CHAPTER XVIII

MORE ABOUT THE USE OF FOOD BY THE BODY

We have now brought our description of the various things that happen in our bodies up to the point where we may begin to make some kind of summary of them; particularly in respect to what goes on within our individual cells. We have seen how some of our cells are muscle cells by which we make motions; others are the cells of sense organs by which we get the necessary information for guiding our activities; still others are in the nervous system and carry on the adjustments by which we act in accordance with the indications of our surroundings and also of our past experience; finally we have the cells which manufacture materials, as in the various digestive glands or in the glands which make hormones. We have also looked into the kind of materials which these cells require, where they come from, and how they are prepared in the digestive tract, and we have seen how these prepared materials and oxygen are conveyed to the different parts of the body where the cells can get them and finally how the waste products which all these cells give off are gotten rid of. We are now to turn to the process of metabolism itself as it takes place within the cells. In Chapters III and IV we looked into the use of food for power development and for repairing the wastage of protoplasm, as well as for the making of new protoplasm in growth. We have seen also that the body contains much nonliving material, as in the bones and teeth, which must come from the food and which must be put in place as the result of metabolism on the part of living cells.

The first thing which we wish to take up here is the use of protein in the repair of protoplasmic wastage and in growth. We saw in Chapter III that protein is manufactured originally by the living cells of green plants. We have also seen that protein is the only material that can repair protoplasmic wastage or that can make new protoplasm. We have omitted to say thus far that the most important place from which we can obtain protein is from living protoplasm. It is true that most seeds store up within themselves nonliving protein to be used by the young sprout as it forms, and seeds make up a large part of our diet; but except in grain and other seed foods we obtain our supplies of protein by eating protoplasm. This protein is to be used by us for repairing our protoplasmic wastage or, in parts of us that are growing, for making new protoplasm. We have already seen that the protein which we eat must go through a process of digestion before we can use it for these purposes, and our present task is to explain just why this is necessary and to show how the protein is actually used in our bodies.

A thing about protein which fits it specially to be the chief material of living protoplasm is that it is very much the same sort of substance wherever we find it and yet can differ enough to account for the differences that exist among animals and plants. In spite of the fact that protoplasm analyzes about the same, no matter where it comes from, we are bound to believe that the difference between a dog and an oak tree is at bottom a chemical difference; they are unlike because the protoplasm of one is not the same substance chemically as the protoplasm of the other, and the difference is a difference in the proteins. We can come even nearer home than that and say that the differences between the races of mankind are probably chemical differences between their proteins. Human protein is undoubtedly different from the protein of beef or pork or mutton. What we have then is a substance which can be at the same time similar and different; also since we can make human protein out of beef protein or any other kind which we happen to eat it must be fairly easy to change one into the other.

Protein is about the most complex substance that we know anything about; it is made up of a number of organic acids combined chemically. These organic acids all contain nitrogen, which puts them into a class to which is given the name of _amino acids_. To the chemist the name amino acid shows a certain kind of chemical formation; to us it need mean no more than an organic acid which contains nitrogen. The proteins which are in our bodies are as complex as any that exist and some of them are made up of as many as eighteen different amino acids. The same eighteen acids are present in the proteins of all the higher animals. When we eat lean beef or pork we get exactly the same eighteen amino acids that are in our own proteins, but not put together in precisely the same way. What we have to do with these proteins is to break them up into the amino acids of which they are composed and then put these together again in the combination which makes up human protein. The breaking up of the proteins is carried on in the digestive organs; we have said a good deal about it in Chapter XV; what were called in that chapter digestion products of proteins we now see are amino acids. These are taken up into the blood stream, carried around the body to the tissue fluids and by the living cells taken up to be built into human protein. Exactly the same thing happens to any proteins that we eat.

One of the great differences between animal protein and plant protein is that the percentages of the different amino acids are very different. Some of the amino acids that make up a large proportion of animal protein are very scantily represented in plant protein. To make human protein we must not only have all the amino acid ingredients, but we must also have enough of every one. On a purely vegetarian diet to get enough of those amino acids which are scantily present in plants we have to eat a large surplus of those amino acids which are specially abundant. In this respect plants as providers of amino acids are less economical than animals, because animal proteins have more nearly the same proportions of the different amino acids as do our own human proteins. Of course, as we will see at once, in theory cannibalism is the most economical way of getting protein; if we were to eat human protein we would have exactly the correct proportion of the different amino acids and so could get along with a minimum amount. This is not to be interpreted as an argument for the practice of cannibalism among human beings, although we may as well face the fact that there is no physiological or dietary reason for avoiding the practice. In some of the lower animals, particularly in rats, cannibalism is a regular part of the life habit. Rats do not have the instinct of storing up food supplies as do squirrels and some other kinds of animals. When food is abundant they multiply very rapidly and then when food becomes scarce the stronger feed upon the weaker. It is largely for this reason that the unsanitary and extremely expensive rat nuisance is so hard to abate.

The amino acids that are circulating in the blood stream after every meal are primarily to be used for repairing protoplasmic wastage; also they serve for the manufacture of new protoplasm, provided growth is going on. In theory an adult who is through with all his growth except in the skin and one or two other minor tissues should be able to get along with just the amount of protein which will make good his protoplasmic wastage. Since protein is an expensive food and likely to be hard to get in times of scarcity, the question of how much protein should be eaten is of great practical importance. There are several ways of studying the problem; one is by the actual study of diets to find out how much protein people do habitually eat; another is by finding out how much the daily protoplasmic wastage amounts to. If no more protein is being eaten than is necessary for the protoplasmic wastage, these two figures should be about the same. The way of finding out how much protoplasmic wastage there is is to go on a diet which contains abundant starch and fat for energy supplies, but no protein or amino acids. When one is on that kind of diet he knows that he will not have to burn up any of his own tissues to supply him with the energy for his metabolism; whatever breakdown of protoplasm occurs on such a diet is the natural wastage of the body and not the result of using tissues for fuel, as is the case in complete starvation. It is fairly easy to keep track of the decomposition of protein in the body, because protein contains nitrogen and the nitrogen is given off almost wholly in the waste products that are passed out from the kidneys. By collecting the urine and analyzing it for nitrogen the amount of protoplasmic breakdown in the body can be determined, provided no nitrogen-containing compounds were taken in with the food. Otherwise, of course, one could not be sure that nitrogen appearing in the urine had actually come from the wastage of protoplasm. The fact is that when an average-sized human being goes on a diet which contains no protein, but is ample in other respects, he loses daily from his body about an ounce of protein. This is proven by the occurrence in the urine of an amount of nitrogen which stands for that much protein. The person may be gaining or losing weight meanwhile; if his consumption of fats, starches, and sugars is excessive, he may deposit some fat, in which case he might gain weight in spite of the loss of some of his actual living protoplasm. Usually though, in experiments of this kind, there is a steady loss of weight made up of the ounce of protein and of three or four ounces of water. We have to remember that living protoplasm is three-fourths or more water, so that whenever any protoplasm wastes away, some water will be lost as well as protein. It is a very interesting fact that this protoplasmic wastage goes on steadily at the rate of about an ounce a day whether the body is active or inactive; this means that the wastage is a matter of the basic metabolism and not of the functional metabolism. The former goes on all the time day and night, in sleep and in waking, and in connection with it the living protoplasm shows this small amount of wastage. Functional metabolism does not, at least under ordinary conditions, increase the amount. Speaking of the body as though it were a machine, we would say that it rusts out just as fast as it wears out. This is one of the features in which the living machine differs from most mechanical devices of human manufacture.

Although the loss of protein due to wastage is only about an ounce a day, nobody can get along on a diet which contains no more than that amount. Between three and four ounces of protein is the average daily consumption of adults in this country. We should not forget that our diet consists of meat, bread, vegetables, fruits, etc., which are mixtures of proteins with the other food materials and with a large percentage of water, so that in order to get three or four ounces of protein we have to eat four or five times that weight of ordinary foodstuffs. There has been much debate as to whether it is necessary or even desirable for adults to eat three or four times as much protein as the body requires for making up its wastage. The decision will have to rest in part on what use the body makes of the surplus. Since from time immemorial human beings have habitually eaten every day this large surplus, it is evident that they have been wasting enormous amounts of good food or else that some use is made of it even though it does not serve its purpose of repairing the body waste. The surplus materials are present in the body in the form of amino acids, since what the cells do in repairing their wastage is to take up from the whole quantity of amino acids in the tissue fluids as much as they require for making good their loss. The mixture of amino acids that is left over will make perfectly good fuel provided the nitrogen that is in it is gotten rid of, and this is what happens in the body. All the amino acids in excess of the amount needed for restoring the tissues are decomposed in such a way that the nitrogen is abstracted in the form of ammonia and the substance that is left, which is a starchlike compound, joins with the other starch products and fats to be burned in the body as a source of energy. We do not know certainly which tissues have the power of decomposing the surplus amino acids. At the present time it is believed that all or nearly all of them can do it, so that as they take up from the tissue fluids the particular amino acids which they need for making good their own wastage they take up also the surplus which they decompose, utilizing the starchlike part for fuel and turning the ammonia back into the body fluid as a waste product.

Ammonia is a very poisonous substance and it quickly poisons the body if allowed to accumulate in the tissue fluids. This is prevented by the action of the liver in changing the poisonous ammonia into a harmless substance known as urea. This urea is carried by the blood stream from the liver to the kidneys where it is passed out to become the chief organic substance in urine. The more protein one eats the more surplus amino acids will there be, and so the more urea will be formed and passed out of the body. Flesh-eating animals and men (Eskimos for an example of the latter) eat a very large surplus of proteins, the fuel for their metabolism being furnished almost altogether either from the usable remains of the decomposed amino acids or the fats that were in the flesh they ate. Some people have been inclined to believe the production of so much ammonia and its subsequent conversion into correspondingly large amounts of urea to be injurious. As a matter of fact, there is no particular reason for thinking this to be the case; it is part of the duty of the liver to change all the ammonia that comes to it to urea and of the kidneys to pass out all the urea that comes to it; so long as these organs are healthy they are able to fulfill these duties effectively, so this does not seem to be a good reason for cutting down the percentage of meat in the diet. It is generally believed that meat has special effects upon the nervous system, such as to incite to cruelty and bloodthirstiness. There is no real scientific proof as to whether this is true or not. The scientific fact is that man is fitted for a mixed diet, neither exclusively of flesh nor exclusively vegetarian. He has lived for thousands of years on that kind of diet and can apparently go on for thousands of years more. We need to remember that the various dietary fads which come into great prominence from time to time are rarely based on a well-established scientific foundation nor have any of them any long experience back of them. On the other hand, the common mixed diet which all of us eat in accordance with custom and the dictates of our appetites has the sanction of thousands of years of successful maintenance of the human race. It is quite true that one can get along on almost any kind of a diet provided it contains enough protein to make good the daily body wastage and enough fuel material to provide for the demands of metabolism. Anyone who is disposed to adopt for himself a dietary fad will rarely suffer seriously from it; on the other hand, those who prefer to eat as our fore-fathers have eaten need not feel conscience-stricken because there is agitation against the commonly accepted diet.

While on the topic of diet, a word should be said about cooking. In a previous chapter the advantage of good cookery as an aid to digestion was emphasized. We would add here merely the comment that in defending the ancestral diet we do not intend to imply that their cookery was always what it should have been. Over most of America there has prevailed from pioneer days a habit of frying food in preference to other means of cooking it. Our hardy pioneer ancestors throve on fried meats; an outdoor life of muscular toil makes almost any kind of cookery both acceptable and digestible. As labor-saving machines tend more and more to diminish the amount of muscular labor that most of us do, we find it harder and harder to maintain good digestion on fried foods. The objection to frying is simple; fats are the hardest of all foods to digest and fried foods are smeared all over with fat. It is only logical to expect fried foods to be harder to digest than other kinds; it is undeniable that the flavor of many fried foods is so agreeable that we would be unwilling to omit them from the diet altogether. What is realty objectionable is the practice of smearing all the food with fat in the process of cooking it.

We have finished what we have to say about the use of food for the repair of bodily wastage. While we are on the topic, a few words about the use of food in growth will come in well, since the growth process is closely related to the process of tissue repair. The chief difference between them is that the process of growth comes to an end in all but a few of our body tissues as soon as we become adults. The tissues which go on growing are the layers of the skin just under the surface, the reproductive tissues, and the blood-corpuscle-forming tissues. Connective tissue has the power to grow at any time during life, although it does not actually keep on growing as does the skin. Whenever an injury is suffered which actually destroys muscular tissue or the deep layers of the skin, there is no growth of new tissue to take the place of either. Repair is made by a growth of connective tissue to fill up the space. The result is the formation of a scar. If the edges of the injury can be brought together skillfully enough, the outer layers of the skin which do have the power of growth may bridge across the space so that no scar will result.

A second thing to be mentioned about growth is a discovery which has attracted a great deal of attention of late years; we will realize of course that the chief thing in the making of new protoplasm is the building together of protein out of amino acids. It is evident that for the manufacture of new protein all the eighteen amino acids must be present in sufficient proportion to give enough of each. There are some plant proteins which lack one or two of the amino acids that are present in human proteins and when a growing animal goes on a diet in which these are the only proteins present it at once stops growing. Most of the experiments proving this have been done on white rats, and it has been found possible to keep a rat for more than a year at the size it was when only a few weeks old simply by feeding it proteins in which one or two amino acids were lacking. The fact that the animal lived during this time proves that his protoplasmic wastage was made good and therefore that there are proteins which can replace the body wastage but cannot manufacture new protein. There is only one conclusion to be drawn from this, namely that the daily wastage which the protein suffers does not include all of it; some amino acids when once built in are there for good; others, on the other hand, are constantly being lost in the process of wastage, and these are the ones which must be replaced. There is a common protein, gelatin, which is used a great deal for food, but which by itself will not serve either for repair or growth because it not only lacks some of the amino acids that are in living protoplasm, but also some that are lost in the process of wastage. Gelatin is useful as a food, therefore, only in combination with other proteins that contain the necessary amino acids; or after the nitrogen has been taken out, it becomes a good fuel.

One interesting question that was settled by the experiments described above was whether the ability to grow is exclusively a matter of youth; as we know, under ordinary conditions only young animals grow. When they reach a certain age, they become mature and thereafter no increase in size takes place. In the case of these white rats which were kept for more than a year at the size of partially grown rats it was found that as soon as their diet was changed to one in which all the necessary amino acids were present they would begin at once to grow and grow to full size, when they would stop growing just as they would have when young. This proves that growth is not a matter of age, but is a matter of achieving a certain size, and is controlled by factors which we do not at present understand. An animal that has not been able to attain

full growth because it has been denied the amino acids necessary for making new protein retains the power of growth, so that even though it may be long past the ordinary age of maturity it can go on growing as soon as the necessary materials are provided. The dependence of growth on certain dietary accessories was spoken of in Chapter IV and need not be repeated here.

The final use of food is as a source of energy for carrying on metabolism. A good deal was said about that in an earlier chapter, but there are a few additional points to be brought out here. 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. It passes from the digestive tract into the blood stream in the form known as an _emulsion_; all this means is that it is broken up into very tiny particles which are kept from running together by some sort of a film. In the case of the fats in the blood it is likely that this film is composed of ordinary soap. In this state the fat circulates in the blood stream until it is taken out by the tissues and burned. In the process of this burning some very poisonous substances are likely to be produced, but this happens only when large amounts of fat are being burned by themselves. If sugar is present and is being burned at the same time, there is no danger. Ordinarily in the body sugar is always present, but in a certain disease, diabetes, about which more will be said later, the body is not able to burn sugar as well as it ordinarily does, and under these circumstances poisoning from the products of the fat burning is apt to happen. This makes up, in fact, the serious danger in diabetes. As we all know, fat is chiefly important as the form in which food is stored in the body against a time of future need. We shall return to the way in which the body does this after we have spoken of the use of sugar as fuel.

Sugar is the great fuel substance of the body. About two-thirds or our food ordinarily consists of starchy materials which are digested into sugar. When we add to this the sugarlike remains of all the protein food which is not actually used for the repair of tissues we see that this substance makes up the great bulk of the material which is carried by the blood to the tissues. This material is handled in the body in an interesting way which depends on the curious fact that although sugar is the most important fuel for living cells they cannot endure its presence in them or in the fluids surrounding them except in very small amounts. Sugar, as we all know, is very soluble in water, and it would be perfectly possible for the blood to dissolve all the sugar that enters it from the digestive tract and simply carry it in solution until the tissues withdrew it for their needs; but this would mean that immediately after every meal the percentage of sugar in the blood would mount up to a high point from which it would gradually sink as the sugar was taken out, to mount again after the next meal when absorption began again. This does not actually happen because it is prevented by the liver, which has as the most important of its many functions that of storing the sugar that is taken up by the blood from the digestive tract and dealing it out little by little as the cells of the body need it. Back in the chapter on digestion we saw that all the blood that passes through the intestinal tract is gathered up by the portal vein and passes through the capillaries of the liver. It is during this passage through the liver that the sugar is taken out of the blood and stored in the form of a less easily dissolved material known as animal starch or _glycogen_. The liver cells have the ability to convert sugar into glycogen and they do this whenever the amount of sugar in the blood passing through them is greater than the very small amount which is suitable for the body cells. The blood that leaves the liver carries in it only this small percentage. The liver cells have the ability to change glycogen back into sugar, and this they do whenever the blood that enters them is deficient in it, so that the blood leaving the liver tends always to have the same amount of sugar in it. Whenever it enters with more, there is a conversion of sugar into glycogen; whenever it enters with less, there is a conversion of glycogen back into sugar.

So important is the protection of the body cells against having too much sugar in the fluids surrounding them that the kidney acts to prevent undue accumulation; this it does by withdrawing from the blood and passing out into the urine any sugar that may be in the blood in excess of the small amount which the tissues are able to endure. Thus we see that if the liver did not have its function of converting the sugar into glycogen we would have to change our eating habits completely, taking only a little food at a time instead of eating it in three meals, since otherwise most of our food would be wasted by being passed out from the kidneys as fast as it was poured into the blood from the digestive organs. There is a limit to the ability of the liver to change sugar into glycogen. If the amount in the portal vein at any one time goes above a certain figure, not all will be saved: a part will escape into the blood stream, and since this will raise the percentage, the kidney comes into action and passes it out. In order for this to happen, there must be a very large amount of digested sugar in the small intestine leading to rapid absorption. Since some starchy foods are easier to digest than others some diets are more likely to result in the appearance of sugar in the urine than others. Some of our common foods, notably honey and corn sirup, consist largely of the kind of sugar the body uses. These require no digestion at all, but are ready for absorption as soon as they enter the small intestine. Naturally, if they are eaten in any quantity, they are likely to flood the liver with sugar beyond the amount which it can change to glycogen. Common table sugar and the sugar of milk have to be digested before they are absorbed and so are less likely to flood the liver. It is true, however, that either of them if taken in very large amounts may do this. The digestion of starch goes on much more slowly and so is absorbed more gradually and it is doubtful whether the liver is ever flooded on a starch diet. Since the presence of sugar in the urine is a common indication of diabetes, it is necessary to know that other conditions may bring it about. Obviously, as in the case of an examination for life insurance, a perfectly healthy person might be rejected on account of the presence of sugar in his urine, if it were not that the examining physician knows of this other possibility and is on his guard against it.

The liver ordinarily turns glycogen back into sugar at just the rate necessary to keep the amount in the blood constant. This means that when functional metabolism is going on glycogen is being turned into sugar more rapidly than when the body is quiet. One of the very interesting discoveries of recent years is that in times of strong excitement leading to the outpouring of adrenalin into the blood the rate of change of glycogen into sugar is much increased, so that instead of the usual small amount there is present in the blood a large concentration of sugar. This is evidently advantageous in insuring ample fuel supplies to the muscles at the time of an emergency. It is wasteful in that a large part of the surplus sugar is passed out by the kidneys. The fact is illustrated that in marshaling the bodily functions for meeting an emergency economy is lost sight of.

In addition to these healthy conditions in which sugar may appear in the urine there is the disease diabetes, in which the presence of sugar in the urine is a conspicuous symptom. In diabetes there is a serious disturbance of the whole fuel-supplying mechanism; the liver does not carry on its function of changing sugar to glycogen and glycogen back to sugar as perfectly as it should, and what is of much more importance the muscles which are the chief users of sugar as fuel cannot use sugar in anything like their usual manner. In fact in severe cases they appear to be almost wholly unable to use sugar as fuel. Since the protein from which the nitrogen has been removed classes itself with sugar in this regard, the muscles are thrown back upon fat as their only source of fuel, and this confronts the body with the danger already mentioned that in the burning of fat when little or no sugar is being burned along with it very poisonous products may be formed. Medical investigators have devoted a vast amount of labor to the attempts to find a diet that can be successfully eaten by diabetics. Starches and, almost equally, proteins are not serviceable because they simply flood the tissues with sugar, making an environment which is not good for the cells, and keeping the kidneys busy getting rid of the surplus. Fats are dangerous for the reason just stated. Quite recently real progress has been made by means of the discovery that when the body is living on its own tissue there will be no accumulation of sugar in the body fluids nor outpouring of it from the kidneys. One who is being starved is living on his own tissues and so by simply starving a diabetic his symptoms can be relieved. This is not in itself a very promising expedient, since evidently without food one cannot go on living very long. The point of the treatment is that after starvation has proceeded until the body is actually living on its own tissue it is possible to begin feeding proteins cautiously until little by little a protein diet can be established in which there is little or no indication of surplus sugar. In other words it seems that when the body is compelled to live upon its own tissue it uses proteins efficiently and will then go on using them efficiently when they are supplied to it in the diet.

In an earlier section of the chapter we talked about the amount of protein that the body requires; now we have to take up the matter of the amounts of energy-yielding foods. 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. In order to talk intelligently about the use of foods for metabolism we must have a word by which to express a definite amount of it. The word that we use for this is “calory.” Primarily this word stands for a certain amount of energy in the form of heat; since one kind of energy can be transformed into another kind without changing the actual energy value we can use this word as a measure of all kinds of energy and this has become the custom. The calory as a unit of energy means really the amount of energy in the form of heat required to raise the temperature of 1,000 grams of water by 1 degree centigrade. We can translate it into more familiar terms by stating that it equals almost exactly the amount of energy required to raise a weight of 300 pounds to a height of 10 feet, or 30 pounds to a height of 100 feet, or any other combination of weight in pounds multiplied by distance in feet which figures up to 3,000.

The total metabolism, as we said a moment ago, varies greatly day by day because the extent to which we use our muscles is so different. 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; this includes the motions that are necessary for eating and swallowing food, since life cannot go on indefinitely without making those motions, but supposes that all other activities are done away with. This figure, as we said a moment ago, holds pretty steady day in and day out. To find the total metabolism on any day we have simply to add to it the figure for the energy expenditure from the use of the muscles, which will vary with the amount of work that is done. In figuring this we have to reckon with the fact that the muscles are like other machines in working at what is called in engineering a low efficiency; by that we mean that the amount of energy that can be gotten out in the form of useful work is less than the energy that is actually consumed; there is a waste of energy which takes the form of heat. Our muscles are under ordinary circumstances about twenty per cent efficient, which means that for every calory-worth of actual muscular work we do we use up five calories-worth of fuel; the energy of four wasted calories takes the form of heat and we all know from experience that the amount of heat thus produced inside our bodies warms us up very quickly, when we are using our muscles actively.

The actual muscular work done in a day by individuals varies from almost nothing in the case of invalids confined to bed through about forty calories for a person of decidedly sedentary habits and about 120 calories for the average clerical or professional man up to from 300 to 400 in the case of manual laborers. These figures are for the actual muscular work done; to obtain the energy expenditure we have to multiply each of them by five on account of the inefficiency of the muscles. If we do this and then add to each product the constant figure of 1,900 for the metabolism of rest we obtain for the total metabolism of a decidedly sedentary person an average of about 2,100 calories; for an average clerical or professional man 2,500 calories, for manual laborers from 3,200 to 4,000 calories. Of course, individuals may exceed even these latter figures. It is believed that athletes in extreme competitions such as for example a six-day bicycle race may liberate energy at the rate of 10,000 calories a day, although they probably cannot keep this up long enough actually to do that amount in a single day.

In order to satisfy the requirements of metabolism the food that is eaten must yield corresponding amounts of energy. If it does not do so enough of the tissues will be consumed to make up the deficiency. Of these the first to be drawn upon will be the stored glycogen in the liver and secondly the body fat. Only when the deficiency is great enough so that all these are used up, does the protoplasm itself begin to be drawn upon as a source of fuel. This happens in cases of prolonged starvation and it is interesting to note that in this case the tissues drawn upon are the muscles. When one wastes away as the result of starvation, the only tissues that suffer serious loss at first are the muscles; the rest of the body is fed at their expense. In this process no muscle cells are actually destroyed--apparently each can sacrifice a little of its protoplasm without being itself injured; the material thus obtained is converted into amino acids and from most of these the nitrogen is removed, leaving a fuel material which can be burned in the cells all over the body to keep them going. It is only after extreme starvation, when the muscles can no longer yield of their substance without being themselves destroyed, that the other tissues begin to show serious wastage. This explains why the brain of a starving man remains clear almost up to the end.

If a surplus of food is eaten over the energy requirements, the liver will store it in the form of glycogen so far as it is able; but if this will not suffice, the excess will be changed into fat and stored in the body in what are called adipose tissues. These are located in various regions, one of the most important being directly under the skin. It is the loading of this with fat that causes the bodily enlargement of fat people. We do not know exactly how the fat is made; we do know that it is not ordinarily food fat that has been simply transported to these tissues and deposited there. There is abundant proof that the body can manufacture fat even if there is none in the diet. Grazing cows that get no fat of any kind produce milk with its regular percentage of fat in the form of cream and do this day in and day out, showing that the fat that the body makes does not have to come from fat in the food. Since body fat represents ordinarily a storage of fuel against a future need, we ordinarily think of it as made whenever the temporary storage in the form of glycogen becomes inadequate to take care of the surplus of food over the amount consumed in carrying on the metabolism. Since the vast majority of people are neither gaining nor losing weight, the amount of food that they take in each day must balance the average metabolism. This is interesting because the amount of food that is eaten is regulated chiefly by the sense of hunger, or by the hunger and appetite together, and it is remarkable that these should cause us to eat so accurately just the amount of food our metabolism requires. In order that we may get some idea of how much energy is furnished in our common foods a table is given below. The figures are for the numbers of calories in a pound of the food material as purchased in the market. In most vegetables and meats there is a loss of about ten per cent in preparing them for the table, or, in the case of meat, in the bones. The figures were prepared by officials of the United States Government in arranging dietaries for the Army.

CALORIES PER POUND AS PURCHASED

Apples, fresh 219 Bacon 2,979 Bananas 298 Beans, dried 1,603 Beef, fresh 1,009 Bread 1,300 Butter 3,478 Cabbage 124 Celery 70 Cheese, American 1,948 Chocolate 2,858 Eggs 614 Fish, salmon, canned 679 Fish, fresh 368 Flour 1,651 Lard and substitutes 4,218 Milk 302 Pork, salt 2,948 Potatoes, white 311 Rice 1,631 Sugar 1,860 Tomatoes 106

Very few of our foodstuffs are exclusively of one kind of material; that is they are not exclusively protein or exclusively fat or exclusively of sugar or starch compounds. Most vegetables are mixtures of starch with protein, fruits are mixtures of starch and sugar with a little protein, both fruits and vegetables contain so much water that their actual fuel value per pound amounts to little. Meats are mixtures of proteins with fats. Milk is a mixture of proteins, sugar, and fat. Table sugar and butter are as near pure unmixed foodstuffs as any of the things we commonly eat. In an earlier part of the chapter we saw that about two-thirds of our ordinary diet is of starch or starchlike materials. This figure is given, not in weight of material, but in the energy value. What it means is that about two-thirds of the energy for our metabolism comes from the starch and sugar that we eat; the other one-third is divided between fat and protein in the proportion of about two to one. The energy we get from fat being about double that which we get from protein--since a given weight of fat has about twice the energy value of the same weight of protein--we actually eat about the same amount of protein as of fat. The combined weight of the starch and sugar is between four and five times that of the protein. These, of course, are average figures representing not what we eat at any one meal, but the way in which the foodstuffs are found to be divided in our diet taken as a whole. Where meals have to be planned on a large scale, as in armies or in institutions where the persons to eat the food have not much choice in selecting them, it is necessary that the diets be arranged both to give the proper amounts of material and also to furnish them in about the correct proportions. The dietary experts who have charge of these matters do this with tables similar to the one given above. In domestic feeding arrangements, although usually the choice of foods is determined by the state of the markets, the preferences of the various members of the family, and the abilities of the cook, it is astonishing how closely the result will correspond in the long run to the figures here given. Most of us, without any effort on our part to do so, eat a diet which is made up week in and week out in just about the proportions here given. The experience of ourselves and of all our ancestors indicates that these are the correct proportions for the human race.

In talking about metabolism thus far we have spoken as though the metabolism of the body at rest were about the same day in and day out regardless of conditions. This is true in the main for healthy persons; there is one condition which may bring about a change in the resting metabolism of health which must be mentioned, and one or two coming under the category of disease about which also something must be said. The change in resting metabolism that comes about in health is one that is seen when the diet contains an especially large percentage of protein. For some reason which we do not understand the digestion, absorption, and utilization of protein stimulate the resting metabolism so that during the time that this protein is being used the metabolism is higher than at other times. The curious thing about it is that the increase of metabolism is not just enough to take care of the protein itself, but goes so far to cause sugars or even fats to be burned at a more rapid rate than usual. This fact is taken advantage of in treatment for reducing flesh; where one lays on flesh it is evident that more food has been taken than was required for metabolism so that the surplus has been stored in the form of fat. The only way to reduce the weight is to compel the body to burn up that stored fat. In theory the simplest way to do this is simply to starve. If starvation is combined with very vigorous exercise, the reduction of weight is bound to be rapid, since metabolism cannot be carried on without fuel, and if not enough fuel is supplied in the form of food, the body will have to furnish it and the stored fat is the place from which it will be taken. Unfortunately this is much simpler in theory than it is in practice. A good deal of discomfort and sometimes even disturbance of health results from too drastic efforts to reduce the weight by starvation. It is perfectly feasible to do it by adopting and sticking to a practice of never eating quite enough. This is a perfectly successful method, but requires great strength of will to carry it out. Probably the easiest way to reduce weight is to combine self-denial with a diet which consists chiefly of protein. Thus the stimulating effect upon metabolism is obtained and the result will be a gradual burning away of the body fat. The selection of a diet to fit any particular individual can best be made under competent medical advice, since personal peculiarities have to be taken into account in selecting among the various foods those best adapted for the purpose.

The variations in resting metabolism that fall under the head of disease are, first, the increase of metabolism in fever, about which we shall speak in detail in the next chapter, and, secondly, variations in metabolism that result from variations in the activity of the thyroid gland. The thyroid gland is an organ at the front of the neck; when it is enlarged, as it is in some people, we have the condition known as goiter. This gland is now known to manufacture and pour out into the blood a hormone which is a regulator of metabolism. When it is produced in normal amounts, the metabolism goes on at the rate that we find in healthy people. If the gland is inactive and does not secrete enough of the hormone, there is a reduction in the metabolism. Since this implies a lowering of the vigor of the life processes, we might expect it to have important effects. The most marked of these are in the nervous system. Persons whose thyroid glands are relatively inactive are mentally sluggish; the less active the gland is, the more marked is this sluggishness, and in case of practically complete absence of the hormone the condition amounts to idiocy. Occasionally a child is born without an active thyroid gland; this child is doomed for life, unless artificial aid can be procured, not only to complete idiocy but to all the other results of lowered metabolism; these show themselves in dwarfishness and a misshapen body. One of the conspicuous and beneficent discoveries of medicine in comparatively recent years has been that extracts of the thyroid glands of meat animals, when eaten by persons whose own thyroids are not sufficiently active, supply the lack, and so they may be restored to the normal condition. There are many people alive to-day who are in all respects normal, but who, if they were to discontinue taking thyroid extract, would relapse rapidly into a condition of idiocy.

The thyroid gland may sometimes become overactive as well as underactive; when the former happens we have an increase in the resting metabolism and a group of symptoms that indicate, so far as the nervous system is concerned, a condition of overexcitability. Unfortunately this does not mean exceptional mental power but rather mental instability. The victims of this condition are exceptionally quick nervously, but they are quick to take offense and quick to be disturbed by all sorts of conditions. If the overactivity of the gland becomes too pronounced, mental instability or even marked insanity results. Another fact of the heightened metabolism is that large amounts of food must be eaten to carry it on. Sufferers from overactivity of the thyroid gland eat voraciously, but, in spite of doing so, are thin or even emaciated. They have rapid heartbeat, high blood pressure, and other symptoms indicative of too great activity of the gland. The successful treatment of this condition depends on the removal by surgery of enough of the thyroid gland to reduce the outpouring of the hormone to the normal amount. This feat is now accomplished successfully by our most skillful surgeons, and the result has been the restoring to health and happiness of large numbers of people whose lives were rendered miserable through no fault of their own, but because their thyroid glands had become unduly active. We do not know how the thyroid gland itself is controlled; there is evidently something which causes it in the vast majority of us to produce its hormone at the rate which keeps the metabolism steady at what we look upon as the level of health. Deficient thyroid is, at least in some cases, hereditary. Excessive thyroid activity seems to be rather the secondary result of some preceding disturbance of the nervous system, but as to that we cannot say with confidence.