Health may be maintained and work done on diets which depart widely from the one which we have selected as a standard. Darwin found the Gauchos of South America living exclusively on meat. Nansen and Johannsen, when seeking the North Pole, lived for months on meat and blubber. Millions of the inhabitants of India abstain from meat and meat-fat, their diet consisting of rice, buttermilk, and a little fruit. In the case of all persons with whom the price of food is an important consideration, carbohydrates are preferred to proteins and fats. Oatmeal is very much cheaper per unit of energy than meat. A man may be a meat-eater or a vegetarian, although he is probably unwise in overlooking the obvious teaching of his teeth and digestive organs, which are those of an omnivorous animal. His prehistoric human ancestors lived chiefly on the harvest of their spears and tomahawks. If we insist upon looking back still farther, we discern a cleavage of the race into the arboreal fruit-eaters, which still retain pre-human characters, and the more enterprising and energetic troglodyte hunters from whom the human race was evolved.
A man may vary his diet within wide limits. Innumerable considerations lead certain individuals to desire to depart from the diet which we have termed “normal”—i.e., typical of inhabitants of the temperate zone. One man rebels against the expense of living; he would fain reduce the quantity and the cost of food. Another, having to traverse regions in which food is scarce, wishes to ascertain the lightest, and therefore the most portable, combination of its essential elements. A third—and he belongs to a much larger class—tormented with indigestion or harassed by gout, asks, “Why must I consume things which give the stomach trouble, or produce disagreeable and incapacitating after-effects?” Many circumstances prompt to experiments in diet. Much latitude is undoubtedly allowed. But there are limits within which alone health can be maintained and work done. It is of great interest to ascertain exactly how wide these limits are; and especially important is it to find out the lower limit, the minimum of food, and the minimum of each particular kind of food, which will enable the human machine to work. The problems involved are somewhat complicated. If it were possible to live on a single food, it would be as easy to ascertain the irreducible minimum as it is to find out with how much coal or with how much petrol an engine can be made to turn a wheel. But to support the body several different kinds of food are indispensable. It is therefore necessary to determine, not only the minimum quantity of the combined foods, but also the minimum amount of each kind of food, and the effect upon the total of variations in the relative amount of each of its several factors. The problem is complicated, but certain limits are impassably defined. In the first place, with regard to the total amount, the work which the body does cannot under any circumstances be reduced below a certain level. The food consumed must provide a supply of energy equal, at the least, to the performance of the minimum of work. The body must receive each day food of due caloric value. Then with regard to the amount of each several constituent. Many considerations lead us to wish to increase one of them or to diminish another. Some food is cheap, and other food is dear. Economic reasons are in favour of the cheaper food. Even ethical considerations are not without weight. We have, perhaps, a prejudice against sacrificing life to supply the pot. We have doubts as to whether our system can properly digest, metabolize, and excrete meat. We need an unambiguous answer to the question, To what extent can nitrogen-foods be replaced by carbon-foods, and vice versa? A cell, as already said, consists of a framework of bioplasm bathed in cell-juice which contains nutrient substances and manufactured products. The bioplasm is alive; the proteins, carbohydrates, and fats of the cell-juice are the materials with which it is nourished, and upon which it works. Some physiologists incline to the view that non-living substances must enter into the bioplasm before they undergo metabolism. They consider that the molecules of the non-living substance must at the time when they undergo a chemical change be physically and chemically a part of the living substance. Others take the opposite view: that the living substance does not undergo change, but brings about changes in the non-living substance which is in contact with it, enclosed within its meshes. This is a problem which is not likely to be solved, nor is its solution of great importance in relation to the question which we are discussing. Whichever of the two views be justified, we have to distinguish between the bioplasm of the cell—the machine—and its raw materials and manufactured products. The question to which we want an answer is the following: Must the bioplasm undergo change? There seems to be no reason in the nature of things why it should. It is not, as we have already pointed out, subject to wear and tear. A perfect machine would in the absence of friction, which rubs down its steel and brass, continue to turn out its products so long as it was supplied with raw materials and the energy needed to manufacture them. We could imagine the bioplasm as indestructible, receiving energy from a portion of the foods, and expending this energy in the production of chemical change in the remainder. We could imagine that when once the tissues had attained their full growth they would require no more protein for their own nutrition; they would be occupied in producing heat and motion from the non-nitrogenous foods. But observation shows clearly that this is not the case. The force which energizes the bioplasm, enabling it to evoke metabolism in non-living substance, is obtained at the cost of its own destruction. The bioplasm wastes unless constantly supplied with proteid food.
Under ordinary circumstances the amount of urea excreted varies directly as the quantity of nitrogen contained in the food. Since urea contains 45 per cent. of nitrogen, and protein 15 per cent., every gramme of urea excreted represents 3 grammes of dry protein consumed; or, in terms of nitrogen, every gramme of nitrogen excreted represents 6·25 grammes of protein consumed. If all food is withheld, the excretion of nitrogen falls, but it never reaches zero. Many observations have been made on fasting men. On the second day of fasting the nitrogen excreted falls to about 13 grammes, representing 80 grammes of protein used up. It is generally thought that by the second day all “floating proteins” are exhausted, and that therefore nitrogenous metabolism is reduced, as it were, to a business basis. So long as the supply of food is abundant, the body has a luxurious habit of using proteins in preference to non-nitrogenous food. But after a day’s starvation there is no longer any fancy metabolism, no consumption of proteins as fuel when cheaper fats and sugar would answer equally well. In the case of Succi, who fasted for thirty days, the nitrogen excreted fell to 6·7 grammes on the tenth day, to 4·3 grammes on the twentieth, and to 3·2 grammes on the last day. Clearly, we have to make a distinction, when all food is cut off, between the oxidation of the protein which, failing all other material, is withdrawn from the tissues for the purpose of supplying the force absolutely necessary to maintain respiration and such other movements as are inevitable, and to keep up the temperature of the body—force which under other circumstances might be supplied by non-nitrogenous food—and the oxidation to which bioplasm is inevitably subject, so long as it is alive. The oxidation of bioplasm under ordinary circumstances of course supplies force; but it does not follow that this is sufficient to maintain the respiratory movements and the contraction of the heart. When a herbivorous animal is starved, it not infrequently excretes more urea at the commencement of the starvation period than it was excreting when well fed. Its activities did not come to a standstill when carbohydrate food was cut off. For a time they were maintained at the expense of its own tissues. On the other hand, the results obtained from the observation of the man who went without food for thirty days show that Nature is able to economize force by reducing the metabolism of living substance below the normal. It might be supposed that the irreducible metabolism could be ascertained by giving a nitrogen-starved animal non-nitrogenous food, but it is found that this scarcely affects the tissue-waste. Becoming more active, the tissues, while saved from the necessity of supplying fuel for the production of heat and motion, suffer more waste. Again, it might be expected that if to an animal which had been starved for a few days, until its urea had fallen to the starvation limit, exactly sufficient protein were given to supply this amount, the tissues would be saved. It is found, on the contrary, that nearly twice as much urea is excreted as before. If the quantity of protein be steadily increased, equilibrium is at last established, but not until the amount of nitrogen in the protein given is two and a half times as great as the amount excreted during the starvation period. Additional food at once gives rise to additional waste. The tissues which during the period of scarcity had reduced their oxidation to a minimum become more active at the first hint of returning plenty.
This last experiment illustrates a general law. An increase of proteid food within certain limits increases the metabolic activity of the tissues—provokes them to extravagance. It is possible, by adding protein to a mixed diet which sufficed for the maintenance of body-weight and nitrogenous equilibrium, to bring about a nitrogen deficit and to reduce the body-weight. Or, if the body is gaining in weight, owing to the accumulation of fat, the substitution of protein for carbohydrate (weight for weight, since their caloric values are the same) will lead to its reduction. It is difficult to avoid the use of fanciful language in accounting for these results. The animal economy is like an over-careful housekeeper, who, when meat is scarce, doles out porridge also with a thrifty hand. When meat is plentiful she is prodigal with every article of diet. Protein is the most costly of foods. Any indication that it is scarce leads to a shutting-down of activity. On the other hand, no other food is so readily absorbed (unless the digestive organs be protein-sick); none is so quickly incorporated in the bioplasm; none is so easy to decompose. When fed with protein the machinery hums. The insatiable appetite for beef and eggs which overtakes a man of sedentary habits after a long morning in a boat or on a bicycle does not indicate that his muscular tissue is suffering from wear and tear. It does not prove that he is setting free energy by oxidizing proteid food. It shows that he is asking certain tissues which are accustomed to a quiet life to exhibit prodigious energy. They will not shake off their customary sloth unless he stimulates them with sumptuous fare. At the end of a week he finds that proteins are not the best fuel for steady work. If he consumes sufficient to supply all the energy needed by his muscles, he is hampered by a quantity of nitrogenous residues which have to be reduced to urea and eliminated by the kidneys. He goes back approximately to his old regimen, so far as proteins are concerned, and consumes more carbohydrates for the supply of the force which his increased muscular activity demands.
It is possible to live on meat alone, but the quantity required is very great, involving the digestive organs, the liver, and the kidneys in an excessive amount of work. On the other hand, it is possible to reduce the consumption of proteins to a minimum by substituting for them fats and carbohydrates. But, again, after the proper balance is disturbed, the substitution ceases to be a simple problem in arithmetic. The carbon-food has to be increased out of all proportion to the protein which it replaces. If a dog which is being fed on a diet natural to it—chiefly meat—is in a condition of nitrogenous equilibrium, carbohydrate may be substituted for some of the meat. But from the very beginning it is found that, if nitrogenous equilibrium is not to be disturbed (if the dog is not to be induced to consume its own tissues), a weight of carbohydrate must be given considerably greater than the weight of the protein withdrawn. The disproportion increases as the experiment proceeds, until perhaps 12 to 15 grammes of carbohydrate have to be substituted for every gramme of protein. The proteid food has now come down to 1·5 gramme per kilogramme of the animal’s weight. Owing to the increase of carbohydrate, the caloric value of the total food, nitrogenous and non-nitrogenous, is several times as great as the animal requires. The surplus is oxidized without any equivalent in work. At about this point the experiment is brought to an end, owing to the failure of the digestive organs to deal with so large a mass of food.
The value of gelatin as an article of diet is of interest in this connection. Gelatin is not, strictly speaking, a protein, and it cannot be built up into the tissues. It does not prevent, nor even delay, starvation. Yet up to a certain point it can be used as a substitute for proteid food. In the observation just referred to, protein might be withdrawn at any stage, without disturbing nitrogenous equilibrium, by substituting about 2 grammes of gelatin for every gramme of protein withdrawn. It spares protein, although it does not take its place. It is said that the minimum of protein necessary for the maintenance of nitrogenous equilibrium may be reduced to about one-half by the substitution of gelatin. This has been interpreted as indicating that when we have reduced the oxidation of nitrogenous substance to its smallest amount the nitrogen comes from two sources in about equal proportions—(a) the bioplasm; (b) the food-proteins in contact with it. It is inferred that gelatin, although it cannot be built up into bioplasm, may take the place of proteins present in the cell-juice. It appears to be impossible to starve the cell until it consists of a bioplasm framework bathed in nitrogen-free cell-juice. As the non-living proteins of cell-juice are removed, they are, if no nitrogenous food be given, renewed by the breaking down of bioplasm. When gelatin is absorbed, it takes its place in the cell-juice, and the breaking down of bioplasm is no longer necessary. When digestion is impaired, or vitality lowered, decoctions of meat which contain extractives of low calorific value, useless, without synthesis ([cf. p. 144]), for the purposes of tissue-repair, may to a certain extent save tissue-waste. In the same way, gelatin, which is very rapidly digested in the stomach, may cover the consumption of proteins, although it cannot take their place.
To sum up: The requisite daily income of energy must come from both nitrogenous and non-nitrogenous food. It is impossible to reduce the nitrogenous factor below a certain minimum. From this minimum upwards, until a certain level is reached, every additional unit of nitrogenous food enables the system to dispense with more than its equivalent of non-nitrogenous food. When the proper balance of foods is attained, there is no waste either of labour involved in digestion, or of labour involved in metabolism and excretion.
The Liver.—The liver weighs from 3 to 3½ pounds. It lies beneath the diaphragm, more on the right side than on the left. Its posterior border, which rests against the last three ribs (separated from them by the diaphragm), is about 3 inches thick. Its anterior border is thin, and keeps close along the line of the ribs. If the organ is neither unduly enlarged nor squeezed out of its place owing to the use of a tight corset, it does not project below the ribs, save where it crosses the space between the rib-cartilages below the end of the breast-bone.
