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.
CHAPTER XIX
KEEPING WARM IN WINTER AND COOL IN SUMMER
A GOOD deal of what has been said thus far in the book applies to nearly all kinds of animals about as well as it does to man. We have now to take up a feature found only in two great groups of the animal kingdom—birds, and the four-legged animals, to which are given the name of mammals. This peculiarity is commonly called warm-bloodedness. What we really mean when we say that an animal is warm-blooded is that the temperature of its body runs about the same summer and winter, day in and day out. An animal that we call cold-blooded, on the other hand, cools down when in a cold place, but is warm when in a warm place. It therefore has a very variable temperature as compared with the almost constant temperature of warm-blooded animals. The maintaining of the warm-blooded condition, which means really the maintaining of a constant temperature, involves only one thing, namely that the amount of heat lost from the body shall exactly balance the amount of heat produced in it. If more heat is produced than is lost, the temperature must rise; if more heat is lost than is produced, the temperature must fall. These are simple facts of physics which apply as well to the body of an animal as to any other source of heat.