Virginia M. Schelar Norfhern Illinois University DeKalb
rhermochemistrv , and Animal Metabolism A historical survey
The history of metabolism offers opportunity for restudy, but not because it has been neglected. The role of Liebig and his group and the contribution of the law of conservation of energy have been overemphasized, while the contributions of the thennochemists Hess, Thomsen, and Berthelot have been largely ignored. The problems of "animal heat" and muscular power were important ones historically because their solution led to the overthrow of the idea of "vital forces" as an explanation of life processes. An othe~wiseexcellent early history written by Graham Lusk ignores the work of Eward Frankland "On the origin of muscular power" (1). This work has been completely neglected in histories of metabolism apparently since many of these seem to be based on the Lusk review. The solution of the problem of the source of muscular power will therefore be emphasized in this review' and other aspects of metabolism will be ~p
Presented before the Division of History of Chemistry at the 142nd Meeting of the American Chemical Society, Atlantic City, N. J., September, 1962. Based on work done at Harvard University, Cambridge, Mass. ' An attempt has been made to cite only readily accessible literature, which often means secondary sources. A more extensive bibliography of 48 references is avail~blefrom the author.
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considered primarily in relation to their contribution to solution of this problem. Today the underlying conception of metabolism is so familiar that we tend to forget that the word "metabolism" is scarcely more than one hundred years old. i\letabolic studies were possible only on the basis of fundamental physical laws and the recognition that these laws operated in animate as well as inanimate nature. Lavoisier
Lavoisier opened the modein era of the study of energy metabolism in 1780. On the basis of very early experiments by Erasistratus and by Sanctorius, Lavoisier was aware that the weight taken in by the ingestion of food was lost in part in the urine, feces, and the invisible exhalations from the body. Lavoisier's experiments carried out with Laplace and Seguin are well known. He concluded that the combination of oxygen with carbon and hydrogen in the body t o form carbon dioxide and water was the source of the mysterious "animal heat" which Stahl had still explained as a result of the friction of the blood in the vessels. He was not the first to conduct calorimetric studies. Crawford bad made calorimetric studies on animal heat in 1778. I n
1777 Crawford found, after burning wax or charcoal, or by the respiration of a guinea pig in his water calorimeter, that for every 100 oz of oxygen used the water temperature was raised 2.1, 1.93, and 1.73 degrees respectively. Though he interpreted his work in terms of phlogiston, his conclusion that the quantity of heat produced when a given quantity of oxygen was altered by animal respiration was nearly equal to that produced when the same quantity of oxygen was used up in the combustion of wax or charcoal is noteworthy (8). Lavoisier also equated the heat produced by an animal in a calorimeter with the heat which the same quantity of oxygen as that respired by the animal would have produced when combined with carbon outside the body. Lavoisier, working with Seguin as a subject as well as with animals, established that of every 100 g of oxygen absorbed in respiration only 81 parts reappeared as carbon dioxde. He accounted for this hy stating that a part of the absorbed oxygen was utilized to oxidize hydrogen in the lungs. This oxidation would produce additional heat and account for the discrepancy between the heat directly measured for a guinea pig and the heat calculated as coming from oxidation of carbon by oxygen. With Seguin as subject, Lavoisier showed that oxygen was absorbed and carbon dioxide expired in proportion to the work performed. The amount of oxygen utilized during respiration as the result of work rose to two or three times that absorbed when he was taking his ordinary amount of food and resting. Lavoisier's method suffers from his failure to realize that the heat produced by burning one gram of elemental carbon is different from that produced by one gram of combined carbon. Eighty years later Bischoff and Voit were still ignoring the effects of bonding and using carbon and hydrogen equivalents in the manner of Lavoisier. Herr's Law
A series of "Thermochemical Investigations" by G. H. Hess m-as published in the years 183942. Most famous of Hess's generalizations is his law of constant heat summation: "The quantity of heat produced in the formation of a compound is a constant, whether the compound is formed directly or indirectly." It is this law which justifies equating the results of calorimetric measurements outside the body with values for combustion of food inside the body without knowledge of the metabolic pathway. Hess also showed how thermochemical equations could be treated algebraically to calculate heats indirectly. Despite the fact that Hess published in Poggendorf's Annakn and therefore his work was available to Liebig, the near implication of Lusk in his history is that Liebig discovered the law of constant heat summation. I t is clear that his contribution was its application to metabolic studies. When Dumas and Cahours in 1842 (3) published a formula for calculating the heat of combustion of protein indirectly, Liebig attacked Dumas for copying him. Liebig was to publish his "Animal Chemistry" later that same year. Dumas' formula is clearly no more than an application of Hess's law. Signifirantly, both parties to the controversy dropped any claim to priority after a heated interchange.
Much emphasis has been placed on the role of the law of conservation of energy in the overthrow of the concept of vital force. It should be noted that Hess's law follows from the principle of conservation of energy and is in fact a limited statement of that law. Hess's law was enunciated as an empirical generalization from experiment, however, even before the law of conservation of energy had been formulated. Two of the discovers of the law, Mayer and Joule, were brought to their discovery a t least in part by consideration of animal heat. Helmholz's "On the interaction of natural forces," published in 1848, has a passage that is very reminiscent of a passage in Matteuci's publication of 1847 concerning Hess's law. Hess's law of 1840 was to play a much greater role in experimental investigations of metabolic processes than the more general statement of the law of conservation of energy. For the measurements actually made were of weight and heat, and generally the mechanical equivalent of heat was calculated indirectly from the heat, if it was calculated at all. Boussingault and Barral
Organic analysis, founded by Lavoisier, further advanced by Gay-Lussac and Thenard (1810-15), by Berzelius (1814), and perfected by Liebig in 1830, was the foundation for the work of Boussingault. Boussingault made a number of studies on materials balance as early as 1839 (4). He compared the C, H, 0, and N in the milk, urine, and feces and considered that the differences would be available for the products of respiratory exchange. He also drew up a table of nutritive equivalents. These equivalents, however, were based on the nitrogen equivalent of fodders. Thus, 13.5 kg of hay was accounted the nutritive equivalent of 54 kg of beets or 27 kg of potatoes. It is evident that a t that time there was no real understanding of the nature of different foodstuffs. Although Boussingault's studies scarcely advanced understanding of the metabolic processes, he did for the first time apply the technics of the chemistry laboratory to determine the magnitude of the changes in food substauces in terms of carbon and nitrogen. Barral (5) in 1849 applied the principle of Boussingault to studies of the metabolism of membersof hisfamily and arrived a t reasonable results for total calorie production. He stated, "Ihowing the amount and the elementary composition of the food, both solid and liquid, taken each day, determining the elementary composition of the excreta, and perspiration, one may calculate the gains and losses of the human body." Taking the difference in water, salts, chlorine, carbon, hydrogen, nitrogen, and oxygen in the food and in the excreta and perspiration, he calculated the heat production in terms of the carbon and hydrogen differences and the heats of combustion of elemental carbon and hydrogen. Liebig's Influence
I n 1842 Liebig published his "Animal Chemistry." He clearly stated that every production of motion in an animal involved a proportionate disintegration of muscular substance. Thus, proteins, in his view, were the source of muscular power. The nitrogen of the musVolume 47, Number 4, April 7964
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cular substance metabolized should thus be found in the urine as urea, and the quantity of nitrogen in the urine could then be used t o calculate the mass of protein destroyed. Differences in the quantity of urea secreted in these and similar experiments are explained by the condition of the animals in regard to the amount of muscular movement permitted. Every movement increases the amount of organized tissue which undergoes metamorphosis. Thus, after a. walk, the secretion of urine in man is invariably increased (6).
He believed that inspired oxygen was the direct cause of the oxidation of fat and carbohydrate in the body and that the heat generated by oxidation explained the maintenance of a constant hody temperature. Liehig calculated the following values for the oxidation of various foods in the body: 100 liters of oxygen combine with 120.2 e of starch 48.8 g of fat
and they w a r n liters of water from 0" to 37°C 28.356 27.647
He also calculated the caloric value of meat and prepared a table of isodynamic equivalents determined on the basis of their nitrogen content, which are given below, contrasted with values given later by Rnbner (7) :
Fat Starch Cane-sugar Dried meat
Liebig (1846)
Ruhner (1885)
100 242 249 300
100 232 234 243
I t should be noted that only a few albuminous substances were known to Liehig and Mulder and their predecessors. (Apparently Liehig gained his interest in the chemistry of proteins from the work and speculations of Mulder (8). I n 1836 Mulder stated that bread and other foods which contain protein supply the most essential constituents of the animal body in forms suitable for utilization and without having to undergo changes in digestion. Liehig accepted and publicized this idea.) These early investigators necessarily dealt with preparations which were rather crude and impure when judged by the criteria of later chemists. After the middle of the 19th centusy a number of important discoveries were made of technics for separating and purifying individual proteins from the complex mixtures in which they occur in naturalproducts. Liebig's restatement that all heat produced in the body arose from oxidation led t o many investigations on respiratory metabolism since, if this were the case, the carbon dioxide exhaled by a n animal was an index of heat production. Those of Regnault and of Bidder and Schmidt are particularly noteworthy (9). Liebig's doctrine that muscular force is produced only by the oxidation of proteins was seriously shaken by the investigations of Edward Smith in 1857-59 (10). He showed that the production of carbon dioxide in the human hody may be increased tenfold by muscular exertion, while the excretion of urea changed very little. These studies failed t o secure the attention they deserved because of Liebig's great authority. From a treadwheel experiment with human subjects, Smith calculated the average external work; the average nitrogen output; and the weight of dry muscle required to produce the nitrogen eliminated per m n per day. 228
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But Smith did not estimate the energy equivalent of dry muscle, and therefore, his data did not furnish a direct disproof of Liebig's theory. The fact that carbon dioxide output increased dramatically as work was performed, while excretion of urea did not, had beennotedpreviously, for example, by Bischoff and Voit (5). Bischoff and Voit had also observed that the urea output was proportional to the nitrogen taken in. But their conclusions from the data were in error due to their adherence to Liebig's theory. They later changed their views. Fick and Wislicenus
A. Fick and F. J. Wislicenus (11) submitted Liebig's theory to a crucial test. They confined themselves to a non-nitrogenous diet and ascended the Faulhorn, taking strict account of the greatest possible muscular oxidation by determining the amount of nitrogen expelled from the hody of each person before, during, and after the ascent of the mountain. Particulars of the experiment appeared in the Philosophical Magaiine for June 1866. They had no nitrogenous food for seventeen hours before ascending the 1656-meter peak. The climb took six hours and during this time and for seven more hours, they ate only carbohydrates and fats. They collected their urines and analyzed them for nitrogen content. I n the 13-hr period, Fick lost 5.74 grams of nitrogen and Wislicenus lost 5.54 grams. No adequate experimental data was available for the quantity of heat generated when a gram of muscle substance was burned to the products in which its constituent elements leave the human hody. They therefore used the upper limit of the amount of heat, that is, the amount which would be obtained if the combustible elements contained in a gram of albumen were burned separately. This was in keeping with the current methods of using the elemental constituents as a basis for heat of combustion calculations. They concluded: The burning of prot,ein substances cannot be the only source of muscular power; for we have here two cssea in which men performed more measurable work than the equivalent of the amount of heat which, taken a t a most absurdly high figure, could be calculated to result from the burning of albumen . . . the substances by the burning of which forceis generated in the muscles are not the albuminous constituents of those tissues, hut non-nitrogenous suhstances, either fats or hydrates of carbon ( 1 1 ) .
Almost immediately following the publication of Fick and Wislicenus, Lawes and Gilbert cited their 1852 report (12) concerning the source of muscular power. The report had stated that they had evidence that during exercise of muscular force, an animal requires non-nitrogenous rather than nitrogenous food. The basis for their conclusion was the observation of the immediate and pronounced response of the respiratory system to effect more rapid and deeper breathing when physical effort was made. Their experiment on two pigs which showed that the nitrogen eliminated as urea paralleled the protein intake and was independent of the muscular activity caused them to assert that nitrogen eliminated could not he considered a measure of muscular work. But a more important discussion of the mountain climbing experiment was contributed by Frankland.
Frankland's Colorimetry
Edward Frankland (1) determined the energy of muscle and its chief products of oxidation: urea, uric acid, and hippuric acid. The difference between the amounts of energy in the original muscle and in the products leaving the hody gave the maximum energy that could be developed. I n his calculations, Frankland used the Joule mechanical equivalent of heat. I n order to interpret the data of Fick and Wislicenus, he calculated the actual energy in kg meters of force developed by oxidation of one gram of beef muscle, beef fat, purified albumen, hippuric acid, uric acid, and urea. His determination showed that the maximum work capable of being evolved from the sources, even under the most favorable assumptions, was less than one-half the work actually performed. These results proved that the estimated force values of muscle made by Fick and Wislicenus were much too high, and consequently their results were far more conclusive than they believed them t o be. The introduction of the quantitative energy concept of foods and body tissues by Frankland confirmed the conclusions already reached by Lawes and Gilbert, Smith, and Fick and Wislicenus. Muscles work a t the expense of energy derived from the oxidation of non-nitrogenous foods. Thus for the first time the actual energy developed by the combustion of protein and other foods was determined calorimetrically. Heretofore the heat value had been calculated from knowledge of the elementary composition and known heats of combustion of carbon and hydrogen. The determination of the heat of combustion of protein was difficult. Also previous investigators were not aware of the difference in values obtained by the two methods. The thermochemists again supplied the necessary background. Berthelot used potassium chlorate for the first time to accomplish oxidations not possible with oxygen gas. Frankland used potassium chlorate also. I n a publication a year earlier than Frankland's, Berthelot (15) argued concerning the differences in the quantities of heat produced when equal weights of carbohydrate and fat are oxidized in the body. He pointed out that it is impossible to determine the heat production in the body by Lavoisier's method because 44 g of carbon dioxide produced from the oxidation of carbon yield 94 cal, whereas the same amount produced from methane yields 210,000 cal. He concluded that the quantity of heat liberated in the incomplete oxidation of a mbstance is equal to the diRerence between the tots1 caloric value of the substanre and that of the products formed.
Thomsen's numerous thermochemical investigations of carbon compounds, published continuously after 1854, also testified that the heat of combustion is dependent not only on the number of carbon and hydrogen a t o m but also on the nature of the atomic linkages. Voit and Pettenkofer (7) later undertook experiments in Pettenkofer's famous respiratory apparatus which was large enough for experiments on a man. Further research was done involving balance studies of food intake minus losses due to excretion; they confirmed Frankland's findings. Voit and Pettenkofer are given credit, by Lusk and others, for overthrowing Liehig's theory of muscular work. Yet it is apparent that Voit was aware of Frankland's work as Lusk mentions
that Voit made a trip to England and brought back a Frankland calorimeter prior to the time of Voit's experiments (14). The methods of computing the metabolism used by those who employed the Pettenkofer-Voit respiration apparatus also show a heavy debt to Bidder and Schmidt who attempted to compute the total metabolism of the cat prior to 1852. Rubner, a student of Voit, carried on his work and is known for his isodynamic law. He recognized that carbohydrate and fat are interchangeable in metabolism on the basis of energy equivalents. One hundred calories in fat are the nutritive equivalent of the same number of calories in carbohydrates. The "law of Rubner" was implicit in Frankland's determinations of the caloric value of foods and in the work of the thermochemists. The animal calorimeter which Rubner evolved and which enabled him to calculate heat production directly as well as indirectly gave him results which agreed within a fraction of a per cent. This accurate quantitative work of Rubner entitles him a place in demonstrating the truth of the law of conservation of energy as applied to man (7). Thus we have seen that the extension of chemistry to cover many vit,al changes was accompanied by advances made in applying physical principles to metabolic problems. The change of opinion away from vitalism, which was begun by progress in organic chemistry, was reinforced on the physical side by the work of the thermochemists. To examine the question of the applicability of the law of conservation of energy t o life processes, it was necessary to measure the intake of energy in food and the output in muscular work, heat and excrement. These balance studies were entirely in accord with the spirit of the age. The general accordance with the principle of conservation of energy showed that the activities of the hody could be ultimately traced t o the chemical and thermal energy of the food taken in. The establishment of the laws of the thermochemists were important not only in isolated studies of metabolism, but also because these studies helped t o throw doubt on the prevalent theories of vitalism. It was seen that the fundamental laws of physics and chemistry could be applied to much of the life process. Literature Cited
E., "Experimental Researches in Pure, A p (1) FRANKLAND, plied and Physical Chemistry," John Van Voorst, London, England, 1877. A., "Experiments and Observations on Animal (2) CRAWFORD, Heat," London, England, 1778. (3) DUMAS;J. B., AND CAHOURS, A,, Compl. ad., 15, 976 (1x42) \----,.
J. B., Ann. Chim.Phgs., 71, 113 (1839). (4) BOUSSINGAULT, E. H., Ciba Symposia, 6, 1814 (1941). (5) ACKERKNECAT, (6) LIFBIG,J., "Animal Chemistry," JOHNOWEN,translator, Cambridge, England, 1842. (7) BARKER,L. F., "Endocrinology and Metabolism," D. Appleton and Co., New York, 1922, Vol. 3. E. V., '(A History of Nutrition," Houghton (8) MCCOLLUM, Mifflin, Boston, 1957. T. M., J . Am. Diet.Assoc., 25,837 (1949). (9) CARPENTER, (10) SMITH,E., Philos. Tram.,149, 681 (1859). F. J., Philm. Mag. (4th series), (11) FICK,A., AND WISLICENUS, 31,159 (1866). J. H., Philos. Mag. (4th series), (12) LAWES,J. B., AND GLLBERT, 32, 55 (1466). (13) BERT HE LO^, M., J. Pham., 2,189 (1865). (14) LuSK, G., "Nutrition," P. B. Hoeber, New Yark, 1933. Volume 41, Number
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