What is the nature of the material used up in connection with muscle work? As can readily be seen, this is an important question, for on its answer depends, in some measure at least, the character of the proper intake, or food, to be supplied in order to make good the loss. If the energy of mechanical work, the energy of muscle contraction, comes from the breaking down of proteid matter alone, then obviously excessive muscular work would need to be accompanied, or followed, by a generous supply of proteid food. If, on the other hand, external work means liberation of energy solely from non-nitrogenous materials, then it is equally clear that fats and carbohydrates are the proper foods to offset the drain incidental to vigorous muscular action.
The views of Liebig, briefly referred to in a previous chapter, held sway over physiologists for many years. His dictum that proteid foods were true plastic foods, entering into the structure of the tissues of the body, and that they alone were the real sources of muscular energy, met for a time with no opposition. It was not until the advent of a more critical spirit, accompanied by a fuller appreciation of the necessity of experimental evidence, that physiologists began to test with scientific accuracy the validity of the current views. It is worthy of note that long prior to this time, even before oxygen was discovered, the far-sighted and resourceful John Mayow, in his work with the various “spirits” of the body and their relation to respiration, etc., evolved the view that muscular power has its origin in the combustion of fat brought to the muscles by the blood and burned there by aid of a gas or “spirit” taken from the air by the lungs, and likewise carried to the muscles by the circulating blood. Considering the time when Mayow lived and the dearth of true scientific knowledge as we measure it to-day, his hypothesis was a wonderful forestalling of present views.
It is quite obvious that the views of Liebig, if true, admit of easy proof; since, if the energy of muscular power comes from the breaking down of proteid, there should be a certain parallelism between the output of nitrogen from the body and the amount of muscular work accomplished, everything else being equal. As stated in a previous chapter, such study of this question as was made soon disclosed the fact that the one element above all others that seemed to influence the output of nitrogen was the intake of proteid food. Thus, the English investigators, Lawes and Gilbert, found by experimenting with animals that when the latter were kept under uniform conditions of muscular work, the amount of nitrogen excreted ran parallel with the intake of nitrogen. Further, in the early experiments of Voit, the results obtained clearly showed that variations in the amount of work performed were practically without influence on the excretion of nitrogenous waste products.
The experiment, however, that came as a death blow to the theories of Liebig was that of Fick and Wislicenus,[43] who in 1865 made an ascent of the Faulhorn, 6500 feet high, using a diet wholly non-nitrogenous. From the nitrogen excreted they were able, of course, to calculate the amount of proteid oxidized in the body during the period of work, and found that the proteid consumed could not have furnished, at the most, more than one-half the energy required to lift the weights of their bodies to the top of the high peak. Further, they observed that neither during the work period, nor immediately after, was there any noticeable increase in the excretion of nitrogen. Obviously, as they state, the oxidation of proteid matter in the body cannot be the exclusive source of the energy of muscular contraction, since the measurable amount of external work performed in the ascent of the mountain was far greater than the equivalent of the energy capable of being furnished by the proteid actually burned. To which may be added the fact that considerable energy, not measurable in their experiment, must have been employed in the work of the involuntary muscles of the body; thus increasing by so much the difference between the muscular work actually accomplished and the available energy from proteid consumed. It is true that minor criticisms regarding certain details of the experiment can be offered to-day, such as the fact that the men were, in a measure, in a state of “nitrogen starvation,” etc., but these criticisms do not in any degree militate against the main thesis that the energy of muscular contraction does not come exclusively from the consumption or breaking down of proteid, either of food or tissue. Vigorous and even severe muscular work does not necessarily increase the decomposition of proteid material. Dogs made to run in large treadmills, with the same diet as on resting days, were found to excrete practically no more nitrogen than during the days of rest. Occasionally, however, in some one experiment the output of nitrogen would show an increase over the output on resting days. Further, experiments made with horses led to essentially the same result, except that greater increase in the excretion of nitrogen was observed than with dogs. This increase in nitrogen output, however, as a concomitant of increased muscular activity, could be prevented by adding to the amount of carbohydrate food.
While experiments of this nature, on man and animals, all tended to show little or no increase in the excretion of nitrogen, as a result of muscle work; and likewise no increase in the output of sulphur and phosphorus, thus strengthening the view that muscular energy is not the result of proteid disintegration, there was observed marked increase in the consumption of oxygen, and in the excretion of carbon dioxide. Non-nitrogenous matter was thus at once suggested as the material with which muscle chiefly does its work. There is to-day no question of the general truth of this statement, yet there are other aspects of the problem to be considered before we can lay it aside. Pflüger, working with dogs, and Argutinsky, experimenting on himself by arduous mountain climbing, reached conclusions seemingly quite opposed to what has just been said. Their results, however, admit of quite a different interpretation from what they were disposed to attach to them. Thus, Pflüger[44] would go back to the old view that all muscle work is at the expense of proteid material, because lean dogs fed mainly, or entirely, on meat and made to do an excessive amount of work were found by him to excrete nitrogen somewhat in proportion to the amount of work done. Argutinsky,[45] likewise, in his mountain climbing carried to the point of fatigue, and with a high proteid intake likewise, saw in the increased output of nitrogen a suggestion of the same idea. In reality, however, their results merely prove that, under some circumstances, proteid may serve as the chief source of muscular energy; as when the body is poor in fat and carbohydrate, or when the intake consists solely of proteid matter. In other words, muscular work may result in an increased excretion of nitrogen when the work is very severe, and there is not a corresponding increase in the fats or carbohydrates (fuel ingredients) of the food. In the words of Bunge,[46] “we might assume à priori, on teleological grounds, that in the performance of its most important functions the organism is to a certain extent independent of the quality of its food. As long as non-nitrogenous food is supplied in adequate quantity or is stored up in the tissues, muscular work is chiefly maintained from this store. When it is gone the proteids are attacked.”
There is no question that the energy of muscular contraction can come from all three classes of organic foodstuffs. Voluntary muscular movement is under the control of the nervous system, and when the stimulus is applied the muscle is bound to contract, provided of course there is sufficient energy-containing material present to furnish the means. Muscle tissue, like other tissues and organs, has a certain power of adaptability, by which it is able to do its work, even though it is not adequately supplied with its preferred nutrient. While proteid is plainly not the material from which the energy of muscular contraction is ordinarily derived, it is equally evident that in emergency, as when the usual store of carbohydrate and fat is wanting, proteid can be drawn upon, and in such cases vigorous work may be attended with increased nitrogen output. In harmony with this statement, we find on record in recent years many experiments, both with man and animals, where severe muscular labor is accompanied by an excretion of nitrogen beyond what occurs on days of rest; but by simply adding to the intake of non-nitrogenous food this increased outgo of nitrogen is at once checked. With moderate work, the nitrogen outgo is rarely influenced; it is only when the work becomes excessive, or the store of non-nitrogenous reserve is small and the intake of the latter food is limited, that proteid matter is drawn upon to supply the required energy.
Recalling what has been said regarding the significance of the respiratory quotient, it is obvious that we have here a means of acquiring information as to the character of the material that is burned up in the body during muscular work. Increased metabolism of carbohydrate will necessarily result in raising the respiratory quotient, and if the latter food material alone is involved the respiratory quotient must naturally approach 1.0. Zuntz, however, has clearly shown that vigorous muscular activity does not materially change the respiratory quotient; except in cases of very severe work, where the oxygen-supply of the muscles is interfered with. Indeed, the muscles may be made to do work sufficient to increase the consumption of oxygen threefold or more, without any change in the respiratory quotient being observed. And as there is frequently no change whatever in the output of nitrogen under these conditions, it follows that the energy of the muscle work must have come from the decomposition of non-nitrogenous material. If carbohydrates alone were involved, the respiratory quotient would obviously undergo change. Since, however, this remains practically stationary, we are led to the conclusion that fat must be involved in large degree, in addition to carbohydrate.
In this connection, it is a significant fact that with fasting animals, where the store of carbohydrate material is more or less used up, severe muscle work may be accomplished without any appreciable increase in nitrogen output, thus showing that proteid material is not involved and clearly pointing to fat as the source of the muscular energy. Thus, in an experiment referred to by Leathes, a dog on the sixth and seventh day of starvation was made to do work in a treadmill equivalent to climbing to a height of 1400 meters, yet the output of nitrogen was increased from six to only six and a half grams. Obviously, not much of the energy of this muscle work could have come from the breaking down of proteid, but it must have been derived mainly from the oxidation of fat. There is abundant evidence that fat can be used as a source of energy by muscles, as well as carbohydrates and proteids, and there is every reason for believing that the yield of work for a given amount of chemical energy in the form of fat is as good as in the case of either of the other two substances. In fact, the observations of Zuntz show that fat can be used just as economically by the body for muscle work as either carbohydrates or proteid. Thus, in one experiment,[47] he determined the oxygen-consumption and respiratory quotient in a man resting and working on three different diets—one principally fat, one principally carbohydrate, and the other principally proteid—and found that slightly less oxygen and energy were required to do work on the fat diet than on the others. This is clearly shown in the following table:
| Diet Principally. | Resting. | Working. | Kilo- | Per Kilogram-meter | |||
|---|---|---|---|---|---|---|---|
| Oxygen Used per Minute. | Respira- | Oxygen | Respira- | Oxygen | Calories. | ||
c.c. | c.c. | c.c. | |||||
| Fat | 319 | 0.72 | 1029 | 0.72 | 354 | 2.01 | 9.39 |
| Carbohydrate | 277 | 0.90 | 1029 | 0.90 | 346 | 2.17 | 10.41 |
| Proteid | 306 | 0.80 | 1127 | 0.80 | 345 | 2.38 | 11.35 |
From these data, we see that per kilogram-meter of work less energy was required and less oxygen consumed with fat than with either of the other two foodstuffs; but practically, fat and carbohydrate as sources of muscle energy have about the same value.