulation and the extra heat production brought about through the ingestion of food.

Since the extra heat production after food ingestion could be utilized instead of heat from chemical regulation, Rubner perceived that the true increase through specific dynamic action could be measured best at the temperature of 330, where there was no reflex increase in metabolism through chemical regulation.

It was especially important to make experiments regarding the action of food-stuffs at a temperature of 330, for that is the temperature with which man surrounds his skin. By means of clothes and artificial heating man constantly tries to remove himself from the influence of chemical regulation. His daily life is practically under the influence of a tropical climate. His metabolism is unchanged from the normal when he is immersed in a bath at 330.1

1 Rubner: "Energiegesetze," p. 145.