In considering these figures bearing on the output of carbon dioxide under the conditions specified, we note at once a correspondence with the total energy exchange, as indicated in the preceding table. As previously stated, we are at present dealing simply with generalities, and the important point to be observed here is that muscular work—7 A. M. to 7 P. M.—in the work experiments, increases enormously the output of carbon dioxide. We see clearly emphasized a connection between the total energy exchange of the body, as expressed in calories or heat units, and the oxidation of carbonaceous material, of which carbon dioxide is the natural oxidation product. We note that on the cessation of work—7 P. M. to 7 A. M.—the output of carbon dioxide tends to drop back to the level characteristic of the corresponding period in rest, with or without food. In the experiment with “extra severe muscular work,” the results are different simply because here the subject worked sixteen hours, necessitating a portion of the work being done at night-time. Finally, it should be mentioned that the differences in output of carbon dioxide in these experiments are somewhat greater than in many experiments of this type, although all show the same general characteristics. This may be explained, as stated by the authors from whom the data are taken, “by the fact that J. C. W. was a larger and heavier man than any of the others; that the differences in diet were wider, and that the amounts of external muscular work were larger in these experiments than in those with the other subjects.”

If we pass from experiments of this type, conducted in a calorimeter, to those cases where competitive trials of endurance are held by trained athletes, i. e., where external muscular activity is pushed to the extreme limit, we then see even more strikingly displayed the effect of work in increasing the energy exchange of the body. One of the best illustrations of this type of experiment is to be found in the observations made in connection with the six-day bicycle race held in New York City, at the Madison Square Garden, in December, 1898.39 The observations in question were made upon three of the athletes, one of whom withdrew early in the fourth day, while the others continued until the close of the race—142 consecutive hours—winning the first and fourth places, respectively. The following table gives the computation of energy of the material metabolized, exclusive of body-fat lost:

Miller, the winner of the race, who averaged a daily energy exchange of 4820 calories, rode 2007 miles during the week, and finished the race without physical or mental weakness resulting from the fatigue and strain. During the first five days, he rode about 21 hours a day and slept only 1 hour. Albert, who weighed a few pounds less than Miller, covered 1822 miles in 109 hours, with an average daily exchange of 6074 calories. We may add a table (on the following page) showing the balance of income and outgo of nitrogen in these three subjects, as being of general interest in this connection. The figures given are averages per day.

The special significance of these data, as bearing upon the topic under discussion, is that apparently all three of the subjects were drawing in a measure upon their body material. As stated by Atwater and Sherman, Pilkington lost per day 5.1 grams of nitrogen; that is to say, the total nitrogen excreted exceeded the total nitrogen of the food by 5.1 grams per day, corresponding to 33 grams of proteid, which must have been drawn from the supply in the body. If we assume that lean flesh contains 25 per cent of proteid, this would mean about 4 3/4 ounces per day. The other two subjects, Miller and Albert, lost from the body per day 8.6 grams and 7.1 grams respectively of nitrogen, which would imply a loss of about 54 grams and 44 grams of body proteid respectively, or 8 ounces and 6 1/4 ounces of lean flesh per day. It is evident, therefore, that none of the three subjects consumed sufficient food to avoid loss of body proteid, under the existing conditions of muscular activity. Indeed, it may be noted in Miller’s case that the average fuel value of the food per day was 4770 calories, while the average expenditure of energy per day was 4820 calories. We should naturally expect, however, that any small deficiency in fuel value would be made good by a call upon body fat. “Why the body should use its own substance under such circumstances is a question which at present cannot be satisfactorily answered. The fact that such was the case, each of the contestants who finished the race consuming during the period body protein equivalent to 2 or 3 pounds of lean flesh, and that no injury resulted therefrom, would seem to indicate that these men had stores of protein which could be metabolized to aid in meeting the demands put upon the body by the severe exertion, without robbing any of the working parts, and at the same time relieving the system of a part of the labor of digestion. Possibly, the ability to carry such a store of available protein is one of the factors which make for physical endurance.”40 This possibility we shall have occasion to discuss in another connection. At present, the facts presented are to be accepted as accentuating the general law that the energy exchange of the body, everything else being equal, is increased proportionally to increase in the extent of external muscular activity. It may be noted that Albert, who did considerably less work than Miller, showed a much larger exchange of energy than the latter athlete. This, however, is to be connected with the fact that his fuel intake was 1300 calories larger per day than Miller’s; in other words, the conditions were not equal. This fact also calls to mind the observations of Schnyder,41 who, studying the relationship between muscular activity and the production of carbon dioxide, maintained that the quantity of this excretory product formed depends less upon the amount of work accomplished than upon the intensity of the exertion; efficiency in muscular work varying greatly with the condition of the subject, and his familiarity with the particular task involved.

From what has been said, it is obvious that oxygen consumption, as well as output of carbon dioxide, must vary enormously with variations in the muscular activity of the body. The one important factor influencing the quantities of oxygen and carbon dioxide exchanged in the lungs, i. e., the extent of the respiratory interchange, is muscular activity; and since, as we have seen, carbonaceous material is the substance mainly oxidized in muscle work, it follows, as carbon dioxide is excreted principally through the lungs, that the respiratory interchange becomes in good measure an indicator of the extent of chemical decomposition incidental to external work. If we recall that man, on an average, at each inspiration draws in about 500 cubic centimeters of air (30 cubic inches), and that for the 24 hours he averages 15 breaths a minute, it is easy to see that in one minute the average man will inspire 7.5 litres of air, or 450 litres an hour, with a total of 10,800 litres for the entire day, which is equivalent to about 380 cubic feet. This would be a volume of air just filling a room 7 1/3 feet in length, width, and height. Inspired air loses to the body 4.78 volumes per cent of oxygen, while expired air contains an excess of 4.34 volumes per cent of carbon dioxide. In muscular work, respiration is increased in frequency and in depth. The volume of air exchanged in the lungs during severe labor may be increased sevenfold, while oxygen consumption and carbon dioxide excretion are frequently increased 7–10 times. The following figures, being values for one minute, show the effect on oxygen consumption of walking on a level and climbing, the subject being a man of 55.5 kilos body-weight:42

Remembering that these figures represent the oxygen consumption for only one minute of time, it is easy to see the striking effect of moderate and vigorous exercise on respiratory interchange. Simply walking along a level suffices to increase the consumption of oxygen threefold over what occurs when the body stands at rest. When the more vigorous exercise attendant on lifting the body up a steep incline is attempted, most striking is the great increase in the amount of oxygen consumed. We thus see another forcible illustration of the influence of muscular activity upon the exchange of matter in the body, and a further confirmation of the statement, so many times made, that oxidation—especially the oxidation of fats and carbohydrates by which large quantities of heat are set free, easily convertible into mechanical energy—is a primary factor in the metabolic processes, by which the machinery of the living man is able to work so efficiently.

Finally, we cannot avoid the conclusion that the outgoings of the body, in the form of matter and energy, are subject to great variation, incidental to the degree of activity of the day or hour. The ordinary vicissitudes of life, bringing days of physical inaction, followed perhaps by periods of unusual activity; changes in climatic conditions, with their influence upon heat production in the body; alterations in the character and amount of the daily dietary, etc.,—all seemingly combine as natural obstacles to the maintenance of a true nutritive balance. Outgo, however, must be met by adequate amounts of proper intake if there is to be an approach toward a balance of nutrition. In some way the normal, healthy man does maintain, approximately at least, a condition of balance; not necessarily for every hour or for every day, but the intake and outgo if measured for a definite period, not too short, say for a week or two, will be found to approach each other very closely. Body equilibrium and approximate nitrogen balance may be reasonably looked for, as well as a balance of total energy, in the case of a healthy man leading a life which conforms to ordinary physiological requirements. The man who, on the other hand, consciously or unconsciously, continues an intake way beyond the outgo, whose daily income of nitrogen and total fuel value far exceeds the requirements of his body, obviously lives with an accumulating plus balance, which ordinarily shows itself in increasing body-weight and with a storing away of fat.

Equally conspicuous is the effect of an inadequate income of proper nutriment; a food supply which persistently fails to furnish the available nitrogen and total energy value called for by the body under the conditions prevailing, will inevitably result in a minus balance, which, if continued too long, must of necessity tax the body’s store to the danger limit. At the same time, the well-nourished individual, without being unduly burdened by a bulky store of energy-containing material, is always supplied with a sufficient surplus to meet all rational demands, when from any cause the intake fails, for brief periods of time, to be commensurate with the needs of the body. It is reasonable to believe, however, that in the maintenance of good health, and the preservation of a high degree of efficiency, the body should be kept in a condition approaching a true nutritive balance.