Tones: Body equilibrium. Nitrogen equilibrium. Carbon equilibrium. Loss of nitrogen during fasting. Influence of previous diet on loss of nitrogen in fasting. Output of carbon during fasting. Influence of pure proteid diet on output of nitrogen. Influence of fat on proteid metabolism. Effect of carbohydrate on nitrogen metabolism. Storing up of proteid by the body. Transformation of energy in the body. Respiration calorimeter. Basal energy exchange of the body. Circumstances influencing energy exchange. Effect of food on heat production. Respiratory quotient and its significance. Influence of muscle work on energy exchange. Elimination of carbon dioxide during work and with different diets. Effect of excessive muscular work on energy exchange. Oxygen consumption under different conditions. Output of matter and energy subject to great variation. Body equilibrium and approximate nitrogen balance to be expected in health.

MAN, strictly speaking, is always in a condition of unequilibrium. If placed upon a large and sensitive pair of scales with the opposite side exactly counterpoised, he will be found to lose weight constantly until water or food are taken, when the losses of an hour or two may be made good, or perchance more than balanced. The human body is a maelstrom of chemical changes; chemical decompositions are taking place continuously at the expense of the proteids, fats, and carbohydrates of the tissues and of the food, the stored-up energy of these organic compounds being thereby transformed into the active or "kinetic" forms of heat and motion; while carbon dioxide, water, urea, and some few other nitrogenous substances are being continually formed as the normal waste products of these tissue changes, and constantly or intermittently excreted. In other words, the body is in a perpetual condition of chemical oscillation, constantly consuming its own substance, rejecting the waste products which result, and giving off energy in the several forms characteristic of living beings. The condition of the body plainly depends upon the relation which it is able to maintain between the income and the expenditure of matter and energy. If the income equals the output, the body is kept in a condition approaching equilibrium; if the intake exceeds the outgo, the body adds to its capital of matter and energy; while if the expenditure is greater than the income, the accumulated capital is drawn upon; and this, if continued indefinitely, results in a drain upon the bank which must eventually end in disaster. It is comparatively easy, however, for man to maintain his body in a condition of equilibrium from day to day; i. e., the losses of the morning can be made good at luncheon, or the expenditures of an entire day counterbalanced by a corresponding addition to capital the following day, in which case the body may be said to be in balance. It is necessary, however, to discriminate between body equilibrium, meaning thereby the maintenance from day to day of a constant body-weight, and nitrogen equilibrium, or carbon equilibrium. In the latter cases, what is meant is that the intake of nitrogen, or of carbon, exactly equals the output of these two elements. It is quite possible, however, to have a condition of nitrogen equilibrium without the body being in a state of balance, as when the outgo of carbon exceeds the intake of carbon, or when there is an increased output of water.

As a rule, it may be stated that when a man puts out less carbon and less nitrogen than he takes in he must be gaining in weight; the only exception being the possible case of an increased excretion of water, which might more than counterbalance the gain. On the other hand, if he gives off more carbon and more nitrogen than he takes in, the body must lose in weight. Where the output of carbon is beyond the amount of carbon ingested, the lost carbon represents a drain upon body fat. In a reversal of this condition, i. e., where the carbon taken in is in excess of the outgo, the body is gaining in fat. Theoretically, gain or loss of carbon may mean gain or loss of either carbohydrate or fat but practically stored-up carbon generally stands for accumulated fat; and, correspondingly, loss of carbon represents a withdrawal from the store of adipose tissue, since glycogen and sugar from a quantitative standpoint figure only slightly in these metabolic processes. When the body excretes more nitrogren than is taken in during a given period, there is only one interpretation possible, viz., that the body is losing proteid or flesh. If, on the other hand, the nitrogen import excceds the outgo, then the body must be gaining flesh. Here, again, there is the theoretical possibility that gain or loss of nitrogen might represent increase or decrease of proteid in some glandular organ, or even in the blood; but practically it is the relatively bulky muscle tissue, with its high content of proteid matter, that is most subject to change in metabolism. Finally, it is easy to see how, knowing the percentage of nitrogen in proteid and the percentage of carbon in fat, one can calculate from the nitrogen and carbon lost or gained the amounts of proteid or fat added to the capital stock, or withdrawn from the store of nutritive material.

When there is no income, as in fasting, the body loses rapidly, living during the hunger period upon its store of energy-containing material. Many careful observations have been made upon people who have fasted for long periods, some as long as thirty days, the income consisting solely of water. The following figures1 show the daily excretion of nitrogen in several notable cases:

In Succi's case, the fasting was continued for thirty days. The daily average loss of nitrogen from the 11th to the 15th day was 5.8 grams; from the 16th to the 20th day, 5.3 grams; from the 20th to the 25th day, 4.7 grams; and from the 26th to the 30th day, 5.3 grams. A daily loss of 5.3 grams of nitrogen means a breaking down, or using up, of 33 grams of proteid, or a little more than one ounce. On the sixth day of fasting, all three of these subjects showed essentially the same daily loss of nitrogen; viz., 10 grams, which implies a using up of 62.5 grams of proteid material. We must not be led astray by these figures, however, or draw too hasty conclusions therefrom regarding the requirements of the body for proteid food. Noting the close agreement in the nitrogen output of the three subjects on the sixth day, combined with the fact that their body-weight was essentially the same, we might infer that 62.5 grams of proteid matter represents the amount of nitrogenous food necessary to maintain nitrogen equilibrium and beep the body in a condition of balance. Such a conclusion, however, would be quite erroneous for several reasons. First, a man fasting, if he was in an ordinary condition of nutrition prior to the fast, has in bis tissues a large store of fat. It is considered that in fasting only about 10-12 per cent of the total energy of the body is derived from tissue proteid; the major part comes from the fat stored up. When there is no income to make good the loss, the body must naturally draw upon its own store. A certain amount of proteid must be used up daily, but in addition there are the energy requirements to be considered. These are met mainly by fat and carbohydrate, and so long as fat endures proteid will be drawn upon only, or mainly, to meet the nitrogen requirement; but if the fat gives out, then proteid must be used in larger quantity, as a source of energy. Hence in fasting, the daily loss of nitrogen will be governed largely by the condition of the body as regards fat. Thus, Munk has reported the case of a well-nourished and fat person, suffering from disease of the brain, who gave off daily in the later stages of starvation only one-third the amount of nitrogen voided by Cetti, who had been poorly nourished. Obviously, in fasting, as soon as the adipose tissue of the body has been largely used up, there will be an increase in the amount of tissue proteid consumed, since under such conditions the heat of the body and the energy of muscular work (work of the heart and involuntary muscles) must come from the decomposition of proteid. In harmony with this statement, it is frequently observed that in cases of starvation there comes toward the end a sudden and marked increase in the output of nitrogen.

1 Taken from Landergren: Untersuchungen iiber die Eiweissumsetzung des Menschen. Skandinavisches Archir fur Phytiologie, Band 14, p. 112; and from A. Magnua-Lery: r. Noorden'f Handbnch der Pathologie des Stoffwech-sels, 1906, p. 312.