During the period of gestation the fetus is supplied with nourishment from the mother through the placenta and no muscular movement or exertion of any kind is required to secure food. After birth much of the muscular activity of infants is a direct or indirect effort to secure nourishment. This includes not only the act of suckling but also the muscular activity and crying due to hunger. Thus the apparently anomalous condition exists of an expenditure of a considerable amount of energy for the purpose of obtaining energy for vital processes.

With animals of prey there is usually a period of intense muscular activity prior to feeding. The chase, the attack, the act of killing and tearing apart of the prey, all make demands upon the energy supply of the animal. After feeding, there is usually a relatively long period of muscular repose, although, as will be seen later, immediately after eating there is invariably an increase in the internal cellular activity incident to the process of digestion.

Even with non-predatory animals and birds a considerable amount of energy is necessary to secure food. The trails to the feeding-grounds, "salt-licks," and watering-places often lead over considerable distances. Birds fly enormous distances to special feeding-grounds, while with birds of prey the chase and attack are comparable to those of animals of prey.

An exception to this general activity in securing food is the serpent, which, instead of having to chase its prey, lies in wait for it. When the victim is captured, the serpent kills it either by poison, which of itself requires no muscular activity other than the act of striking, or by constriction, which usually continues but a short time. Probably no living organism secures its food with so economical a consumption of energy as the serpent does, not only because of its extraordinarily low metabolism, which permits it to live for a long time without food, but also because of the minimum amount of muscular activity expended in obtaining the food. With the human infant the muscular activity incidental to securing food plays a very important role. To what extent irritation and discomfort, accompanied by vigorous muscular exercise and crying, may be directly charged to a desire for nourishment is problematical, but in any event it is certain that a large part of the physical activity of an infant is due to an effort to secure food.

In human civilization it is rarely that an individual must pursue, attack, gather, and prepare his food prior to eating, as the food materials are gathered by harvesters, hunters, or fishermen, brought to the dwelling by transportation agencies, prepared by some member of the household, and finally placed upon the table ready for consumption. With humans the exertion necessary to secure food is no longer individual, but represents the serious occupation of a large number of persons devoted to this service only. But even after the food has been prepared and placed before the individual, there are certain muscular processes necessary to prepare it for digestion; these are admirably classified by Armsby in the following paragraph:

"In the process of digestion we are probably safe in assuming that the muscular work of prehension, mastication, deglutition, rumination, peristalsis, etc., constitutes an important source of heat production."1

Entirely aside from the external muscular activity incident to the procuring and preparing of food and its introduction into the mouth, we have internal processes other than those of mastication, primarily the movements of the stomach and intestinal tract, which may be grouped under the general term of peristalsis. These movements, certainly in ruminants, are very considerable in amount. While with humans rumination does not occur, yet the admirable X-ray observations of Cannon2 have demonstrated that with men peristalsis is continuous during digestion. How much the movements of peristalsis and segmentation contribute to or make demands upon the energy of the body is a problem still to be considered. The possibility of there being extensive demands for these processes in man has been carefully considered by Zuntz and his co-workers. These investigators have been influenced in large part by their observations on ruminants and herbivorous animals in general, such animals having a large amount of residue or ballast in the gastro-intestinal tract that must be worked over by the peristaltic movements.

Finally, a considerable demand is made upon the energy of the body for heat to warm the ingesta. Water and many other fluids are commonly taken by man at a temperature considerably lower than the temperature of the body; these must be warmed to body-temperature. Again, certain liquids are taken somewhat above the temperature of the body and therefore may contribute, in part at least, to the heat elimination. The amount of cold ingesta required to be warmed by body heat is invariably much greater than the amount of warm food taken, so that in many instances we have carefully to consider this expenditure of heat. In fact, this has been pointed out as an important path for the output of heat in diabetics with an enormous excretion of urine. If 3 liters of water are taken and excreted as urine in the course of the day, it will be seen that this water may be warmed from an average of 10° C. to the temperature of the body, or 37° C, with an expenditure of 81 calories.1

1Armsby, The principles of animal nutrition, 2d ed., 1906, p. 374. 2Cannon, The mechanical factors of digestion, 1911.

The feeling of warmth following the ingestion of food, familiar to all, is not without significance as being a crude index of a scientific fact which has been well established since the days of Lavoisier and Jurine, i. e., that after food ingestion there is an increase in the metabolism or heat output. At present the main subjects for discussion with physiologists are not as to there being an increase in the heat output, but first, as to its quantitative relations to the ingesta; second, as to the cause of the increase in the heat output.

After an historical examination of the evidence with human subjects which has thus far been accumulated to show that there is an increased heat production following food, the results of an extensive series of observations made under the auspices of the Carnegie Institution of Washington, first in the Department of Chemistry of Wesleyan University, Middletown, Connecticut, and later in the Nutrition Laboratory in Boston, will be presented. These observations, covering a period of 10 years, were made with a variety of methods and somewhat changing technique, so that they are not strictly comparable in all instances. The evidence is, however, so extensive as to throw general light upon the metabolism following ingestion of food and justifies a consideration of the quantitative relations between the energy intake and character of the ingesta and the quantitative increase in the metabolism of man following the ingestion of the various diets.

1Benedict and Joalin, Carnegie Inst. Wash. Pub. No. 136, 1910, p. 230.