Much stress is ordinarily laid upon the importance of a large intake of proteid food whenever the body is called upon to perform severe, or long-continued, muscular work; but in view of what has been stated it may be questioned whether there is any real physiological justification for such custom. The pedestrian Weston,[48] who in 1884 walked 50 miles a day for 100 consecutive days, was found by Blyth during a period of five days to consume in his food 37.2 grams of nitrogen a day, while he excreted only 35.3 grams, leaving a balance of 1.9 grams of nitrogen per day apparently stored in the body. His daily food during this period was composed of 262 grams of proteid, 64.6 grams of fat, and 799 grams of carbohydrate, with an estimated fuel value of 4850 calories. Yet he performed this large amount of work daily, and still laid by a certain amount of proteid on a ration, the energy value of which would not ordinarily be considered high for the muscular work to be done. Fourteen years prior to this, Weston, while in New York, was carefully studied by Dr. Flint during a period of 15 days, on 5 of which he walked a total of 317 miles. His diet was essentially a proteid diet, consisting principally of beef extract, oatmeal gruel, and raw eggs. Nitrogen intake and output were carefully compared during the days of rest and during the days of work, with the results tabulated.
Period. | Occupation. | Duration | Nitrogen. | ||||
|---|---|---|---|---|---|---|---|
In | In | In | Gain + | ||||
days | grams | grams | grams | grams | |||
| Fore period | Comparative rest | 5 | 22.0 | 18.7 | 1.4 | +1.9 | |
| Working period | Walking 62 miles per day |  | 5 | 13.2 | 21.6 | 1.6 | –10.0 |
| After period | Rest | 5 | 28.6 | 22.0 | 2.2 | +4.4 | |
In this case it will be noted that the daily ration was comparatively small, and, further, that during the working period the subject consumed much less proteid than on the resting days. Moreover, when we remember that the total energy value of his diet must have been quite small, it is not at all strange that in the laborious task of walking 62 miles a day he should have temporarily drawn upon his store of body proteid to the extent of 62.5 grams, or 10 grams of nitrogen a day. Such experiences, however, do not by any means constitute proof that in excessive muscular work there is need for the consumption of correspondingly increased quantities of proteid food, or that the energy of muscular work comes preferably from the breaking down of proteid material. Carbohydrate and fat unquestionably take precedence over proteid in this respect, and we may accept as settled the view that in all practical ways carbohydrate and fat stand on an equal footing as sources of muscular energy. Less clear, perhaps, is the question as to how these two radically different types of organic material are utilized by the muscle. It has been a favorite belief among some physiologists that the contracting muscle makes use of only one substance as the direct source of its energy, and that this substance is the sugar dextrose. This view would seemingly imply that fat and proteid must undergo alteration prior to their utilization by the muscle; that, possibly, the carbon of the fat and proteid is transformed into sugar before the muscle can make use of it. So far as fat is concerned, this view is not supported by the facts available, since experiments show that the heat and energy liberated in the utilization of a given amount of fat in muscle work are in harmony with the energy value of the fat; in other words, the fat is apparently burned, or oxidized, directly, without undergoing previous transformation into any form of carbohydrate; or, if transformation does occur, under some conditions, it must take place within the muscle and without loss of energy. The practical significance of these facts is at once apparent, for if fat, in order to be available as a source of muscle energy, must first undergo conversion into sugar, it would be far more economical from a physiological standpoint to replace the fat of the diet with carbohydrate in any attempt to provide suitable nourishment for the working muscle. We may safely conclude, however, that fat and carbohydrate, as previously suggested, are in reality both capable of direct metabolism by the muscular tissue, and that each is of value as a source of muscular energy in proportion to its heat of combustion, yielding substantially the same proportion of its potential energy in the form of mechanical work.
Regarding the utilization of proteid as a source of energy by the muscle, there are many grounds for believing that here the body has to deal with certain alterations, before the proteid can be made available. We may indeed conjecture the transformation of a non-nitrogenous portion of the proteid molecule into carbohydrate, as a necessary step in its utilization for muscle work. It is certainly true that in the ordinary katabolic processes, through which proteid passes, there is a tendency for the nitrogen-containing portion to be quickly split off and eliminated, leaving a carbonaceous residue which may represent as much as 80 per cent of the total energy of the original proteid. This so-called carbon moiety of the proteid molecule is apparently much less rapidly oxidized than the nitrogenous portion, and may indeed be temporarily stored in the body, in the form of fat or carbohydrate.[49] We have very convincing proof that the carbohydrate glycogen can be formed from proteid. Thus, the feeding of proteid to warm-blooded animals may be accompanied by an accumulation of glycogen in the liver. This is interpreted as meaning that in the cleavage of proteid by digestion the various nitrogenous products formed are somewhere, probably in the liver, still further acted upon; the contained nitrogen with some of the carbon being converted into urea, while the non-nitrogenous residue is transformed into glycogen, or sugar. That some such change takes place, or, more specifically, that carbohydrate does result from proteid is more strikingly shown in human beings suffering with diabetes. In severe forms of this disease, all carbohydrate food consumed is rapidly eliminated through the kidneys in the form of sugar, the body having lost the power of burning sugar. If such a person is placed upon a diet composed exclusively of proteid, sugar still continues to be excreted, and there is observed a certain definite relationship between the nitrogen output and the excretion of sugar, thus implying that they have a common origin.
Further, there are certain drugs, such as phloridzin, which, when introduced into the circulation, set up a severe diabetes and glycosuria. Dogs treated in this way, fed solely on proteid or even starved for some time, will continue to excrete sugar, and as in the previous instance, there is observed a certain definite ratio between the nitrogen output and the elimination of sugar; thus leading to the conclusion that both arise from the destruction of the proteid molecule. Careful study of this ratio of dextrose to nitrogen has led Lusk to the conclusion that full 58 per cent of the proteid may undergo conversion into sugar in the body. Hence, it is easy to see how in muscle work, when proteid is the sole source of the energy of muscular contraction, the work accomplished may still result from the direct oxidation of carbohydrate material, indirectly derived from the proteid molecule. It requires no argument, however, to convince one that such a procedure for the normal individual is less economical physiologically than a direct utilization of carbohydrate and fat, introduced as such and duly incorporated with the muscle substance. Consequently, in the nourishment of the body for vigorous muscular work, there is reason in a diet which shall provide an abundance of carbohydrate and fat; proteid being added thereto only in amounts sufficient to meet the ordinary requirements of the body for nitrogen and to furnish, it may be, proper pabulum for the development of fresh muscle fibres, where, as in training, effort is being made to strengthen the muscle tissue and so enable it to do more work. Increase in proteid food may help to make new tissue, but the source of the energy of muscle work is to be found mainly in the breaking down of the non-nitrogenous materials, carbohydrate and fat.
In view of these facts, we may advantageously consider next the real significance of the proteid metabolism of the body. As we have seen, a meal rich in proteid leads at once—within a few hours—to an excretion of urea equivalent to full 50 per cent of the nitrogen of the ingested proteid, while a few hours later finds practically all of the nitrogen of the intake eliminated from the body. Further, it is to be remembered that in a general way this occurs no matter what the condition of the body may be at the time and no matter how large or small the amount of proteid consumed. In other words, there is practically no appreciable storing of nitrogen or proteid for future needs,—at least none that is proportional to the increase in nitrogen intake, even though the body be in a condition approximating to nitrogen starvation. Moreover, it is to be recalled that the increased proteid metabolism attendant on increased intake of proteid food is accompanied by an acceleration of the metabolism of non-nitrogenous matter; thus resulting in a stirring up of tissue change, with consequent oxidation and loss of a certain proportion of accumulated fat and carbohydrate. Coincident with this increased excretion of nitrogen, the output of carbon dioxide is likewise increased somewhat, due as is believed mainly to increased metabolism of the involuntary muscle fibres of the gastro-intestinal tract, by action of which the accelerated peristalsis so characteristic of food intake is accomplished. Further, the increased output of carbon dioxide, under these conditions, is to be attributed also to the greater activity of the digestive and excretory organs, naturally stimulated to greater functional power by the presence of proteid foods and their decomposition products. Still, as stated by Leathes, “the two main end-products of proteid metabolism, urea and carbonic acid, are, to a great extent, produced independently of each other, and the reactions which result in the discharge of the nitrogen are not those in which energy is set free, work done, and carbonic acid produced.” In other words, there is suggested what we have already referred to, viz., that in proteid metabolism a nitrogenous portion of the proteid molecule is quickly split off and gotten rid of, while the non-nitrogenous part may be reserved for future oxidation, serving as a source of muscle energy or for other purposes. This being so, it is plain that “proteid metabolism in so far as it is concerned with the evolution of energy, proteid metabolism in its exothermic stages, may be almost entirely non-nitrogenous metabolism” (Leathes).
Is there any advantage to the body, however, in this carbonaceous residue of the proteid molecule over simple carbohydrate and fat? Can the processes of the body be accomplished more economically, or more advantageously, with a daily diet so constructed that the tissues and organs must depend mainly upon this carbon moiety of the proteid molecule for their energy-yielding material? It has been one of the physiological dogmas of the past, that the tissues and organs of the body, or rather their constituent cells, preferred to use proteid for all their needs whenever it was available. If proteid were wanting, either because of insufficient intake, or because of excessive activity, then the tissue cells would draw upon their store of non-nitrogenous material. Food proteid and tissue proteid, however, were the materials preferred by the organism, so ran the argument, and the large and incessant output of nitrogen which accompanied the intake of proteid was accepted as proof of the general truth of this idea. We might well question wherein lies the great advantage to the body in this continual excretion of nitrogen; whether the loss of energy in handling and removing the nitrogenous portion of the necessarily large proteid intake, in order to render available the non-nitrogenous part of the molecule, might not more than compensate for the supposed gain? But the truly astonishing fact that the output of nitrogen runs parallel with the intake of proteid, that the body cannot store up nitrogen to any large extent, has been taken as conclusive evidence that the organism prefers to use proteid for all of its requirements. Truly, we might just as well argue that this significant rise in the excretion of nitrogen after partaking of a proteid meal is an indication that the body has no need of this excess of nitrogen; that it is indeed a possible source of danger, since the system strives vigorously to rid itself of the surplus, and that the energy-needs of the body can be much more advantageously and economically met from fat and carbohydrate than from the carbonaceous residue resulting from the disruption of the proteid molecule.
In discussing these questions, we shall need to refer to several of the current theories concerning proteid metabolism, notably, the theories of Voit, Pflüger, and Folin. In 1867 Carl Voit,[50] of Munich, advanced the view that the proteid material of the body exists in two distinct forms, viz., as “morphotic” or “organized” proteid, representing proteid which has actually become a part of the living units of the body, i. e., an integral part of the living tissues; and “circulating” proteid, or that which exists in the internal meshes of the tissue, or in the surrounding lymph and circulating blood. The real point of distinction here is that while one portion of the body proteid is raised to the higher plane of living matter, i. e., becomes a component part of the living protoplasm, another and perhaps larger portion is outside of the morphological framework of the tissue, constituting a sort of internal medium which bathes the living cells, and acts as middleman between the blood and lymph on the one side and the living cells on the other. According to Voit’s view, it is this circulating proteid that undergoes metabolism; the proteid of the food after digestion and absorption being carried to the different tissues and organs, and then, without becoming an integral part of the living protoplasm of the cells, it is broken down under the influence of the latter. Obviously, small numbers of tissue cells are constantly dying, their proteid matter passing into solution, where it likewise undergoes metabolism. In other words, according to Voit, the great bulk of the proteid undergoing katabolism is the circulating proteid, derived more or less directly from the food, and which at no time has been a part of the tissue framework; while a smaller, but more constant amount, represents the breaking down of tissue cells. This conception of proteid metabolism is akin to our conception of morphological and physiological destruction. In the words of Foster: “We know that an epithelial cell, as notably in the case of the skin, may be bodily cast off and its place filled by a new cell; and probably a similar disappearance of and renewal of histological units takes place in all the tissues of the body to a variable extent. But in the adult body these histological transformations are, in the cases of most of the tissues, slow and infrequent. A muscle, for instance, may suffer very considerable wasting and recover from that wasting without any loss or renewal of its elementary fibres. And it is obvious that the metabolism of which we are now speaking does not involve any such shifting of histological units. On the other hand, we find these histological units, the muscle fibre or the gland cell, for instance, living on their internal medium, the blood, or rather on the lymph, which is the middleman between themselves and the actual blood flowing in the vascular channels.”
Voit claims that the proteid dissolved in the fluids of the body is more easily decomposable than that which exists combined in organized form, or as more or less insoluble tissue proteid; and it is this soluble and circulating form which, under the influence of the living cells, undergoes destruction or metabolism. We know, as has been previously stated, that oxidation does not take place to any extent in the circulating blood, and similarly there is every reason for believing that proteid metabolism does not occur in this menstrum. Metabolism is limited mainly to the active tissues of the body, but according to the present conception of the matter it does not occur at the expense of the proteid of the living cells, but involves material contained in the fluids bathing the cells; i. e., it is not the organized proteid that undergoes metabolism, but the proteid circulating in and about the internal meshes of the cells and tissues, the living cell being the active agent in controlling the process. Further, this view lessens the difficulty of understanding the elimination of nitrogen after a meal rich in proteid. If it was necessary to assume that all the proteid of our daily food is built up into living protoplasm before katabolism occurs, it would be exceedingly difficult to explain the sudden and rapid elimination of nitrogen which follows the ingestion of proteid. For example, we can hardly imagine that merely eating an excess of proteid food will lead to an actual breaking down of the living framework of the tissues, equivalent to the amount of nitrogen which the body at once eliminates. Voit’s theory, on the other hand, supposes a twofold origin of the nitrogen excreted; one part, the larger and variable portion, comes from the direct metabolism of the circulating proteid, being the immediate result of the ingested food and varying in amount with the quantity of proteid food consumed; the other, smaller and less variable in amount, has its origin in the metabolism of the true tissue proteid, or the actual living framework of the body.
In a fasting animal, the tissues and organs of the body still contain a large proportion of proteid matter, yet only a small fraction of this proteid is eliminated each day, hardly 1 per cent. If, however, proteid is absorbed from the intestine, proteid metabolism is at once increased, and the excretion of nitrogen may be fifteen times greater than during hunger. In other words, the extent of proteid metabolism is not at all proportional to the total amount of proteid contained in the body as a whole, but runs parallel in a general way with the quantity of proteid absorbed from the intestine. Obviously, the newly absorbed proteid is quite different in nature from the proteid which in much larger amounts is deposited throughout the body, since it is not organized and is so much more easily decomposable (Voit). This is the circulating proteid of the body; it exists in solution, and it is a significant fact that, according to Voit, the chemical transformations that characterize proteid katabolism occur only in solution. The organized proteid, on the other hand, is in a state of suspension, and its katabolism, which is relatively very small, is preceded by solution of the proteid in the fluids of the tissue, after which its further breaking down is assumed to be the same as that of the circulating proteid. This latter view is a fundamental part of the Voit theory; in long-continued fasting, for example, the living protoplasm of the various tissues and organs is of necessity drawn upon for the nourishment of the more vital parts of the body, such as the brain, spinal cord, etc., consequently the organized proteid is gradually dissolved and then decomposed, after it has become liquefied and has thus lost its organized structure.