CHAPTER III.ALIMENTARY ENERGETICS.
Various Problems of Alimentation. § 1.Food the source of Energy and Matter.The two forms of Energy afforded by Food—Vital Energy, Thermal Energy. Food the source of Heat. The rôle of Heat.—§ 2.Measure of the output of Energy—by the Calometric Method—by the Chemical Method.—§ 3. The regular type of Food, Biothermogenic, and the irregular type, Thermogenic.—§ 4. Food considered as the source of Heat. The Law of Surfaces. The limits of Isodynamics.—§ 5. Plastic rôle of Food. Preponderance of Nitrogenous Foods.
Among the problems on which energetics has thrown a vivid light we have mentioned alimentation, muscular contraction, and, more general still, the intermittence of vital functional activity. We shall begin with the study of alimentation.
The Different Problems of Alimentation.—What is a food? In what does alimentation consist? The dictionary of theAcadémiewill give us our first answer. It tells us that the word food is applied to “every kind of matter, whatever may be its nature, which habitually serves or may serve for nutrition.” This is very well put, but here again we must know what nutrition is, and that is not a simple matter; in fact, it practically means whatever is usually placed on the table in a civilized and polished society. Butit is just the profound reasons for this traditional practice that we are trying to discover.
The problem of alimentation may be looked at in a thousand ways. It is culinary, no doubt, and gastronomic; but it is also economical and social, agricultural, fiscal, hygienic, medical, and even moral. But first and foremost, it is physiological. It comprises and assumes the knowledge of the general composition of foods, of their transformations in the digestive apparatus, and their comparative utility in the maintenance and the sound functional activity of the organism. To this first group of subjects for our discussion are attached others relating to the effects of inanition, of insufficient alimentation, and of over-feeding. And in order to throw light on all these aspects of the problem of alimentation, we have to lay bare the most intimate and delicate reactions by which the organism is maintained and recruited, and, in the words of a celebrated physiologist, “to penetrate into the kitchen of vital phenomena.” And here neither Apicius, nor Brillat-Savarin, nor Berchoux, nor the moralists, nor the economists are of any use to us as guides. We must appeal to the scientists, who, following the example of Lavoisier, Berzelius, Regnault, and Liebig, have applied to the study of living beings the resources of general science, and have thus foundedchemical biology.
This branch of science developed considerably in the second half of the nineteenth century. It has now its methods, its technique, its chairs at the universities, its laboratories, and its literature. It has particularly applied itself to the study of the “material changes” or themetabolismof living beings, and with that object in view it has done two things. In the firstplace, it has determined the composition of the constituent materials of the organism; then analyzing qualitatively and quantitatively all that penetrates into that organism in a given time—that is to say, all the alimentary or respiratory ingesta, and all that issues from the organism,i.e., all the excreta, all theegesta,—it has drawn upnutritive balance sheets, corresponding to the various conditions of life, whether naturally or artificially created. And thus we can determine the alimentary régimes which give too much, and which give too little, and which finally restore equilibrium.
We do not propose to give a detailed account of this scientific movement. This may be done in monographs. All we wish to indicate here is the most general result of these laborious researches—that is to say, the laws and the doctrines which are derived from them, and the theories to which they have given birth. It is by this alone that they are brought into relation with general science, and may therefore interest the reader. The facts of detail are never lacking to the historian; it is more profitable to show the movement of ideas. The theories of alimentation bring into conflict very different conceptions of the vital functional activity. And here we find a confused medley of opinions on which it is not without interest to endeavour to throw some light.
Definitions of Food.—Before the introduction into physiology of the notion of energy, no one had succeeded in giving an exact idea and a precise definition of food and alimentation. Every physiologist and medical man who attempted it had failed, and this for various reasons.
The general cause of this failure was that most definitions, popular or technical, interposed the condition that the food must be introduced into the digestive apparatus. “It is,” said they, “a substance which when introduced into the digestive tube undergoes, etc., etc.” But plants draw food from the soil, and they possess no digestive apparatus; many animals have no intestinal tube; and in the case of certain rotifera, the females possess a digestive apparatus, while the males have none. Nevertheless all animals feed.
On the other hand, there are other substances than those which use the digestive tract for the purpose of entering the organism, and which are eminently useful or necessary to the maintenance of life. In particular we may mention oxygen.
The distinctive feature of food is itsutility—when conveniently introduced or employed—to the living being. Claude Bernard’s definition is this:—A substance taken in the external medium “necessary for the maintenance of the phenomena of the healthy organism and for the reparation of the losses it constantly suffers.” “A substance which supplies an element necessary for the constitution of the organism, or whichdiminishes its disintegration” (stored-up food); this is the definition of C. Voit, the German physiologist. M. Duclaux says, in his turn, but in far too general terms, that it is a substance which contributes to assure the sound functional activity of any of the organs of the living being. None of these ways of describing food gives a complete idea.
Food, the Source of Energy and Matter.—The intervention of the notion of energy enables us more completely to understand the true nature of food. We must, in fact, have recourse to the energetic conception if we desire to take into account all that the organism requires from food. It not only requiresmatter, but also, and most important of all, energy.
Investigators so far concentrated their thoughts exclusively on the necessity of a supply of matter—that is to say, they only looked upon one side of the problem. The living body presents, at each of its points, an uninterrupted series of disintegrations and reconstitutions, the materials being supplied from without by alimentation, and rejected by excretion. Cuvier gave to this unceasing circulation of ambient matter throughout the vital world the name ofvital vortex, and he rightly saw in it the characteristic of nutrition, and the distinctive feature of life.
This idea of thecycle of matterhas been completed in our own time by that of thecycle of energy. All the phenomena of the universe, and therefore those of life, are conceived of as energetic transformations. We now look at them in their relationship instead of considering them individually as of old. Each has an antecedent and a consequent unity with which it is connected in magnitude by the law of equivalents taught us by contemporary physics. And thus we may conceive of their succession as the cycle of a kind of indestructible agent, which changes only apparently, or assumes another form as it passes from one to the other, but its magnitude remains unaltered. This is energy. Thus, in the living being there is not only a circulation of matter, but also a circulation of energy.
The most general result of research in physiologicalchemistry from the time of Lavoisier down to our own day has been to teach us thatthe antecedent of the vital phenomenon is always a chemical phenomenon. The vital energies are derived from the potential chemical energy accumulated in the immediate constituent principles of the organism. In the same waythe consequent phenomenon of the vital phenomenon is in general a thermal phenomenon. The final form of vital energy is thermal energy. These three assertions as to the nature, the origin, and the final form of vital phenomena constitute the three fundamental principles, the three laws, of biological energetics.
Food, a Source of Heat. It is not quâ source of heat that food is the source of vital energy.—The place of vital energy in the cycle of universal energy is completely determined. It lies between the chemical energy which is its generating form and the thermal energy which is its form of disappearance, of breakdown, the “degraded form,” as the physicists say. Hence we have a result which can be immediately applied in the theory of food—namely, that heat is in the dynamical order an excretum of the animal life rejected by the living being, just as in the substantial order, urea, carbonic acid and water, are the materials used up and again rejected by it. We therefore must not think of the transformation in the animal organism of heat into vital energy, as certain physiologists always do. Nor must we think, with Béclard, of its transformation into muscular movement; or, as others have maintained, into animal electricity. This is not only an error of doctrine but an error of fact. It proceeds from a false interpretation of the principle of the mechanical equivalent of heat and a misunderstanding of Carnot’s principle. Thermal energy doesnot repeat the course of the energetic flux in the animal organism. The heat is not transformed into anything. It is simply dissipated.
The Part played by Animal Heat as a Condition of Physiological Manifestations.—Does this mean that heat is useless to life in the very beings in which it is most abundantly produced—i.e., in man and in the warm-blooded vertebrates? So far from this being so, it is necessary to life. But its utility has a peculiar character which must neither be misunderstood nor exaggerated. It is not transformed into chemical or vital reactions, but merely creates for them a favourable condition.
According to the first principle of energetics, for the vital fact to be derived from the thermal fact, the heat must be preliminarily transformed into chemical energy, since chemical energy is necessarily an antecedent and generating form of vital energy. Now this regressive transformation is impossible according to the current theories of general physics. The part played by heat in the act of chemical combination is that of a primer to the reaction. It consists in placing the reacting bodies, by changing their state or by modifying their temperature, in the condition in which they ought to be for the chemical forces to come into play. For example, in the combination of hydrogen and oxygen by setting light to an explosive mixture, heat only acts as a primer to the phenomenon, because the two gases which are passive at ordinary temperatures, require to be raised to 400° C. before chemical affinity comes into play. And so it is with the reactions which go on in the organism. They have a maximum temperature, and the part played by animal heat is to furnish them with it.
It follows that heat intervenes in animal life in two capacities—first and foremost asexcretum, or end of the vital phenomenon, ofphysiological work; and on the other hand, as aconditionorprimerof the chemical reactions of the organism; and generally, as a favourable condition for the appearance of the physiological manifestations of living matter. Thus, it is not dissipated in sheer waste.
I was led to adopt these views some years ago from certain experiments on the rôle played in food by alcohol. I did not then know that they had already been expressed by one of the masters of contemporary physiology, M. A. Chauveau, and that they were related in his mind to a series of conceptions and of researches of great interest, in the development of which I have since then taken a share.
Two Forms of Energy supplied to Animals by Food.—To say that food is simultaneously a supply of energy and a supply of matter, is really to express in a single sentence the fundamental conception of biology, in virtue of which life brings into play no substratum or characteristic dynamism. According to this, the living being appears to us as the seat of an incessant circulation of matter and energy, starting from the external world and returning to it. All food is nothing but this matter and this energy. All its characteristics, our views as to its rôle, its evolution, all the rules of alimentation are simple consequences of this principle, interpreted by the light of energetics.
And first of all, let us ask what forms of energy are afforded by food? It is easy to see that there are two—food is essentially a source of chemical energy; and secondarily and accessorily, it is a source of heat.Chemical energy is the only energy, according to the second law of energetics, which may be transformed into vital energy. It is true at any rate for animals; for in plants it is otherwise. There the vital cycle has neither the same point of departure nor the same final position. The circulation of energy does not take place in the same manner.
On the other hand, and this we are taught by the third law, energy brought into play in vital phenomena is finally liberated and restored to the physical world in the form of heat. We have just said that this release of heat is employed in raising the temperature of the living being. It is animal heat.
Thus there are two forms of energy supplied by food, chemical and thermal.
It must be added that these are not the only forms, but the principal, and by far the most important. It is not absolutely true that heat is the only outcome of the vital cycle. It is only so in the subject in repose, contented to live idly without doing external mechanical work, without lifting a tool or a weight, even that of its own body. And again, speaking in this way, we neglect all the movements and all the mechanical work which is done without exercise of the volition, by the beating of the heart and of the arteries, the movements of respiration, and the contractions of the digestive tube.
Mechanical work is, in fact, another possible termination of the cycle of energy. But there is no longer anything necessary or inevitable in this, since motion and the use of force are in a certain measure subordinated to the capricious volition of the animal.[11]
At other times, again, it is an electrical phenomenon which terminates the vital cycle, and it is, in fact, in this way that things happen in the functional activity of the nerves and muscles in all animals, and in the functional activity of the electrical organ in fish, such as the ray and the torpedo. Finally, the termination may be a photic phenomenon, and this is what happens in phosphorescent animals.
It is idle to diminish the power of these principles by proceeding to enumerate the whole of the exceptions to their validity. We know perfectly well that there are no absolute principles in nature. Let us say, then, that the energy which temporarily animates the living being is furnished to it by the external world under the exclusive form of potential chemical energy; but that, if there is only one door of entry, there are two exits. It may return to the external world in the principal form of thermal energy and in the accessory form of mechanical energy.
Calorimetric Method.—From what has preceded it is clear that if the energeticfluxwhich circulates through the animal emerges,in toto, in the state of heat, the measurement of this heat becomes themeasurement of the vital energy itself, for the origin of which we must go back to the food. If the flux is divided into two currents, mechanical and thermal, they must both be measured and the sum of their values taken. If the animal does not produce mechanical work, and all ends in heat, we have only to capture, by means of a calorimeter, this energetic flux as it emerges, and thus measure in magnitude and numerically the energy in motion in the living being. Physiologists use for this purpose various types of apparatus. Lavoisier and Laplace used an ice calorimeter—that is to say, a block of ice in which they shut up a small animal, such as a guinea-pig; they then measured its thermal production by the quantity of ice it caused to melt. In one of their experiments, for instance, they found that a guinea-pig had melted 341 grammes of ice in the space of ten hours, and had therefore set free 27 Calories.
But since those days more perfect instruments have been invented. M. d’Arsonval employed an air calorimeter, which is nothing but a differential thermometer very ingeniously arranged, and giving an automatic record. Messrs. Rosenthal, Richet, Hirn and Kaufmann, and Lefèvre have used more or less simplified or complicated air calorimeters. Others, following the example of Dulong and Despretz, have used calorimeters of air and mercury, or with Liebermester, Winternitz, and J. Lefèvre (of Havre), have had recourse to baths. Here, then, there is a considerable movement of research which has led to the discovery of very interesting facts.
Measurement of the Supply of Alimentary Energy by the Chemical Method.—We may again reach our result in another way. Instead of surprising the current of energy as it emerges and in the form of heat, we may try and capture it at its entry in the form of potential chemical energy.
The evaluation of potential chemical energy may be effected with the same unit of measurement as the preceding—that is to say, the Calorie. If we consider man and mammals, for example, we know that there is only apparently an infinite variety in their foods. We may say that they feed on only three substances. It is a very remarkable fact that all the complexity and multiplicity of foods, fruits, grains, leaves, animal tissues, and vegetable products of which use is made, reduce to so great a simplicity and uniformity, that all these substances are of three types only: albuminoids, such as albumen or white of egg—foods of animal origin or varieties of albumen; carbo-hydrates, which are more or less disguised varieties of sugar; and finally, fats.
Here, then, from the chemical point of view, leaving out certain mineral substances, are the principal categories of alimentary substances. Here, with the oxygen that is brought in by respiration, is everything that penetrates the organism.
And now, what comes out of the organism? Three things only, water, carbonic acid, and urea. But the former are the products of the combustion of the latter. If we consider an adult organism in perfect equilibrium, which varies throughout the experiment neither in weight nor in composition, we may say that the receipts balance the expenditure. Albumen, sugar, fat, plus the oxygen brought in, balance quantitatively the water, carbonic acid, and urea expelled. Things happen, in fact, as if the foods of the three categories were burned up more or less completely by the oxygen.
It is this combustion that we have known since the days of Lavoisier to be the source of animal heat. We can easily determine the quantity of heat left by albumen passing into the state of urea, and by the starch, the sugars, and the fats reduced to the state of water and carbonic acid. This quantity of heat does not depend on the variety of the unknown intermediary products which have been formed in the organism. Berthelot has shown that this quantity of heat which measures the chemical energy liberated by these substances is identical with the quantity obtained by burning the sugar and the fats in a chemical apparatus, in a calorimetric bomb, until we get carbonic acid and water, and by burning albumen till we get urea. This result is a consequence of Berthelot’sprinciple of initial and final states. The liberated heat only depends on the initial and final states, and not on the intermediary states. The heat left in the economy by the food being the same as that left in the calorimetric bomb, it is easy for the chemist to determine it. It has thus been discovered that one gramme of albumen produces 4.8 Calories, one gramme of sugar 4.2 Calories, and one gramme of fat 9.4 Calories. We thus gather what a given ration—a mixture in certain proportions of these different kinds of foods—supplies to the organism and what energy it gives it, measured in Calories.
The calculation may be carried out to a high degree of accuracy if, instead of confining ourselves to the broad features of the problem, we enter into rigorous detail. It is only, in fact, approximately that we have reduced all foods to albumen, sugar, and fat, and all excreta to water, carbonic acid, and urea.
The reality is a little more complicated. Thereare varieties of albumen, carbo-hydrates, and fatty bodies, the heats of combustion of which in the organism oscillate in the neighbourhood of the numbers 4.8, 4.2, and 9.4. Each of these bodies has been individually examined, and numerical tables have been drawn up by Berthelot, Rubner, Stohmann, Van Noorden, etc. The tables exhibit the thermal value or energetic value of very different kinds of foods.
In our climate, the adult average man, doing no laborious work, daily consumes a maintenance ration composed, as a rule, of 100 grammes of albuminoids, 49 grammes of fats, and 403 grammes of carbo-hydrates. This ration has an energetic value of 2,600 Calories.
It is therefore, thanks to the victories won in the field of thermo-chemistry, and to the principles laid down since 1864 by M. Berthelot, that this second method of attack on nutritive dynamism has been rendered possible. Physiologists, by the aid of these methods, have drawn upbalance-sheets of energyfor living beings just as they had previously establishedbalance-sheets of matter.
Now, it is precisely researches of this kind that we have indicated here as a consequence of biological energetics, which in reality have helped to build up that principle. These researches have shown us that, in conformity with theprinciples of thermodynamics, there was not, in fact, in the organism, any transformation of heat into mechanical work, as the physiologists for a short time supposed, on the authority of Berthelot. With the help of our theory this mistake is no longer possible. The doctrine of energetics shows us in fact the current of energy dividing itself, as it issues from the living being, intotwo divergent branches, the one thermal and the other mechanical, external the one to the other although both issuing from the same common trunk, and having between them no relation but this, that the sum of their discharges represents the total of the energy in motion. Let us now translate these very simple notions into the more or less barbarous jargon in use in physiology. We shall be convinced as we go on of the truth of the saying of Buffon, that “the language of science is more difficult to learn than the science itself.” We shall say, then, that chemical energy, that the unit of weight of the food which may be placed in the organism, constitutes the alimentarypotential, theenergetic valueof this substance, itsdynamogenic power. It is measured in units of heat, in Calories, which the substance may leave in the organism. The evaluation is made according to the principles of thermo-chemistry, by means of the numerical tables of Berthelot, Rubner, and Stohmann. The same number also expresses thethermogenic power, virtual or theoretical, of the alimentary substance. This energy being destined to be transformed intovital energies(Chauveau’sphysiological work,physiological energy), the dynamogenic or thermogenic value of the food is at the same time its biogenetic value. Two weights of different foods which supply the organism with the same number of Calories,—i.e.for which these numerical values are the same,—will be calledisodynamicorisodynamogenic,isobiogenetic,isoenergeticweights. They will be equivalent from the point of view of their alimentary value. And finally, if, as is usually the case, the cycle of energy ends in the production of heat, the food which has been utilized for this purpose has a realthermogenic value,identical with its theoretical thermogenic value. In this case it might be determined experimentally by direct calorimetry, measuring the heat produced by the animal supposed absolutely unchanged and identical before and after the consumption of the food.
Food is a source of thermal energy for the organism because it is decomposed within it, and undergoes within it a chemical degradation. Physiological chemistry tells us that whatever be the manner in which it is broken up, it always results in the same body and always sets free the same quantity of heat. But if the point of departure and the point of arrival are the same, it is possible that the path pursued is not constantly identical. For example, one gramme of fat will always give the same quantity of heat, 9.4 Calories, and will always come to its final state of carbonic acid and water; but from the fat to the mixture of carbonic acid gas and water there are many different intermediaries. In a word we get the conception of varied cycles of alimentary evolutions.
From the point of view of the heat produced it has just been said that these cycles are equivalent. But are they equivalent from the vital point of view? This is an essential question.
Let us imagine the most ordinary alternative. Food passes from the natural to the final state after being incorporated with the elements of the tissues,and after having taken part in the vital operations. The chemical potential only passes into thermal energy after having passed through a certain intermediary phase of vital energy. This is the normal case,the regular type of alimentary evolution. It may be said in this case that the food has fulfilled the whole of its function, it has served for the vital functional activity before producing heat. It has beenbiothermogenic.
The irregular or pure thermogenic type.—And now let us conceive of the most simpleirregular or aberrant type. Food passes from the initial to the final state without incorporation in the living cells of the organism, and without taking part in the vital functional activity. It remains confined in the blood and the circulating liquids, but it undergoes in the end, however, the same molecular disintegration as before, and sets free the same quantity of heat Its chemical energy changes at once into thermal energy. Food is apure thermogen. It has fulfilled only one part of its work. It has been of slight vital utility.
Does this ever occur in reality? Are there foods which would be onlypure thermogens—that is to say, which would not in reality be incorporated with the living anatomical elements, which would form no part of them either in a state of provisory constituents of the living protoplasm, or in the state of reserve-stuff; which would remain in the internal medium, in the blood and the lymph, and would there undergo their chemical evolution? Or again, if the whole of the food does not escape assimilation, would it be possible for part to escape it? Would it be possible for one part of the same alimentary substance to beincorporated, and for the rest to be kept in the blood or the lymph, in the circulating liquidsad limina corporis, so to speak? In other words, can the same food be according to circumstances abiothermogenor apure thermogen? Some physiologists—Fick of Wurzburg, for instance—have claimed that this is really the case for most nitrogenous elements, carbohydrates, and fats; all would be capable of evolving according to the two types. On the other hand, Zuntz and von Mering have absolutely denied the existence of the aberrant or pure thermogenic type. No substances would be directly decomposed in the organic liquids apart from the functional intervention of the histological elements. Finally, other authors teach that there is a small number of alimentary substances which thus undergoes direct combustion, and among them is alcohol.
Liebig’s Superfluous Consumption.—Liebig’stheory of superfluous consumptionand Voit’stheory of the circulating albumenassert that the proteid foods undergo partial direct combustion in the blood vessels. The organism only incorporates what is necessary for physiological requirements. As for the surplus of the food that is offered it, it accepts it, and, so to speak, squanders it; it burns it directly; and we have a “sumptuary” consumption, consumptionde luxe.
In this connection arose a celebrated discussion which still divides physiologists. If we disengage the essential body of the discussion from all that envelops it, we see that it is fundamentally a question of deciding whether a food always follows the same evolution whatever the circumstances may be, and particularly when it is introduced in great excess.Liebig thought that the superabundant part, escaping the ordinary process, was destroyed by direct combustion. He affirmed, for instance, that nitrogenous substances in excess were directly burned in the blood instead of passing through their usual cycle of vital operations. We might express the same idea by saying that they then undergo an accelerated evolution. Instead of passing through the blood in the anatomical element, to return in the dismembered form from the anatomical element to the blood, their breaking up takes place in the blood itself. They save a displacement, and therefore in reality remain external to the construction of the living edifice. Their energy, crossing the intermediary vital stage, passes with a leap from the chemical to the thermal form. Liebig’s doctrine reduced to this fundamental idea deserved to survive, but mistakes in minor details involved its ruin.
Voit’s Circulating Albumen.—A few years later C. Voit, a celebrated physiological chemist of Munich, revived it in a more extravagant form. He held that almost the whole of the albuminoid element is burned directly in the blood. He interpreted certain experiments on the utilization of nitrogenous foods by imagining that these substances when introduced into the blood were divided as a result of digestion into two parts: the one very small, which was incorporated with the living elements, and passed into the stage oforganized albumen, the other, corresponding to the greater part of the alimentary albumen, remained mingled with the blood and lymph, and was subjected in this medium to direct combustion. This wascirculating albumen. In this theory the tissues are almost stable; the organicliquids alone are subjected to oxydizing transformations, to nutritive metabolism. The accelerated evolution, which Liebig considered as an exceptional case, was to C. Voit the rule.
Current Ideas as to the Rôle of Foods.—The ideas of to-day are not those of Voit; but they do not, however, differ from them essentially. We no longer admit that the greater part of the ingested and digested albumen remains confined in the circulating medium external to the anatomical elements. It is held, with Pflüger and the school of Bonn, that it penetrates the anatomical element and is incorporated in it; but in agreement with Voit it is believed that a very small part is assimilated to the really living matter, to the protoplasm properly so called; the greater part is deposited in the cellular element as reserve-stuff. The material, properly so called, of the living machine does not undergo destruction and reparation as extensively as our predecessors supposed. There is no need for great reparation. On the contrary, the physiological activity consumes to a great extent the reserve-stuff. And the greater part of the food, after having undergone suitable elaboration, serves to replace the reserve-stuff destroyed in each anatomical element by the vital functional activity.
Experimental Facts.—Among the facts which brought physiologists of the school of Voit to believe that most foods do not get beyond the internal medium, there is one which may well be mentioned here. It has been observed that the consumption of oxygen in respiration increases notably (about a fifth of its value) immediately after a meal. What does this mean? The interval is too short for the digestedalimentary substances to have been elaborated and incorporated in the living cells. It is supposed that an appreciable time is required for this complete assimilation. The products of alimentary digestion are therefore in all probability still in the blood, and in the interstitial liquids in communication with it. The increase of oxygen consumed would show that a considerable portion of these nutritive substances absorbed and passed into the blood would be oxydized and then and there destroyed. But this interpretation, however probable it may be, does not really fit in with the facts in such a way that we may consider it as proved. Certain experiments by Zuntz and Mering are opposed to the idea that combustion in the blood is easy. These physiologists injected certain oxydizable substances into the vessels without being able to detect any instantaneous oxidation. It is only fair to add that against these fruitless attempts other more fortunate experiments may be quoted.
Category of Purely Thermogenic Foods, with Accelerated Evolution. Alcohol. Acids of Fruits.—The accelerated evolution of foods—an evolution which takes place in the blood, that is to say outside the really living elements—remains, therefore, very uncertain as far as ordinary food is concerned. It has been thought that it was a little less uncertain as far as the special category of alcohol, acids of fruits, and glycerine is concerned.
Some authors consider these bodies as pure thermogens. When alcohol is ingested in moderate doses, they say that about a tenth of the quantity absorbed becomes fixed in the living tissues; the rest is “circulating alcohol.” It is oxidizeddirectly in the blood and in the lymph, without intervening in the vital functions other than by the heat it produces. From the point of view of the energetic theory these are not real foods, because their potential energy is not transformed into any kind of vital energy, but passes at once to the thermal form. On the other hand, other physiologists look upon alcohol as really a food. According to them everything is called a food which is transformed in the organism with the production of heat; and they measure the nutritive value of a substance by the number of Calories it can give up to the organism. So that alcohol would be a better food than carbohydrated and nitrogenous substances. A definite quantity of alcohol, a gramme for instance, is equivalent from the thermal point of view to 1.66 grammes of sugar, 1.44 of albumen, or 0.73 of fat. These quantities would beisodynamic.
Experiment has not entirely decided for or against this theory. However, the first tests have not been very favourable to it. The researches of C. von Noorden and his pupils, Stammreich and Miura, have clearly and directly established that alcohol cannot be substituted in a maintenance ration for an exactly isodynamic quantity of carbohydrates. If the substitution is effected, a ration only just capable of maintaining the organism in equilibrium becomes insufficient. The animal decreases in weight. It loses more nitrogenous matter than it can recover from its diet, and this situation cannot be sustained for long. On the other hand, the celebrated researches of the American physiologist, Atwater, would plead, on the contrary, in favour of almost isodynamic substitution. Finally, Duclaux has shown that alcoholis a real food, biothermogenic for certain vegetable organisms. But urea is also a food formicrococcus ureæ. It does not follow that it is a food for mammals. We have not reached the solution yet—adhuc sub judice.
Conclusion: The Energetic Character of Food.—To sum up we have confined ourselves, in what has been said, to the consideration of a single character of food, and really the most essential, its energetic character. Food must furnish energy to the organism, and for that purpose it is decomposed and broken up within it, and issues from it simplified. It is thus, for instance, that the fats, which from the chemical point of view are complicated molecular edifices, escape in the form of carbonic acid and water. And so it is with carbo-hydrates, starchy and sugary substances. This is because these compounds descend to a lower degree of complexity during their passage through the organism, and by this drop, as it were, they get rid of the chemical energy which they contained in the potential state. Thermo-chemistry enables us to deduce from the comparison of the initial and final states the value of the energy absorbed by the living being. This energetic, dynamogenic or thermogenic value, thus gives a measure of the alimentary capacity of the substance. A gramme of fat, for instance, gives to the organism a quantity of energy equivalent to 9.4 Calories; the thermogenic value of the albumenoids is 4.8 Calories. The thermogenic or thermal value of carbohydrates is less than 4.7 calories. This being so, we understand why the animal is nourished by foods which are products very high in the scale of chemical complexity.
We have seen that food is, in the first place, a source ofchemical energy; and, in the second place, a source ofvital energy—finally, and consequently, a source of thermal energy. It is this last point of view which has exclusively struck the attention of certain physiologists, and hence has arisen a peculiar manner of conceiving the rôle of food. It consists in looking on food as a source of thermal energy.
This conception is easily applied to warm-blooded animals, but to them exclusively—and this is where it first fails. The animal is warmer than the environment in general. It is constantly giving out heat to it. To repair this loss of heat it takes in food in exact proportion to the loss it sustains. When it is a question of cold-blooded vertebrates, which live in water and in most cases have an internal temperature which is not distinguishable from that of the environment, we see less clearly the thermal rôle of food. It seems then that the production of heat is an episodic phenomenon, not existing for itself.
However that may be, food is in the second place a source of thermal energy for the organism. Can it be said, inversely, that every substance which we introduce into the economy, and which is there broken up and gives off heat, is a food? This is a moot point. We dealt just now with purely thermogenic foods. However, most physiologists are inclined to give a positive answer. In their eyes the idea of food cannot be considered apart from the fact of the production of heat. They take the effect for thecause. To these physiologists everything ingested is called food, if it gives off heat within the body.
To be heated by food is, indeed, an imperious necessity for the higher animals. If this need be not satisfied the functional activities become enervated; the animal falls into a state of torpor; and if it is capable of attenuated, of more or less latent, life it sleeps in a state of hibernation; but if it is not capable of this, it dies. The warm-blooded animal with a fixed temperature is so organized that this constancy of temperature is necessary to the exercise and to the conservation of life. To maintain this indispensable temperature there must be a continual supply of thermal energy. According to this, the necessity of alimentation is confused with the necessity of a supply of heat to cover the deficit which is due to the inevitable cooling of the organism. This is the point of view taken up by theorists, and we cannot say that they have no right to do so. We can only protest against the exaggeration of this principle, and the subordination of the other rôles of food to this single role as a thermogen. It is the magnitude of the thermal losses which, according to these physiologists, determines the need for food, and regulates the total value of the maintenance ration. From the quantitative view it is approximately true. From the qualitative point of view it is false.
Such is the theory opposed to the theory of chemical and vital energy. It has on its side a large number of experts, among whom are Rubner, Stohmann, and von Noorden. It has been defended in an article in theDictionnaire de Physiologieby Ch. Richet and Lapicque. They hold that thermogenesis absolutely dominates the play of nutritive exchanges;and it is the need for the production of heat that regulates the total demand for Calories which every organism requires from its ration. It is not because it produces too much heat that the organism gets rid of it peripherally: it is rather because it inevitably disperses it that it is adapted to produce it.
Rubner’s Experiments.—This conception of the rôle of alimentation is based on two arguments. The first is furnished by Rubner’s last experiment (1893). A dog in a calorimeter is kept alive for a rather long period (two to twelve days); the quantity of heat produced in this lapse of time is measured, and it is compared with the heat afforded by the food. In all cases the agreement is remarkable. But is it possible that there should be no such agreement? Clearly no, because there is a well-known regulating mechanism which always exactly proportions the losses and the gains of heat to the necessity of maintaining the fixed internal temperature. This first argument is, therefore, not conclusive.
The second argument is drawn from what has been called thelaw of surfaces, clearly perceived by Regnault and Reiset in their celebrated memoir in 1849, formulated by Rubner in 1884, and beautifully demonstrated by Ch. Richet. In comparing the maintenance rations for subjects of very different weights, placed under very different conditions, it is found that the food always introduces the same number of Calories for the same extent of skin—i.e., for the same cooling surface. The numerical data collected by E. Voit show that, under identical conditions, warm-blooded animals daily expend the same quantity of heat per unit of surface—namely, 1.036 Calories per square yard. The average ration introduces exactly the amount of food which gives off sensibly this number of Calories. Now, this is an interesting fact, but, like the preceding, it has no demonstrative force.
Objections. The Limits of Isodynamism.—On the contrary, there are serious objections. The thermal value of the nutritive principles only represents one feature of their physiological rôle. In fact, animals and man are capable of extracting the same profit and the same results from rations in which one of the foods is replaced by anisodynamicproportion of the other two—that is to say, a proportion developing the same quantity of heat. But this substitution has very narrow limits. Isodynamism—that is to say, the faculty that food has of supplyingpro ratâits thermal values—is limited all round by exceptions. In the first place, there are a few nitrogenous foods that no other nutritive principle can supply; and besides, beyond this minimum, when the supply takes place, it is not perfect. Lying between the albuminoids and the carbohydrates relatively to the fats, it is not between these two categories relatively to nitrogenous substances if the thermal power of food were the only thing that had to be considered in it, the isodynamic supply would not fail in a whole category of principles such as alcohol, glycerin, and the fatty acids. Finally, if the thermal power of a food is the sole measure of its physiological utility, we are compelled to ask why a dose of food may not be replaced by a dose of heat. External warming might take the place of the internal warming given by food. We might be ambitious enough to substitute for rations of sugar and fat an isodynamic quantity of heat-giving coal, and so nourish the man by suitably warming his room. Inreality, food has many other offices to fulfill than that of warming the body and of giving it energy—that is to say, of providing for the functional activity of the living machine. It must also serve to provide for wear and tear. The organism needs a suitable quantity of certain fixed principles, organic and mineral. These substances are evidently intended to replace those which have been involved in the cycle of matter, and to reconstitute the organic material. To these materials we may give the name ofhistogeneticfoods (repairing the tissues), or ofplasticfoods.
Opinions of the Early Physiologists.—It is from this point of view that the ancients regarded the rôle of alimentation. Hippocrates, Aristotle, and Galen believed in the existence of a unique nutritive substance, existing in all the infinitely different bodies that man and the animals utilize for their nourishment. It was Lavoisier who first had the idea of a dynamogenic or thermal rôle of foods. Finally, the general view of these two species of attributes and their marked distinction is due to J. Liebig, who called themplasticanddynamogenicfoods. In addition he thought that the same substance should accumulate the same attributes, and that this was the case with the albuminoid foods, which were at onceplasticanddynamogenic.
Preponderance of Nitrogenous Foods.—Magendie, in 1836, was the pioneer who introduced in this interminable list of foods the first simple division. He divided them into proteid substances, still calledalbuminoids, nitrogenous, quaternary, andternary substances. Proteid substances are capable of maintaining life. Hence the preponderant importance given by the eminent physiologist to this order of foods. These results have since been verified. Pflüger, of Bonn, gave a very convincing proof of this a few years ago. He fed a dog, made it work, and finally fattened it, by giving it nothing at all to eat but meat from which had been extracted, as thoroughly as possible, every other substance.[12]The same experiment showed that the organism can manufacture fats and carbo-hydrates at the expense of the nitrogenous food, when it does not find them ready formed in the ration. The albumen will suffice for all the needs of energy and and matter. To sum up, there is no necessary fat, no carbohydrate is necessary; albuminoids alone are indispensable. Theoretically, the animal and man alike could maintain life by the exclusive use of proteid food; but, practically, this is not possible for man, because of the enormous amount of meat which would have to be used (3 kilogrammes a day).
Ordinary alimentation comprises a mixture of three orders of substances, and to this mixture albumen brings the plastic element materially necessary for the reparation of the organism; it also is the source of energy. The two other varieties only bring energy. In this mixed regimen the quantity of albumen must never descend below a certain minimum. The efforts of physiologists of late years have tended to fix with precision this minimum ration of albuminoids—or as we may briefly put it,ofalbumen—below which the organism would perish. Voit had found 118 grammes of albumen necessary for the average adult man weighing 70 kilos. This figure is certainly too high. The Japanese doctors, Mori, Tsuboï, and Murato, have shown that a considerable portion of the population of Japan is content with a diet much poorer in nitrogen, and suffers no inconvenience. The Abyssinians, according to Lapicque, ingest, on the average, only 67 grammes of albumen per day. A Scandinavian physiologist, Siven, experimenting on himself, found that he could reduce the ration of albumen necessary to the maintenance and equilibrium of the organism to the lowest figures which have been yet reached—namely, from 35 to 46 grammes a day. These experiments, however, must be confirmed and interpreted. Besides, it is important to point out that the most advantageous ration of albumen requires to be a good deal above the strictly sufficient quantity.
It only remains to refer to several other recent researches. The most important of many are those published by M. Chauveau, on the reciprocal transformation of the immediate principles in the organism according to the conditions of its functioning and the circumstances of its activity. To deal with these researches with as much detail as they deserve, we must study the physiology of muscular contraction and of movement—that is to say, of muscular energetics.