Metabolism

“ The energy for metabolism can come from any of the foodstuffs; these are present in the blood stream in the form of sugar or fat or amino acids. A moment ago we saw that in the case of the amino acids the nitrogen is removed before they are ready for use as fuel. After this has happened the part that remains is so similar to sugar that it can be thought of as the same material and will be so considered by us. We have then to follow only the two food substances, sugar and fat, through their use by the body cells. Fat will be dealt with first since we have less to say about it. ”

“ First of all, in describing the various kinds of metabolism that cells may show, we have the metabolism of rest. By this we mean the power development that is going on when the cell is doing nothing more than keeping alive; neither growing nor showing any special activity. This is evidently the minimum amount that any cell can show, so it is often referred to as the basic metabolism. We know of at least two things that may change the amount of basic metabolism; the first of these is a change in temperature; when a cell is cold, its basic metabolism is less than when it is warm. ”

“ It is evident that the amounts of these depend upon the amount of metabolism; the total metabolism is made up of the basic metabolism which is steady, shifting little day in and day out, plus the functional metabolism which depends upon how actively the body works. The main functional metabolism is that of the muscles and it is that which varies from day to day. In the case of children there is the additional metabolism of growth for which energy is required and for which food must be eaten. ”

“ Repair and Elimination of Waste (Metabolism)
Every part of the body is constantly changing. Its work never stops. If kept in thorough repair it must be torn down and rebuilt incessantly. These chemical changes are called collectively metabolism. They are divided into two groups: the chemical process of building up complex substances from simple ones is known as anabolism; the chemical process of oxidizing and breaking down the complex substances into simple ones, so that they are in a state to be excreted, is catabolism. ”

“ First we note that metabolism consists of two correlative systems, one of anabolism, the other of catabolism; that is, the one builds up, the other tears down. They are not only correlative, but to a large extent they are contemporaneous. In fact, there can be no building up without tearing down; that is, no placement which is not displacement, except that material may be stored adjacent to organs as fatty substances, to be used as needed after it has thus been stored. ”

“ There are two kinds of outside factors of fundamental importance for all metabolic processes: food is one, and oxygen is the other. And metabolism as a whole is of two different aspects also: it both serves for assimilation proper—that is, building up—and it supplies the energy for driving the functional machine. It is clear that food alone—together of course with the assimilating means of the organism, can account for the first type of metabolism, while both food and oxygen, or some sort of substitute for the latter, as in certain bacteria, supply functional energy. ”

“ Endogenous or tissue metabolism. Exogenous or intermediate metabolism. Needs of the body for proteid food possibly satisfied by quantity sufficient to meet the demands of tissue or endogenous metabolism. Bearings of Folin’s views on current theories and general facts of proteid metabolism. Large proteid reserve and voluminous exogenous metabolism probably not needed. Importance of feeding experiments in determining the true value of different views.
As we have already seen, every form of muscular activity begets an increase in the energy exchange of the body. ”

“ When the metabolism is perfect, any carbohydrates consumed in excess of the ordinary requirements are converted into glycogen and fat and stored for providing fuel at a future date. When stored as fats they are completely oxidized in the simplest compounds, carbon dioxid and water, and are eliminated through the lungs and skin, possibly part of the water so formed acting as a solvent for the urinary compounds. ”

“ The basic metabolism is very steady and the functional metabolism of the vital processes like breathing, the heartbeat, etc., is also pretty steady, so that the metabolism for one who makes no use of his muscles is fairly uniform. Of course it differs in large people as compared with small, although curiously enough the difference is not proportional to the weight but to the body surface. Just why this is so we do not know. The average figure for the total metabolism of a resting man is about 1,900 calories a day ”

“ Metabolism of Matter and Energy.—The processes of nutrition thus consist largely of the transformation of food into body material and the conversion of the potential energy of both food and body material into the kinetic energy of heat and muscular work and other forms of energy. These various processes are generally designated by the term metabolism. The metabolism of matter in the body is governed largely by the needs of the body for energy. ”

“ In the preservation of metabolic equilibrium, we have a process before us, the principle of which is nowadays restricted to living substance. In my “Biogen hypothesis,”75 I have associated the self-regulation of metabolism with the chemical equilibrium in interreacting masses. I have considered the metabolic self-regulation as the expression of the formation of a mass equilibrium between the quantity of foodstuffs and the quantity of a hypothetical combination of living substance, the biogen, which continuously disintegrates and builds up again of its own accord. ”

“ Metabolism, it must never be forgotten, is the general scheme within which all the processes of life in a given living organism go on; but metabolism means nothing else, at least if we use the word in its descriptive and unpretentious meaning, than change in the physical or chemical characteristics of the single constituents of that organism. ”

“ It is becoming evident that special paths of metabolism exist, not only for proteins, fats and carbohydrates as such, but that even the individual primary fractions of the protein molecule follow their several catabolic paths, and are dealt with in successive stages by series of enzymes until the final products of catabolism are formed. ”

“ They pass into the blood numerous end-products of their metabolism. Further, some cells decay here and there, and partial products reach the blood which are out of harmony with the species, and, of course, entirely so with the plasma and the cell. The whole regulation of tho normal metabolism is seriously injured. The cells of the gut-wall will still be on the watch to prevent any disharmonious material from entering the organism; and the single cells of the body will still struggle to supply the blood only with properly altered substances. ”

“ As already intimated, we do not know just why in some persons and at a certain time a disturbance of metabolism results in perverted cell growth, but there is, of course, a reason, and the physician’s duty is so to regulate the patient’s whole life that the metabolism is kept in the state of perfect health. ”

“ There will be found also the digestion products of the other classes of foodstuffs. The body fluids contain also the waste products of cell metabolism on the way to be discharged from the body and the special products, such as the hormones, which are manufactured by certain cells and carried through the blood stream to act upon other cells. Of course we realize that not all of the materials that are manufactured by cells are poured out into the blood stream. ”

“ This increase of the nitrogen metabolism in its totality is not nearly as great as that of the breaking down of nitrogen-free substances, but it is, nevertheless, present and shows us that functional metabolism cannot experience a lasting excitation without being followed by secondary results in the entire cytoplastic metabolism. This fact is even more strikingly illustrated in the alteration of the entire volume of a living organism as produced by the lengthened duration of functional stimulation. ”

“ A trifling amount of urea is also eliminated in the sweat and in the breath as well as in the feces. Proteins increase nitrogenous metabolism and also the metabolism of other foods, but the amount of nitrogen eliminated is just equal to that taken in. Probably some comes from the tissues themselves and not from the food. The oxidation of carbohydrates and fats is measured by the amount of carbon excreted. At first as much is given off as is taken in, but after a while the carbohydrate is stored up as glycogen in the liver and the fats are stored as fat. ”

“ The second thing that influences the amount of basic metabolism is the percentage of water in the protoplasm of the cell. Highly organized animals, like ourselves, are destroyed if the cells lose more than a small fraction of their water, but there are many of the lower animals that can be dried until their bodies contain only a very little water and still live. ”

“ METABOLISM is the process which is constantly going on in the human system, whereby the cells that have been consumed by oxidation are removed through the excreta—the faeces, the urine, the perspiration, and the exhalations from the lungs—to be replaced by new ones. ”

“ It cannot be claimed that the complex chain of transformations which organic chemistry has demonstrated in the destructive metamorphosis of albumen and starch in the laboratory is represented in the vital chemistry of the body. All that can be said in the present state of knowledge is, that the metabolism of food is in its nature a chemical analysis, modified and regulated by vital force, and resulting in the building up of tissues and in the conversion of potential into active energy. ”

“ It may be impracticable to outline in terms of metabolism the precise processes of waste and repair in organs and organisms, or to define the relative periods of action and assimilation (or of catabolism and anabolism) best adapted to the development of motile tissue ”

“ These somewhat general statements, with the illustrations given, will serve in a brief way to emphasize some of the essential features of metabolism in the fasting individual; where there is no income of energy-containing material, and where the body must draw entirely upon its store of accumulated fat and proteid to keep the machinery in motion, maintain body temperature, and do the tasks of every-day life. ”

“ Whenever either of these things happens the protoplasm takes up water again and the former rate of metabolism is resumed. It is only by means of this reduction in rate of metabolism that many kinds of animals are able to persist, for in large parts of the globe there is a period of each year when conditions become so unfavorable that the usual rate of metabolism could not possibly be maintained. ”

“ The conception of ascending steps of constructive metabolism resulting in the formation of protoplasm and the storing of energy, with correlated descending steps, by which protoplasm is transformed into less complex compounds (destructive metabolism) , with a liberation of energy, serves as a key to the complex processes of nutrition which enables us to trace their conformity to general laws that are readily recognized, and clears up the obscurity arising from the multiplicity of details which from other points of view could not be brought into consistent relations. ”

“ In the higher animals the distinction between basic and functional metabolism is sharp, and, necessarily so, for the well-being of any of the higher animals requires that he shall have pretty complete control over the activities of his protoplasm, and this he could not have if the functional metabolism were blended in with the basic. In other words, it is as important for bodily well-being that the cells be able to become inactive as that they be capable of activity. ”

“ But all our experience, hitherto, has proved, that even those cells which are constructed on a simple plan morphologically do, when studied from a purely chemical point of view, show exceedingly complicated relations. Indeed, the study of the processes of metabolism in unicellular forms of life is a study all the more difficult, in comparison with that of more complicated organisms, in that, in the former, it is so difficult to separate the actually absorbed materials from the metabolic by-products, and these again from the secretory or excretory products. ”

“ A chemical can be excreted from the body without metabolism, in which case the parent compounds may be detectable in the urine, breath, or fecal material. In other cases, the chemical may be metabolized through oxidation, reduction, hydrolysis, or a combination of these processes, often followed by conjugation with an endogenous substrate. Conjugation of a chemical or metabolite is a pathway for excretion. ”

“ We may confidently expect that a thorough study of the physiological action of all the varied katabolic products formed in the body will result in a decided expansion of our knowledge regarding the part these substances may play in normal and abnormal metabolism, and in nutrition in general.
Just here, reference may be made to the many problems in the broad field of nutrition that confront the physiological chemist of the present day. The maintenance of life on a sound physiological basis is one of the practical problems in physiological chemistry, and its solution is not yet attained. ”