THE BALANCE OF NUTRITION

Topics: 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 nitrogen 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 exceeds 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 figures22 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 keep 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 his 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.

Secondly, the elimination of nitrogen during the earlier days of fasting is governed in large measure by the character and extent of the diet on the days just preceding the fast. This is well illustrated by some experiments conducted by C Voit on a dog. In the first series of experiments, the dog received daily 2500 grams of meat prior to fasting; in the second series, 1500 grams of meat were fed daily before the fast; while in the third series, a mixed diet relatively poor in proteid was given. The following figures23 show the amounts of proteid used up by the dog (calculated from the nitrogen excreted) each day of the fasting period, under the different conditions:

We see very clearly in these experiments the effects of the large quantities of proteid fed on the destruction of proteid in the early days of fasting. When the body is rich in proteid from food previously taken, the metabolism of nitrogenous matter is very large at first, as in the first series of experiments. Indeed, in this series, even on the fifth day of fasting, the amount of proteid metabolized was larger than on the second day of the third series. We have here a forcible illustration of the physiological axiom that excess of proteid matter in the tissues, or in the blood, stimulates proteid metabolism; and it affords convincing proof of the contention that in the first days of fasting the output of nitrogen, or the amount of proteid used up, will depend in large measure upon the proteid condition of the body at the time of the fast. Equally noticeable is the fact that there comes a time—the sixth day in the above experiment—when the nitrogen output reaches a common level, irrespective of the previous proteid condition of the body. Further, it is easy to see that the greater loss of nitrogen, i. e., the large breaking down of proteid during the first few days of fasting, in those cases where proteid food has been freely taken, suggests the existence in the tissues of two forms of proteid. We may term them, following the nomenclature of Voit, as circulating and morphotic, or tissue, proteid; or, we may designate them as labile and stable forms of proteid. In other words, following the usually accepted view, this circulating or labile proteid represents reserve or surplus material which is easily decomposed and hence rapidly gotten rid of, while the stable proteid is more slowly oxidized, and its metabolism may be taken as representing more nearly the real necessities of the body. However this may be, it is plainly manifest that the nitrogen output, meaning the metabolism of proteid matter, during hunger or fasting is modified by a variety of circumstances, notably the previous nutritive condition of the body as regards both fat and proteid. It is hardly necessary to add that the amount of muscular work performed is another factor of importance in this connection. Fat in the body represents inert material stored up mainly for nutritive purposes; hence, in hunger it is used largely, and serves to protect more important tissues. Thus, experiments have shown that in long periods of fasting, adipose tissue may be consumed to the extent of 97 per cent of the total amount present, while the heart and nervous tissue will not lose over 3 per cent of their tissue substance. The influence of tissue fat upon the consumption of proteid during hunger can thus be fully appreciated.

The output of carbon during fasting may be illustrated by the following experiment24 made upon a young man, the nitrogen data being included for comparison, and likewise the intake of food, in terms of nitrogen and carbon, preceding the fast and for two days following the fast. The fasting was of five days’ duration.

On the non-fasting days, the intake consisted of an ordinary food mixture of proteids, fats, and carbohydrates, with a small addition of alcohol. The point to be emphasized here, however, is that the carbon-content was more than sufficient to meet the needs of the body. Thus, it will be observed that on all three of the days when food was taken, the income of carbon was far in excess of the output. In other words, on the day preceding the beginning of the fast the body stored up 135 grams of carbon, and on the day following the fast the body retained 169 grams of carbon to help make good the loss. Similarly, the amount of proteid food taken in on the day prior to the fast was considerably in excess of the needs of the body, 5.1 grams of nitrogen equivalent to 31.8 grams of proteid being stored for future use. Plainly, the man was not in either carbon or nitrogen balance prior to the fast, but was taking far more food than the needs of the body called for. This fact may be emphasized by noting that the total fuel value of the daily food, plus the fuel value of the alcohol, amounted on an average to about 4200 large calories, while the fuel value of the material metabolized on the feeding days averaged only 2500 calories. Looking at the figures showing the output of carbon, as well as of nitrogen, during the fasting days, it is to be seen that in the early days of fasting, the metabolism of the body tends to remain at a fairly constant level, especially when figured per kilogram of body-weight.

To fully appreciate what takes place in a man of the above body-weight fasting for five days (though living on a large excess of food prior to the fast), the daily losses of carbon and nitrogen may be translated into terms of fat and proteid. If it is assumed that the total carbon, aside from what necessarily belongs to the proteid indicated by the nitrogen figures, comes from the oxidation of fat, it is easy to compute the amounts of fat and proteid metabolized, or destroyed, each day of the fasting period. These are shown in the following table:

Finally, if from these figures we calculate the fuel value of the proteid and fat oxidized per day, it is possible to gain a fairly clear conception of the part played by these two classes of tissue material during fasting, in furnishing the heat of the body and the energy for muscular motion, etc.

These somewhat general statements, with the illustrations given, will serve in a brief way to emphasize some of the essential features of metabolism in the fasting individual; where there is no income of energy-containing material, and where the body must draw entirely upon its store of accumulated fat and proteid to keep the machinery in motion, maintain body temperature, and do the tasks of every-day life. When it is remembered that persons have fasted for periods of thirty days or longer without succumbing, it is evident that the body of the well-nourished man has a large reserve of nutritive material, which can be drawn upon in cases of emergency. At the same time, the facts presented show us that in the early days of fasting the actual amounts of tissue proteid and body fat consumed are not large. In Cetti’s case, on the sixth day of fasting the metabolized nitrogen amounted to 10 grams, which implies a loss of 62.5 grams of proteid. At this rate of loss, one pound of dry proteid matter in the form of tissue proteid would meet the wants of a man of 130 pounds body-weight for seven and a half days, provided of course there was a reasonable stock of fat to help satisfy the energy requirements. Finally, we may again emphasize the fact that the loss of nitrogen in the fasting man is by no means a measure of the minimal proteid requirement. By feeding fat, or carbohydrate, or both, the output of nitrogen can be materially diminished, although naturally we cannot establish a nitrogen balance by so doing, since the income is free from nitrogen; but we can postpone for a time the approach of nitrogen starvation.

We may next profitably consider the effect of a pure proteid diet—such as lean meat free from fat—on the output of nitrogen. In studying this problem, we at once meet with several important and surprising facts. First, we are led to see that, strange as it may seem, every addition of proteid to the diet results in an increased excretion of nitrogen. In other words, increase of proteid income is followed at once by an increase in the metabolism of proteid, with a corresponding outgo of nitrogen. The hungry or fasting man with his income entirely cut off, and consequently suffering from a heavy drain upon his capital stock, would be expected, when suddenly supplied with fresh capital in the form of meat or other kind of proteid food, to hold on firmly to this all-important foodstuff; but such is not the case. It is impossible, for example, to establish nitrogen equilibrium by an income of proteid equal to what the individual during fasting is found to metabolize. As stated by another, “It is one of the cardinal laws of proteid metabolism that the store of nitrogenous substances in the body is not increased by, or not in proportion to, an increase in the nitrogen intake.” The principle is well illustrated in the fasting experiment just described. On the fifth day of fasting, the nitrogen output amounted to 11.4 grams. On the day following, the man took 35.6 grams of nitrogen in the form of proteid, while the excretion of nitrogen for that day rose to 26.8 grams. In other words, although deprived of all proteid income for five days, and during that period drawing entirely upon his proteid capital, the man was wholly unable to avail himself of the proteid so abundantly supplied at the close of the fast and make good the losses of the preceding days; only a small proportion of the proteid income could be retained. If a dog fed on a definite quantity of meat suddenly has his proteid income increased, there is at once an acceleration of proteid metabolism, and a corresponding increase in the output of nitrogen. Addition of still more proteid to his income is followed by an accumulation of a portion of the proteid; but this tends to decrease gradually, while there is a corresponding daily increase in the excretion of nitrogen. In this manner, there finally results a condition of nitrogenous equilibrium or nitrogen balance.

Again, an animal brought into nitrogen equilibrium by excessive proteid feeding, if suddenly given a small amount of meat per day, tends to put out nitrogen from its own tissues. This tissue loss, however, decreases slowly, and eventually the animal is quite likely to re-establish nitrogen equilibrium at a lower level. There is, in other words, a strong tendency for the body to pass into a condition of nitrogen balance under different conditions of proteid feeding, even after a long period of nitrogen loss and with an abundance of proteid in the intake. The starving body, as we have seen, cannot make use of all the nitrogen fed, although we can well conceive its great need for all the proteid available. A certain amount of the proteid fed, or its contained nitrogen, is at once passed out of the body and lost, even though the organism be gasping, as it were, for proteid to make good the drain incidental to long fasting. A recent writer26 has suggested that some explanation for these anomalies may be found in the supposition “that a long succession of generations in the past, which have lived from choice or necessity on a diet rich in proteids, have handed down to us, as an inheritance, a constitution in which arrangements exist for the removal of nitrogen from a considerable part of this proteid. The fact that the amount of proteid taken is re-adjusted to suit the actual needs of the body, though it makes these arrangements unnecessary, will not necessarily remove them. The denitrifying enzyme, which has been trained to keep guard over the entrances by which nitrogenous substances are admitted into the body, will continue to levy its toll of nitrogen, even when the amount of proteid presented to it is no more than the tissues which it serves actually require.”

As an illustration of how the body behaves with a low nitrogen intake followed by a sudden increase in the income of proteid, some data from an experiment performed by Sivén27 on himself may be cited:

I have ventured to give these data in some detail, because of their exceeding great interest in several directions aside from the point under discussion. Confining our attention to the nitrogen exchange, it is to be observed that for a period of two weeks Sivén lived on less than 3 grams of nitrogen per day, and without any excessive intake of carbohydrate or fat. During this time, the body naturally was in a condition of minus balance as regards nitrogen, the output being considerably larger than the income. The total amount of nitrogen lost in the period, 31 grams, corresponds to a breaking down of 193 grams of tissue proteid, or over one-third of a pound. On increasing the income of nitrogen to 4 grams per day, the nitrogen loss still continued, though at a much lower rate; indeed, the body is seen to approach very closely to a condition of nitrogen equilibrium. Still further increase of the nitrogen income to 13 grams per day was followed at once by a slight accumulation of proteid, and the body showed a decided plus balance of nitrogen, as on November 27. This, however, is seen to decrease gradually with a corresponding daily increase in the outgo of nitrogen, until on December 2 the body was once more practically in nitrogenous equilibrium. On again increasing the nitrogen income, to 23 grams per day, the same process was repeated, although in this case the body more quickly approached a condition of nitrogen balance.

We see in these data striking confirmation of the statement that the nitrogen outgo tends to keep pace with the income of nitrogen, the body always striving to maintain a condition of nitrogen equilibrium. Consequently, the fasting man having lost largely of his store of proteid can replace the latter only slowly, even though he eats abundantly of proteid food. Thus, Sivén in the week ending December 2, though taking over 13 grams of nitrogen a day, retained in his body only 14.5 grams of nitrogen during the entire seven days; while in the six days following, with a daily intake of 23 grams of nitrogen, he gained only about 8 grams additional. The human body does not readily store up proteid, and this is true no matter how greatly the tissues are in need of replenishment.

If the daily income is reinforced by the addition of carbohydrate or fat, there is observed a decided influence on the outgo of nitrogen; the rate or extent of proteid metabolism is at once modified, fat and carbohydrate both having a direct saving effect on proteid. Neither fat nor carbohydrate can prevent the katabolism of proteid, but they can and do decrease it, and thus serve as proteid-sparers. In the fasting body, or where there is only an intake of proteid, the latter material, except for the fat contained in the tissues, must serve the double purpose of meeting the specific nitrogen requirements of the body and furnishing the requisite energy. The energy requirements, however, can be met more advantageously by either of the non-nitrogenous foodstuffs, and just so far as they are oxidized, so far will there be a saving of proteid. Herein lies the philosophy of a mixed diet, with its natural intermingling of proteid, fat, and carbohydrate. For the same reason, the body of a man rich in fat will in fasting lose far less proteid per day than the lean man; or, if fed with a given amount of proteid food, the fat man may attain nitrogen equilibrium, or even store up a little proteid, while on the same diet the lean man will lose proteid. Further, if a man is in nitrogen balance with a given amount of proteid food, the addition of fat or carbohydrate to the diet will permit of a reduction in the amount of proteid necessary to maintain nitrogenous equilibrium. Fat, however, when added to food, does not always protect proteid to the extent possibly suggested by the preceding statements. The following data from oft-quoted experiments by Voit28 on dogs will serve to illustrate:

It is to be observed that in both of these experiments the fairly large addition of fat results in a saving of proteid, but the sparing effect in the first experiment amounts to only 38 grams of proteid for the 150 grams of fat added. In the second experiment, however, there is a saving of 36 grams of proteid, although only 100 grams of fat were fed. The radical point of difference in the two experiments is the amount of proteid ingested. Proteid food stimulates proteid metabolism; it likewise accelerates the metabolism of non-nitrogenous matter, consequently the sparing or protecting effect of fat on proteid is most conspicuous when the intake of proteid is relatively small. Only under such conditions, does fat protect in large degree the consumption of proteid in the body. In the ordinary, daily, dietary of man, with its great variety of food materials and with its proteid-content not exceeding 125 grams, fat is apt to be a conspicuous element, and under such conditions its sparing effect on proteid metabolism is most marked. Further, it must not be forgotten, as Voit originally pointed out, that the adipose tissue of the body acts like the food-fat, and consequently the proteid-sparing effect of the former may be added to that of the latter.

The addition of carbohydrate to a meat diet produces at once a saving in the decomposition of proteid, as shown in the following figures, covering an experiment of two days: