In the account of metabolism during starvation a short description has already been given of the influence of mechanical work on protein metabolism, of the influence of posture on general metabolism, and of the relation of the amount of metabolism to the diurnal variations of human temperature.

The source of mechanical work must be from metabolism. for mechanical energy cannot be derived from nothing The necessary energy might be obtained in one of two ways, either at the expense of a proportionate reduction in the quantity of heat liberated by the resting organism, or by anincrease in the amount of the metabolism. In the former case work would diminish the heat production and might cool the tissues, which is not observed to take place. If work were done at the expense of increased metabolism, and if this increase were completely converted into mechanical effect, then the heat production in the organism might remain the same as in the resting state. If, however, the result of mechanical effort be a stimulation of metabolism to the extent, of not only enabling the body to do work but also causing it to produce more heat thanwhen at rest, then the tendency of the tissues must be to grow warmer, perhaps with a resulting outbreak of sweat to reduce the body temperature through physical regulation. The "last named is the actual process.

Lavoisier's discovery that the absorption of oxygen is in-, creased during mechanical exercise firmly established the fact of a higher metabolism under these conditions.

The first experiments in which the effect of work upon the total metabolism was demonstrated were made upon a man by Pettenkofer and Voit.1 A man turned an ergostatic wheel 7500 revolutions on each working day for a period of nine hours, which afforded sufficient exercise to cause great fatigue at the end of the day. The experiments were made both during hunger and when the man was ingesting a medium mixed diet. The food supplied in the mixed diet contained:

The metabolism of this man, a strong workman, weighing 70 kilograms, at rest and at work, starving or on the medium mixed diet as given above, is presented in the following table:2

Effect Of Mechanical Work On Metabolism In Man

1 Pettenkofer and Voit: "Zeitschrift fur Biologie," 1866, ii, 537.

2 I have multiplied the nitrogen of the ingesta and excreta by 6.25 to obtain the quantity of the protein given and metabolized. The ratio N : C = 1 : 3.28 in protein has been employed. The dry starch has been calculated as containing 44.2 per cent, and the fat as containing 76.5 per cent, of carbon, which were the figures used by Pettenkofer and Voit. Rubner's standard calori-metric values have been used. (See Introductory Chapter).

From these early experiments it was evident that mechanical work did not increase protein metabolism even in starvation, but that the power to do work might readily be supplied by the increased metabolism of fat.

In the earlier editions of this book (1906 and 1909), the question was asked whether energy evoked by the specific dynamic, action of the food-stuffs could be utilized in the production of mechanical work.

The following experiments by Rubner1 show beautifully that there is a Summation of function as regards the extra heat production due to the specific dynamic action of protein and the extra heat production incident to mechanical work:

The Influence Of Diet And Mechanical Work Upon The Metabolism Of A Man 61-63 Kg. In Weight

Rubner observed that mechanical work was performed with greater ease when cane-sugar was the basis of the diet than when protein was given. The temperature of the chamber in which the experiments were performed was about 200 and the humidity was about 40 per cent. Rubner calls especial attention to the fact that when cane-sugar was given the increased heat produced by the body was lost, partly by the increased evaporation of water (62 per cent, of the increased heat production) and partly by an increase in radiation and conduction from the skin (38 per cent, of the increased heat production), whereas after meat had been given the elimination of the entire extra heat production due to work was thrown upon the activity of the sweat-glands. A high protein dietary is therefore contraindicated in athletic contests, especially when the weather is hot and humid.

1 Rubner: "Sitzungsberichte der preussischen Akademie der Wissen-schaften," 1910, xvi, 316.

The 100,000 kilogrammeters of work described above were produced by the action of the arms upon an ergostat. Since this quantity of work is the mechanical equivalent of 234 kilo-calories, and since 850 extra calories of metabolism were invoked in its accomplishment, it is evident that the mechanical efficiency of the human engine under these circumstances was 234/850,or 27.5 per cent.

Benedict and Murschhauser1 confirm the summation of energy increase from food and work in the case of men walking in a horizontal direction.

Recent investigations in my laboratory, however, indicate that a small dog, accomplishing a given amount of mechanical work, shows nearly the same heat production without food as after the ingestion of 70 grams of glucose. This is an important fact.

Rubner2 shows that a man of 70 kilograms weight, developing mechanical energy to the extent of 15,000 kilogram-meters per hour, produces practically the same quantity of carbon dioxid, no matter what the temperature of his environment may be. The results of the experiment are as follows:

1 Benedict, F. G., and Murschhauser: "Energy Transformations During Horizontal Walking," Carnegie Institution Publication 231, 1915, p. 91.

2 Rubner: Von Leyden's Handbuch, "Die Ernahrungstherapie," 1903, Bd. i, P. 74.

This person while at rest and at a temperature of 21.10 excreted 33.6 grams of carbon dioxid and 42 grams of water.

It is clear that during work the metabolism is independent of surrounding temperature or climatic conditions. In other words, during mechanical work the influence of the "chemical regulation" of body temperature may be eliminated (seep. 134). The extra heat production in doing mechanical work is utilized instead of the production of heat which is excited reflexly through cold. These results were forecast by Voit.1