After leaving the stomach and intestines, the foods follow two different routes. Proteins and carbohydrates are carried by the portal vein to the liver. Fats are carried by the thoracic duct to the general circulation. An excess of fat is found in the blood in all parts of the body after a meal rich in fat. The eventual destination and fate of fatty foods is unknown. Under certain circumstances they are added to the fatty deposits in connective tissue; but if no additional fat is being laid down, they go to other tissues, in which they are oxidized into carbonic acid and water. When the amount absorbed is excessive, a certain quantity of fat may be stored in the liver. In the cells of this organ it is housed for a time, in order that it may be distributed to the tissues after they have used up the supplies which first reach them through the general blood-stream.
Proteins are completely lost to sight after they are absorbed into the blood. They take part, of course, in the formation of growing tissue, blood-corpuscles, skin, hair, nails. It is also common to speak of them as making good the wear and tear of active tissues, although it is very doubtful whether we can legitimately speak of the wear and tear of tissues. The protoplasm which does the work of the body is not worn out in the same way as the materials of which a machine is made. There is no friction to rub it down. Proteins, like other foods, are used up as sources of muscular energy and heat. Eventually they are reduced to urea, carbonic acid, and water. Chemists naturally seek for substances intermediate in constitution between proteins and urea. They assume that the degradation of proteins will occur in regular steps; complex, partially oxidized, nitrogenous compounds being formed first—in the muscles, for example—to be further oxidized in the glands. The existence in all organs of nitrogenous “extractives,” which can be separated out when the organ is subjected to chemical analysis, seems to justify the search for stages; but hitherto this search has been singularly unsuccessful. Urea is the final product. It is not found in muscle, nor, indeed, in any tissue other than the liver, which, as already said, has the power of making it, even from salts of ammonia. It is therefore clear that if proteins are destroyed in muscle and other tissues, and if all urea is made by the liver, the antecedents of urea must be carried from the muscles to this organ. The substance which is most characteristic of muscular metabolism is lactic acid. It is not impossible that all the nitrogenous portion of the complex proteid molecule is reduced to ammonia (NH₃), which may be regarded as the simplest of all nitrogenous compounds, and that this, combined with lactic acid (C₃H₆O₃) as lactate of ammonia (NH₄C₃H₅O₃), is carried by the general circulation to the liver, where it is converted into urea. A considerable amount of lactate of ammonia may be injected into a vein without any of it overflowing through the kidneys. It is all reduced to the condition of urea, water, and carbonic acid. If the liver is so diseased as to be functionless, or if by operative measures it is thrown out of action, salts of ammonia are excreted by the kidneys instead of urea. In birds and reptiles uric acid takes the place of urea. Their livers yield uric acid on analysis. If lactate of ammonia be injected into their blood, it is converted into uric acid, so long as the liver is intact.
We know nothing of the forms assumed by the proteins absorbed into the blood, of the organs in which they are stored, or of the higher terms of the series of substances through which they pass before they are finally excreted as urea, water, and carbonic acid. No nitrogenous compounds are found in lymph or blood which can be pointed out with confidence as the products of tissue wear and tear. When considering the sources of muscular energy, we shall have something more to say regarding the part that proteins play in the economy.
If there is great difficulty in following fats and proteins after their absorption, it is quite otherwise when we come to deal with sugar. Carbohydrates are the great sources of energy. Muscular work may be generated by the oxidation of either of the three classes of foods, but undoubtedly the carbohydrate glycogen is its most constant source. Provision is therefore made for the storing of glycogen in the liver, and the distribution to the muscles of a regular supply. After a meal the portal blood, on its way from the intestines to the liver, contains a higher percentage of sugar than the blood in the hepatic vein or in any other vessel. If sections of liver be examined after feeding, and compared with those obtained after a period of starvation, it is found that the cells of the well fed liver contain glancing masses of a substance which takes a port-wine colour with iodine. This is glycogen, or animal starch. It has the same empirical formula as starch (C₆H₁₀O₅)ₙ. In the dry state it is a greyish powder, which, unlike starch, forms an opalescent solution in cold water. Like starch, it is non-diffusible. In the animal kingdom it stands to sugar in the same relation as starch to sugar in plants. If a sheep be killed while it is feeding in the paddock, and its liver removed and weighed, it will be found that it is from one-third to one-half heavier than the liver of a sheep of the same weight obtained from a butcher; for butchers have the stupid practice of starving animals before they kill them. It was long ago discovered that it is unnecessary to feed an animal for a day or two before it is killed, and this option has been elevated into a prohibition. A tradition has grown up that it is undesirable to give food for some time before killing. Not only will the liver of a sheep killed during active digestion be found to be heavier than that of a starved sheep, but it will also prove more succulent; for it is loaded with sugar (into which glycogen is rapidly converted after death), as well as with proteins and fats, which are withdrawn from it when the animal fasts. It appears that the liver cannot secure the whole of the sugar which is absorbed after a full meal. Some of it passes into the general circulation, and is stored in the muscles; but the liver always maintains a considerable reserve. Even after prolonged deprivation of food, it holds on to a certain quantity, especially in carnivora. Glycogen is found in the liver of a dog after a long period of starvation. The muscles lose during activity the glycogen which they contain when at rest.
It has already been pointed out that the body is not entirely dependent upon external agencies for the production of the sugar which it needs. When the supply is inadequate, it manufactures glycogen for itself out of the other constituents of the diet. It can, indeed, make it at the expense of its own proteins. If a dog which has been caused to do muscular work, without a sufficiency of carbohydrate food, until (as judged from a control experiment) all glycogen has disappeared from its liver, be placed under the influence of a narcotic drug, which arrests the activity of its muscles, glycogen reappears.
Dietetics.—Even those who are most ignorant of the science of physiology flatter themselves that they have one piece of information: “The whole of the body is renewed once in every seven years.” I cannot trace the origin of this sapient apothegm, which for generations has passed current. If seven weeks or seventy years were the period allowed for the renewal of the tissues, the statement would be equally near the truth. Judging from the rate at which they are destroyed, it is unlikely that blood-corpuscles live for more than five or six weeks. Hairs are shed about two years after they first appear above the surface. On attaining this age a hair drops off and a new one takes its place. The superficial cells of the skin are shed in great numbers every day, and their place taken by younger cells which come up from the deeper layers. The cells of many glands would seem to have a comparatively short term of life. On the other hand, some tissue-elements are far more permanent. By the time a child is a year old all its nerve-cells are in position. They last as long as the individual lives. When the statement with regard to the renewal of the tissues is understood as meaning, not that the cells are destroyed and replaced by new ones, but that within a period of seven years all the molecules which enter into their protoplasm are extruded from the body and replaced by molecules received as food, the assertion verges on the transcendental. It is unlikely that we shall ever obtain data against which it can be checked.
The essential part of every living cell is its sponge-work of protoplasm. “Bioplasm” is perhaps a better term to use when we are speaking of protoplasm as a structure, since it does not suggest any prejudice with regard to its chemical constitution. Within the meshes of the bioplasm are nutrient materials, as yet unused, and worked up products in various stages. It has always been taken for granted that when treating of nutrition, we have to consider the repair of the bioplasm, as well as the provision of raw material which it can convert into the specific products of the cell. Suppose that the cell belongs to the class of supporting tissues; let it be a cell of cartilage, for example. The bioplasm manufactures a collagenous substance which remains in and around its meshwork. If it be an epidermal cell, it forms horny substance. If a secreting cell, it accumulates secernable products. If a muscle-cell, it develops a large quantity of material, which by a change in form produces movement. In this last case we suppose that the energy set free as muscular force is due to oxidation. More stable bodies take the place of a less stable substance. After contraction the relatively complex contractile material is renewed from the foods stored in the muscle-cell; or if it be not, in the ordinary sense of the word, destroyed, if it has merely parted with certain oxidizable constituents, it obtains a fresh supply of such constituents from the foods which the muscle-cell contains. Even in the case of cartilage or epidermis, we imagine that, since the matrix is “alive,” it is always undergoing molecular change, and consequently always requiring food. The fact that every tissue, however inert, dies when, owing to the blocking of the bloodvessels which irrigate the part, its supply of nutriment is cut off, justifies this belief that all living tissue is undergoing change.
When we make up a balance-sheet of the body as a whole, placing to the debit side the food which it receives, and to its credit side the work done in external movement and in the production of heat, we again find reason for believing that every part of every cell is constantly undergoing change.
The balance-sheet of the body can be drawn out in either of two ways. We can estimate the quantities of nitrogen, carbon, hydrogen, and oxygen supplied to it in the several foods, and compare them with the amounts of each of these four elements given off in urea, carbonic acid, and water, making, of course, a note of the body’s balance in hand at the beginning and at the end of the period of observation. Or, we may estimate the amount of potential energy contained in the food, and ascertain the use to which this energy is put in doing external work, in maintaining the temperature of the body, and in warming the breath and other excreta.
If we are making up the balance-sheet of a fully-grown man, we may take for granted that he is not making fresh tissue. During the period throughout which he is under observation, care is taken to avoid altering the conditions of his life in such a manner as to lead him to develop additional muscle. If he gains in weight while under observation, he is putting on fat. If he loses in weight, he is sacrificing fat.