CHAPTER III
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 figures[22] show the daily excretion of nitrogen in several notable cases:
| Day of Fasting. | Breithaupt. 59.9 Kilos. | Cetti. 56.5 Kilos. | Succi. 62.4 Kilos. |
|---|---|---|---|
| grams | grams | grams | |
| 0 | 13.0 | 13.5 | 16.2 |
| 1 | 10.0 | 13.6 | 13.8 |
| 2 | 9.9 | 12.6 | 11.0 |
| 3 | 13.3 | 13.1 | 13.9 |
| 4 | 12.8 | 12.4 | 12.8 |
| 5 | 11.0 | 10.7 | 12.8 |
| 6 | 9.9 | 10.1 | 10.1 |
| 7 | . . . | 10.9 | 9.4 |
| 8 | . . . | 8.9 | 8.4 |
| 9 | . . . | 10.8 | 7.8 |
| 10 | . . . | 9.5 | 6.7 |
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 figures[23] 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:
| First Series. | Second Series. | Third Series. | |
|---|---|---|---|
| grams | grams | grams | |
| First fasting day | 175 | 77 | 40 |
| Second " " | 72 | 54 | 33 |
| Third " " | 56 | 46 | 30 |
| Fourth " " | 50 | 53 | 36 |
| Fifth " " | 36 | 43 | 35 |
| Sixth " " | 39 | 37 | 37 |
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 experiment[24] 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.
Day. | Body-weight. | Intake. | Output. | ||
|---|---|---|---|---|---|
Carbon. | Nitrogen. | Carbon.[25] | Nitrogen. | ||
kilos | grams | grams | grams | grams | |
2 | 67.4 | 438.7 | 30.96 | 303.4 | 25.81 |
3 | 66.9 | 0 | 0 | 197.6 | 12.17 |
4 | 65.7 | 0 | 0 | 188.8 | 12.85 |
5 | 64.8 | 0 | 0 | 183.2 | 13.61 |
6 | 63.9 | 0 | 0 | 180.8 | 13.69 |
7 | 63.1 | 0 | 0 | 176.2 | 11.47 |
8 | 63.9 | 439.9 | 35.65 | 270.5 | 26.83 |
9 | 65.5 | 391.7 | 23.68 | 258.8 | 19.46 |
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:
| Day. | Proteid metabolized. | Fat metabolized. |
|---|---|---|
| grams | grams | |
| 3 | 76.1 | 206.1 |
| 4 | 80.3 | 191.6 |
| 5 | 85.1 | 181.2 |
| 6 | 85.6 | 177.6 |
| 7 | 71.7 | 181.2 |
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.
| Day. | Fuel Value of the Proteid metabolized. | Fuel Value of the Fat metabolized. | Total Fuel Value. |
|---|---|---|---|
| calories | calories | calories | |
| 3 | 303 | 1916 | 2220 |
| 4 | 320 | 1781 | 2102 |
| 5 | 339 | 1684 | 2024 |
| 6 | 341 | 1651 | 1992 |
| 7 | 286 | 1684 | 1970 |
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 writer[26] 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 Sivén[27] on himself may be cited:
| Date. | Body-weight. | Nitrogen of the Food. | Nitrogen excreted. | Nitrogen Balance. |
|---|---|---|---|---|
| kilos | grams | grams | grams | |
| Nov. 6 | 65.4 | 2.69 | 8.31 | –5.62 |
| 7 | 65.4 | 2.69 | 5.37 | –2.68 |
| 8 | 65.1 | 2.69 | 5.71 | –3.02 |
| 9 | 65.3 | 2.69 | 4.88 | –2.19 |
| 10 | 65.0 | 2.69 | 4.32 | –1.63 |
| 11 | 64.9 | 2.69 | 4.25 | –1.56 |
| 12 | 64.9 | 2.69 | 4.47 | –1.78 |
| 13 | 64.6 | 2.96 | 4.88 | –1.92 |
| 14 | 64.4 | 2.96 | 4.30 | –1.44 |
| 15 | 64.3 | 2.96 | 4.75 | –1.79 |
| 16 | 64.4 | 2.96 | 4.36 | –1.40 |
| 17 | 64.4 | 2.96 | 4.13 | –1.17 |
| 18 | 64.4 | 2.96 | 4.35 | –1.39 |
| 19 | 64.4 | 2.96 | 4.32 | –1.36 |
| 20 | 64.4 | 2.96 | 4.22 | –1.26 |
| 21 | 64.0 | 2.96 | 4.06 | –1.10 |
| –31.31 | ||||
| 22 | 64.1 | 4.02 | 4.22 | –0.20 |
| 23 | 64.4 | 4.02 | 4.35 | –0.33 |
| 24 | 64.4 | 4.02 | 4.21 | –0.19 |
| 25 | 64.4 | 4.02 | 4.40 | –0.38 |
| –1.10 | ||||
| 26 | 64.2 | 8.24 | 6.56 | +1.68 |
| 27 | 64.4 | 13.45 | 8.67 | +4.78 |
| 28 | 64.4 | 13.66 | 10.54 | +3.12 |
| 29 | 64.0 | 13.45 | 11.10 | +2.35 |
| 30 | 64.2 | 13.24 | 12.83 | +0.41 |
| Dec. 1 | 64.2 | 13.24 | 11.70 | +1.54 |
| 2 | 63.9 | 12.61 | 12.00 | +0.61 |
| +14.49 | ||||
| 3 | 64.0 | 22.93 | 16.24 | +6.69 |
| 4 | 63.9 | 22.41 | 21.47 | +0.94 |
| 5 | 63.9 | 22.41 | 23.10 | –0.69 |
| 6 | 63.6 | 23.35 | 23.12 | +0.23 |
| 7 | 63.9 | 23.04 | 22.82 | +0.22 |
| 8 | 63.8 | 22.62 | 22.86 | –0.24 |
| +6.15 |
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 Voit[28] on dogs will serve to illustrate:
Food. | Flesh. | ||
|---|---|---|---|
Meat. | Fat. | Metabolized. | On the Body. |
grams | grams | grams | grams |
1500 | 0 | 1512 | –12 |
1500 | 150 | 1474 | +26 |
| |||
500 | 0 | 556 | –56 |
500 | 100 | 520 | –20 |
Food. | Flesh. | ||
|---|---|---|---|
Meat. | Fat. | Metabolized. | On the Body. |
grams | grams | grams | grams |
1500 | 0 | 1512 | –12 |
1500 | 150 | 1474 | +26 |
| |||
500 | 0 | 556 | –56 |
500 | 100 | 520 | –20 |
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:
| Meat. | Sugar. | Proteid metabolized. |
| 500 grams. | 200 grams. | 502 grams. |
| 500 | 0 | 564 |
Without the sugar, there was a minus balance of 64 grams of proteid, but addition of the carbohydrate caused practically a saving of all of this, with establishment of essentially a nitrogen balance. The sparing of proteid by carbohydrate is greater than by fats, a fact of considerable dietetic importance which is well illustrated by the following experiments (on dogs) taken from Voit:
Food. | Flesh. | ||
|---|---|---|---|
Meat. | Non-nitrogenous Food. | Metabolized. | Balance of the Body. |
grams | grams | grams | grams |
500 | 250 Fat | 558 | –58 |
500 | 300 Sugar | 466 | +34 |
500 | 200 Sugar | 505 | –5 |
800 | 250 Starch | 745 | +55 |
800 | 200 Fat | 773 | +27 |
2000 | 200–300 Starch | 1792 | +208 |
2000 | 250 Fat | 1883 | +117 |
Food. | Flesh. | ||
|---|---|---|---|
Meat. | Non-nitrogenous Food. | Metabolized. | Balance of the Body. |
grams | grams | grams | grams |
500 | 250 Fat | 558 | –58 |
500 | 300 Sugar | 466 | +34 |
500 | 200 Sugar | 505 | –5 |
800 | 250 Starch | 745 | +55 |
800 | 200 Fat | 773 | +27 |
2000 | 200–300 Starch | 1792 | +208 |
2000 | 250 Fat | 1883 | +117 |
In considering the results of this experiment, it must be remembered that the calorific or fuel value of fat as compared with carbohydrate is as 9.3 : 4.1; in other words, fats have a fuel value of more than twice that of carbohydrates. In spite of this fact, it is clearly evident that carbohydrates as a class—for the different sugars and starches act alike in this respect—are far more efficient than fats in saving proteid. Thus, with an income of 500 grams of meat and 250 grams of fat, the body of the animal lost 58 grams of proteid, while with a like amount of meat and 300 grams of sugar the body not only saved the 58 grams, but in addition stored 34 grams of proteid, showing a plus balance to that extent. The sparing of proteid by carbohydrate amounts on an average, according to Voit, to 9 per cent—in the highest cases to 15 per cent—of the proteid given, while the saving produced by fat averages only 7 per cent. Further, increasing quantities of carbohydrates in the food diminish the rate of proteid metabolism much more regularly and constantly than increasing quantities of fat. We may attribute this difference in action, in a measure at least, to the greater ease in oxidation and utilization of the carbohydrate. In any event, starches and sugars are most valuable adjuncts to the daily diet, because of this marked proteid-saving power, while their fuel value adds just so much to the total energy intake.
A more striking illustration of the action of carbohydrate in sparing proteid is seen in experiments on man, where the nitrogen intake is reduced to a minimum, so as to constitute a condition of specific nitrogen-hunger. In such a case, increasing amounts of carbohydrate added to the intake reduce enormously the using up of tissue proteid. The following experiment with a young man 22 years old and 71.3 kilos body-weight, reported by Landergren,[29] affords good evidence of the extent to which this proteid sparing power may manifest itself.
We see here the nitrogen consumption fall to the exceedingly low level of 3.34 grams per day, or 0.047 gram per kilo of body-weight. To appreciate the full significance of this drop in the extent of proteid metabolism, we may recall that Succi, with a body-weight of only 62.4 kilos, on the seventh day of fasting excreted 9.4 grams of nitrogen, corresponding to a metabolism of 58.7 grams of tissue proteid. In other words, with an intake of only 5.6 grams of proteid, the addition of 908 grams of carbohydrate, with a total fuel value of 3745 calories, reduced the consumption of tissue proteid to 20.8 grams. The same individual, if fasting, would undoubtedly have used up at least 70 grams of tissue proteid.
Day. | Intake. | Output. | Proteid | ||||
|---|---|---|---|---|---|---|---|
Proteid. | Fat. | Carbo- | Alcohol. | Calories. | Nitrogen | ||
grams | grams | grams | grams | grams | grams | ||
1 | 35.2 | 6.1 | 507 | 26.6 | 2465.9 | 12.16 | 76.0 |
2 | 28.7 | 4.7 | 787 | 26.6 | 3574.3 | 8.37 | 52.3 |
3 | 28.8 | 4.7 | 841 | 26.6 | 3796.1 | 5.02 | 31.3 |
4 | 28.3 | 4.9 | 839 | 13.3 | 3690.5 | 4.50 | 28.1 |
5 | 5.4 | . . | 898 | . . . . | 3703.9 | 4.01 | 25.0 |
6 | 6.0 | . . | 931 | . . . . | 3841.7 | 3.36 | 21.0 |
7 | 5.6 | . . | 908 | . . . . | 3745.8 | 3.34 | 20.8 |
It is evident from what has been said that both of these non-nitrogenous foods, fat and carbohydrate, play a very important part in nutrition, because of their ability to maintain in a measure the integrity of tissue proteid. When we recall that a diet of pure proteid, such as meat or eggs, must be excessive in quantity in order to meet the energy requirements of the body, and that the stimulating action of proteid food serves to whip up body metabolism, we can appreciate at full measure the great physiological economy which results from the addition of carbohydrate and fat to the daily diet. The establishment of nitrogenous equilibrium is made possible at a much lower level by the judicious addition of these two non-nitrogenous foodstuffs. The same principle may be illustrated in another way, viz., by noting the effect on tissue proteid of withdrawal of a portion of the fat or carbohydrate of the intake, in the case of a person nearly or quite in nitrogen balance. The following experiment[30] affords a good example of what will occur under such treatment:
Income. | Output of | Balance of | ||||
|---|---|---|---|---|---|---|
Nitrogen. | Fat. | Carbo- | Calories. | |||
grams | grams | grams | grams | |||
Av. of 3 days | 15.782 | 40.47 | 289.6 | 1955 | 14.927 | +0.862 |
Nov. 30 | 15.782 | 40.34 | 177.3 | 1493 | 14.959 | +0.830 |
Dec. 1 | 15.782 | 40.34 | 177.3 | 1493 | 17.546 | –1.757 |
2 | 15.782 | 40.34 | 177.3 | 1493 | 18.452 | –2.663 |
Average of the last two days . . . . . . . . . -2.210 | ||||||
Starting with the body in a condition of plus nitrogen balance, i. e., with a mixed diet more than sufficient to maintain the tissue proteid intact, the reduction of the fuel value of the food from 1955 to 1493 calories by cutting off 112 grams of carbohydrate per day was followed by a gradual, but marked, increase in the output of nitrogen; indicating thereby the extent to which the body proteid was then drawn upon to make good the loss of energy-containing income. The body showed at the close of the experiment a minus nitrogen balance averaging 2.2 grams per day, or a loss of 13.8 grams of tissue proteid, which would obviously have continued, under the above conditions, until the body was exhausted. In other words, the 112 grams of carbohydrate, if added to the diet on December 3 and the following days, would have quickly saved the daily loss of 2.4 grams of nitrogen, and thus changed the drain of tissue proteid to an actual gain, with consequent establishment of a growing plus balance.
It is obvious from what has been stated, that in man the body can accomplish a storing of proteid only when the intake is reinforced by substantial additions of fat or carbohydrate. It is plainly a matter of great physiological importance that the body should be able to increase at times its reserve of proteid. This, however, cannot apparently be accomplished on a large scale under ordinary conditions. Any storing up of nutritive material in excess, whether it be proteid or fat, necessarily involves overfeeding, i. e., the taking of an amount of food beyond the capacity of the body to metabolize at the time. Fat, as we know, may be stored in large quantities, and it is in cases of overfeeding with non-nitrogenous foods that we find accumulation of fat most marked. Overfeeding with proteid, however, does not lead to corresponding results, owing primarily to the peculiar physiological properties of proteid; its general stimulating effect on metabolism, the tendency of the body to establish nitrogenous equilibrium at different levels, and the fact emphasized by von Noorden that flesh deposition is primarily a function of the specific energy of developing cells. In other words, the protoplasmic cells of the body are more important factors in the storing or holding on to proteid than an excess of proteid-containing food.
It is generally considered as a settled fact, that in man it is impossible to accomplish any large permanent storing or deposition of flesh by overfeeding. Similarly, it is understood that the muscular strength of man cannot be greatly increased by an excessive intake of food. The only conditions under which there is ordinarily any marked and permanent flesh deposition are such as are connected with the regenerative energy of living cells. Thus, as von Noorden has stated, an accumulation or storing of tissue proteid is seen especially in the growing body, where new cells are being rapidly constructed; also in the adult where growth may have ceased, but where increased muscular work has resulted in an hypertrophy or enlargement of the muscular tissue; and lastly in those cases where, owing to previous insufficient food or to the wasting away of the body incidental to disease, the proteid content of the tissues has been more or less diminished, and consequently an abundance of proteid food is called for and duly utilized to make good the loss. In some oft-quoted experiments by Krug, conducted on himself, it was observed that with an abundant food intake, sufficient to furnish 2590 calories per day (44 calories per kilo of body-weight), a condition approaching nitrogenous equilibrium was easily maintained. On then increasing the fuel value of the food to 4300 calories (71 calories per kilo of body-weight) by addition of fat and carbohydrate, there was during a period of fifteen days a sparing of 49.5 grams of nitrogen or 309 grams of proteid, which would correspond to about 1450 grams, or three pounds, of fresh muscle. It is to be noted, however, that of this excess of calories added to the intake only 5 per cent was made use of for flesh deposit, the remaining 95 per cent going to make fat.
Again, we may call attention to the well-known fact that in feeding animals for food, while fat may be laid on in large amounts, flesh cannot be so increased by overfeeding. In this matter, however, race and individuality count for considerable. Thus, there is on record a more recent series of experiments conducted by Dapper[31] on himself which shows some remarkable results. Starting with a daily diet not excessive in amount, he was able by an addition of only 80 grams of starch to accomplish a laying up of 3.32 grams of nitrogen per day for a period of twelve days, or a total gain of 39.8 grams of nitrogen, equal to 248 grams of proteid. It may be said that the gain of proteid or flesh here for the twelve days was no greater than in the preceding case (fifteen days), but the difference lies in the fact that Krug accomplished his gain by increasing the daily intake from 2590 to 4300 calories, an amount which he found too large to be eaten with comfort, while the later investigator raised the fuel value of his daily food from 2930 to only 3250 calories. As the experiments by Dapper contain other points of interest bearing on the question before us, we may advantageously consider them somewhat in detail. The following table gives the more important results:
No. | Duration. | Character of Food. | Food Composition. | Nitrogen | Maxima and Minima | |
|---|---|---|---|---|---|---|
| Nitrogen. | Calories. | |||||
days | grams | grams | grams | |||
1 | 6 | Ordinary mixed diet | 20.25 | 2930 | +2.18 | +3.2 on 4th day. +1.5 on 6th day. |
2 | 12 | Ditto + 80 grams starch | 20.09 | 3250 | +3.32 | +4.75 on 2d day. +4.65 on 12th day. +2.30 on 8th day. |
3 | 9 | Ditto + 80 grams starch, + 40 grams plasmon | 24.58 | 3400 | +2.55 | +5.98 on 1st day. +4.73 on 2d day. +0.50 on 6th day. +1.60 on 9th day. |
As we look at these results, the nitrogen gain for the first and second days of the third experiment and the first day of the second experiment may well attract our attention, since they show an astonishing laying by of proteid, or gain of flesh, under the influence of a comparatively small increase in the fuel value of the food. A gain of 5.98 grams of nitrogen means 37.3 grams of proteid, or more than an ounce; by no means an inconsiderable addition for one day to the store of tissue proteid. In the third experiment, where plasmon (dried, milk proteid) was added to the diet, there is to be noted a gradual falling off in the proteid-sparing power, which may perhaps be interpreted as implying that the body was practically saturated with proteid, and that owing to this fact the body was unable to continue its laying hold of nitrogen. In the entire period of 21 days, however, the body had succeeded in accumulating a store of 62.8 grams of nitrogen, or 392 grams of proteid, and this without adding very largely to the intake of non-nitrogenous matter. This experiment affords a striking illustration of the ability of the body to “fatten on nitrogen,” but it is very doubtful if such results can generally be obtained. Lüthje,[32] however, has reported a large retention of nitrogen on a diet containing 50 grams of nitrogen daily, with a fuel value of 4000 calories. It is more than probable that there existed in these particular cases some personal peculiarity or idiosyncrasy which favored the proteid-sparing power. The personal coefficient of nutrition is not to be ignored; it shows itself in many ways, and the above results are to be counted among those that are exceptional and not the rule. In the words of Magnus-Levy, “a given diet with Cassius may lead to different results than with Anthony.”
For the study of many questions in nutrition, it becomes necessary to determine accurately the transformations of energy within the body as contrasted with the transformation of matter; the total income and outgo of energy, measured in terms of heat, are to be compared one with the other and a balance struck. Further, in studying the metabolism of carbohydrate and fat it is necessary to determine the output of gaseous products through the lungs and skin; to estimate the excretion of carbon dioxide and water, and the intake of oxygen. For these purposes, a special form of apparatus known as a respiration calorimeter is employed. The double name is indicative of the twofold character of the apparatus, viz., a suitably constructed chamber so arranged as to permit of measuring at the same time the respiratory products and the energy given off from the body. The form of apparatus best known to-day, and with which exceedingly satisfactory work has been done, is the Atwater-Rosa apparatus, as modified by Benedict. It consists essentially of a respiration chamber, in reality an air-tight, constant-temperature room (with walls of sheet metal, outside of which are two concentric coverings of wood completely surrounding it, with generous air spaces between), sufficiently large to admit of a man living in it for a week or more at a time. Connected with the chamber is a great variety of complex apparatus for maintaining and analyzing the supply of oxygen, determining the amount of carbon dioxide and of water, etc., etc. As an apparatus for measuring heat, the chamber may be described as “a constant-temperature, continuous-flow water calorimeter, so devised and manipulated that gain or loss of heat through the walls of the chamber is prevented, and the heat generated within the chamber cannot escape in any other way than that provided for carrying it away and measuring it.”[33]
In illustration of the efficiency of an apparatus of this description, and of the close agreement obtainable by direct calorimetric measurement with the estimated energy, as figured from the materials oxidized in the body, we may quote the following data from Dr. Benedict’s report, referred to in the footnote. The subject was a young man who had been fasting for five days. The experiment deals with the metabolism on the first day after the fast, when a diet composed mainly of milk was made use of, containing 53.31 grams of proteid, 211.87 grams of fat, and 75.41 grams of carbohydrate. The following table shows the results of the experiment:
Heat of Combustion of | (d) | (e) | (f) | Heat | Heat Measured | |||
|---|---|---|---|---|---|---|---|---|
(a) | (b) | (c) | Amount. | Propor- | ||||
calories | calories | calories | calories | calories | calories | calories | calories | per cent |
2569 | 149 | 103 | 2317 | +229 | 2088 | 2113 | +25 | +1.2 |
As is seen from the above figures, the total fuel value of the food was 2569 calories. The fuel value of the unoxidized portion of the food contained in the excreta was 149 + 103 calories, leaving as the available energy of the food 2317 calories. This must be further corrected by the fact, mentioned in the footnote, that a portion of the food was stored as fat and glycogen, while the body lost at the same time a small amount of proteid. Making the necessary correction for these causes, we find 2088 calories as the energy from material oxidized in the body. The actual output of energy as measured by the calorimeter was 2113 calories, only 1.2 per cent greater than the estimated amount.
By aid of the respiration calorimeter, many important questions in nutrition can be more or less accurately answered, especially such as relate to the total energy requirements of the body. The law of the conservation of energy obtains in the human body as elsewhere, and if we can measure with accuracy the total heat output, with any energy liberated in the form of work, and at the same time determine the total excretion of carbon dioxide, water, nitrogen, etc., together with the intake of oxygen, it becomes not only possible to ascertain the energy requirements of the body under different conditions, but, aided by data obtainable through study of the exchange of matter, we can draw important conclusions concerning the sources of the energy, i. e., whether from proteid, fat, or carbohydrate.
It is obvious that a man asleep, or lying quietly at rest, in the calorimeter, especially when he has been without food for some hours, furnishes suitable conditions for ascertaining the minimal energy requirements of the body. Under such conditions, bodily activity and heat output are at their lowest, and we are thus afforded the means of determining what is frequently called the basal energy exchange of the body. The following table taken from Magnus-Levy, and embodying results from many sources, shows the heat production during sleep, calculated for 24 hours, of various individuals of different body-weight and of different body surface.
I venture to present these individual results, rather than make a general statement simply, because it is important to recognize the fact that the basal energy exchange differs according to body-weight, extent of body surface, and the condition of the body. In the table, the results are arranged in the order of body-weight, and it is plain to see that the absolute energy exchange is greater with heavy persons than with light, yet the energy exchange does not increase in proportion to increase of body-weight. With a man of 83 kilos body-weight, the basal exchange is only 30–40 per cent higher than in a man of 43 kilos body-weight. In other words, the man of small body-weight has, per kilo, a much higher basal exchange than the heavier man. The energy exchange is more closely proportional to the extent of body surface than to weight.
| Body-weight of the Individual. | Total Calories for 24 Hours. | Calories per Kilo of Body-weight. | Body-weight of the Individual. | Total Calories for 24 Hours. | Calories per Kilo of Body-weight. |
|---|---|---|---|---|---|
| kilos | kilos | ||||
| 43.2 | 1333 | 30.9 | 67.5 | 1608 | 23.8 |
| 48.0 | 1214 | 25.3 | 67.5 | 1621 | 24.0 |
| 50.0 | 1315 | 25.9 | 70.0 | 1661 | 23.7 |
| 53.0 | 1527 | 28.8 | 70.0 | 1620 | 23.1 |
| 55.0 | 1590 | 28.9 | 71.2 | 1787 | 25.1 |
| 56.5 | 1519 | 26.8 | 72.6 | 1550 | 21.3 |
| 57.2 | 1560 | 27.3 | 72.7 | 1657 | 22.8 |
| 58.0 | 1510 | 26.0 | 73.0 | 1584 | 21.7 |
| 62.5 | 1431 | 22.9 | 73.0 | 1630 | 22.4 |
| 63.0 | 1418 | 22.5 | 75.6 | 1670 | 22.1 |
| 63.0 | 1492 | 23.7 | 82.0 | 1556 | 19.0 |
| 64.0 | 1656? | 25.8 | 82.7 | 2030? | 24.5 |
| 64.9 | 1475 | 22.7 | 83.5 | 1670 | 20.0 |
| 65.0 | 1498 | 23.0 | 88.3 | 2019? | 22.9 |
| 65.0 | 1445 | 22.2 | 90.4 | 1773 | 19.6 |
As Richet has expressed it, the basal energy exchange is inversely proportional to the body-weight and directly proportional to the body surface. This is in harmony with the view advanced by v. Hösslin, “that all the important physiological activities of the body, including of course its internal work and the consequent heat production, are substantially proportional to the two-thirds power of its volume, and that since the external surface bears the same ratio to the volume, a proportionality necessarily exists between heat production and surface.”[35]
There are, however, many circumstances that modify, or influence, energy exchange. Thus, the taking of food, with all the attendant processes of digestion, assimilation, etc., involves an expenditure of energy not inconsiderable. This has been experimentally demonstrated on man by several investigators. With fatty food, Magnus-Levy found that his subject lying upon a couch, as completely at rest as possible, produced in the 24 hours 1547 calories when 94 grams of fat were eaten, and 1582 calories when 195 grams of fat were consumed. The increase of heat production over the basal energy exchange was 10 and 58 calories respectively. With a mixed diet, where proteid food is a conspicuous element, the increase in heat production is much more marked. Thus, in some experiments reported from Sweden the following data were obtained:[36]
| Day. | Energy of the Food. | Heat Production. |
|---|---|---|
| calories | calories | |
| First | 4141 | . . . . |
| Second | 4277 | 2705 |
| Third | 0 | 2220 |
| Fourth | 0 | 2102 |
| Fifth | 0 | 2024 |
| Sixth | 0 | 1992 |
| Seventh | 0 | 1970 |
| Eighth | 4355 | 2436 |
| Ninth | 3946 | 2410 |
We see here an increase of 495 calories per day in heat production, due to metabolism of the food ingested. In other words, with a basal energy exchange of 2022 calories, the average of the five fasting days, energy equivalent to 495 calories was expended in taking care of the ingested food. It should be added, however, that the daily ration here was somewhat excessive, 4193 calories being considerably in excess of the requirements of the body. Finally, it should be stated that of the several classes of foods, proteids cause the greatest increase in metabolism and fats the least.
In studying heat production in the body under varying conditions, one of the important aids in drawing conclusions as to the character of the body material burned up is the respiratory quotient. This is the relationship, or ratio, of the oxygen absorbed to the oxygen of the carbon dioxide eliminated, viz., CO2/O2. Carbohydrates (C6H12O6, C12H22O11) all contain hydrogen and oxygen in the proportion to form water, H2O, and in their oxidation they need of oxygen only such quantity as will suffice to oxidize the carbon (C) of the sugar to carbon dioxide (CO2). Carbohydrates, starch and sugars, have a respiratory quotient of 1.00. Fat, on the other hand, has a respiratory quotient of 0.7, and proteid, 0.8. Hence, it is easy to see that the respiratory quotient will approach nearer to unity as the quantity of carbohydrate burned in the body is increased. Similarly, the respiratory quotient will grow smaller the larger the amount of fat burned up. Practically, we never find a respiratory quotient of 1.0 or 0.7, because there is always some oxidation of proteid in the body. If, by way of illustration, we assume that the energy of the body under given conditions comes from proteid to the extent of 15 per cent, while the remaining 85 per cent is derived from the oxidation of carbohydrate, the respiratory quotient will be 0.971. If, however, the 85 per cent of energy comes from fat, the respiratory quotient will change to 0.722. In the resting body, as in the early morning hours, after a night’s sleep and before food is taken, the respiratory quotient is generally in the neighborhood of 0.8. When, however, as sometimes happens, the quotient at this time of day approaches 0.9, it must be assumed that sugar is being burned in the body, presumably from carbohydrate still circulating from the previous day’s intake.
As can easily be seen, any special drain upon either fat or carbohydrate in the processes of the body will be indicated at once by a corresponding change in the respiratory quotient. This we shall have occasion to notice later on, in considering the source of the energy of muscle contraction. Further, the respiratory quotient will naturally change in harmony with transformations in the body which involve alterations in oxygen-content, without the oxygen of the inspired air being necessarily involved; as in the formation of a substance poor in oxygen, such as fat, from a substance rich in oxygen, such as carbohydrate. Moreover, the reversal of this reaction, as in the formation of sugar from proteid with a taking on of oxygen, will produce a corresponding effect upon the respiratory quotient. As Magnus-Levy has clearly pointed out, in the formation of fat from carbohydrate, carbon dioxide is produced in large amount without the oxygen of the inspired air being involved at all. In such a change, 100 grams of starch will yield about 42 grams of fat, while at the same time 45 grams of carbon dioxide will be produced. This might cause the respiratory quotient to rise as high as 1.38. Again, in the formation of sugar from proteid, the respiratory quotient may sink very decidedly, the changes involved being accompanied by a taking on of oxygen from the air, without, however, any corresponding increase of carbon dioxide in the expired air. Assuming a manufacture of 60 grams of dextrose from 100 grams of proteid, i. e., from the non-nitrogenous moiety of the proteid molecule, a respiratory quotient of 0.613 would be possible. Thus, a diabetic patient, living upon a carbohydrate-free diet, consuming only proteid and fat, may show a respiratory quotient of 0.613–0.707. These illustrations will suffice to show how chemical alterations taking place in the body, involving transformations of proteid, fat, and carbohydrate of the tissues and of the food, may produce alterations in the respiratory quotient without necessarily being directly connected with intake of oxygen or output of carbon dioxide through the lungs; and how, conversely, the respiratory quotient becomes a factor of great significance in throwing light upon the character of the nutritive changes taking place in the body.
Among the various conditions that influence the energy exchange of the body, muscle work stands out as the most conspicuous. It needs no argument to convince one that all forms of muscular activity involve liberation of the energy stored up in the tissues of the body; and consequently that all work accomplished means chemical decomposition, in which complex molecules are broken down into simple ones with liberation of the contained energy, the energy exchange being proportional to the amount of work done. As we have seen, the basal energy exchange of the normal individual is ascertained by studying his heat production while at rest—best during sleep—without food, when involuntary muscle activity and heat production are at their lowest. The maximum energy exchange is seen in the individual at hard muscular work. Heat production is then at its highest, as can be ascertained by direct calorimetric observation; or, by studying the output of excretory products, which measure the extent of the oxidative processes from which comes the energy for the accomplishment of the work. As an illustration of the general effect of muscular work on the energy exchange of the body, we may cite a summary of some results reported by Atwater and Benedict,[37] the figures given being average results, from several individuals, and covering different periods of time. Though not strictly comparable in all details, they are sufficiently so to illustrate the main principle.
HEAT GIVEN OFF BY BODY, INCLUDING FOR WORK EXPERIMENTS THE HEAT EQUIVALENT OF THE EXTERNAL MUSCULAR WORK.
Kind of Experiment. | Total | Rates per Hour. | Average | ||||
|---|---|---|---|---|---|---|---|
Day Periods. | Night Periods. | ||||||
7 A. M. to | 1 P. M. to | 7 P. M. to | 1 A. M. to | ||||
calories | calories | calories | calories | calories | calories | ||
Rest experiments | 2262 | 106.3 | 104.4 | 98.3 | 67.9 | 94.3 | |
Work experiments |
| 4225 | 231.7 | 235.6 | 118.1 | 78.4 | 166.6 |
Heat equivalent of |
| 451 | 58.5 | 56.8 | . . . | . . . | . . . |
Total | 4676 | 290.2 | 292.4 | 118.1 | 78.4 | 194.8 | |
The work done in these experiments was on a stationary bicycle in the calorimeter, and the heat equivalent was calculated from measurements made by an ergometer attached to the bicycle. We are not concerned here with details, but simply with the general question of the influence of muscular work upon the energy exchange of the body. We note that the work of the day periods, 7 A. M. to 7 P. M., resulted, in the several cases brought together under the average figures, in an increased heat production amounting to more than 100 per cent. Further, we observe that in the body, as in all machines, only a fraction of the energy liberated by the accelerated chemical decomposition, or oxidation, was manifested as mechanical work, the larger part by far being heat eliminated and lost. Thus, Zuntz has found that, in man, about 35 per cent of the extra energy of the food used in connection with external muscular work is available for that work. This, however, shows a noticeably higher degree of efficiency than is generally obtainable by the best steam or oil engines. Lastly, attention may be called to the fact that after the work of the day was finished at 7 P. M., the next period of six hours still showed an accelerated metabolism, as contrasted with what took place during absolute rest.
As bearing upon the exchange of matter in the body in connection with muscular work, and as showing the relationship which exists here between energy exchange and exchange of matter, we may quote a few data relating to the elimination of carbon dioxide; remembering that this substance represents particularly the final oxidation product in the body of carbonaceous materials, such as fat and carbohydrate. The following data, taken from Atwater and Benedict,[38] being results of experiments upon the subject “J. C. W.,” are of value as showing the variations in output of carbon dioxide that may be expected under the conditions described:
| Period. | Rest Experiments without Food. | Rest Experiments with Food. | Work Experiments with Diet. | Work Experiments with Fat Diet. | Extra Se- vere Work Experiment with Fat Diet. |
|---|---|---|---|---|---|
| grams | grams | grams | grams | grams | |
| 7 A. M. to 1 P. M. | 189.6 | 230.4 | 694.0 | 642.3 | 907.0 |
| 1 P. M. to 7 P. M. | 172.6 | 232.0 | 705.6 | 634.8 | 821.3 |
| 7 P. M. to 1 A. M. | 167.2 | 196.6 | 260.1 | 230.3 | 842.7 |
| 1 A. M. to 7 A. M. | 146.7 | 153.1 | 161.1 | 157.6 | 502.6 |
| Total for 24 hours | 676.1 | 812.1 | 1820.8 | 1665.0 | 3073.6 |
In considering these figures bearing on the output of carbon dioxide under the conditions specified, we note at once a correspondence with the total energy exchange, as indicated in the preceding table. As previously stated, we are at present dealing simply with generalities, and the important point to be observed here is that muscular work—7 A. M. to 7 P. M.—in the work experiments, increases enormously the output of carbon dioxide. We see clearly emphasized a connection between the total energy exchange of the body, as expressed in calories or heat units, and the oxidation of carbonaceous material, of which carbon dioxide is the natural oxidation product. We note that on the cessation of work—7 P. M. to 7 A. M.—the output of carbon dioxide tends to drop back to the level characteristic of the corresponding period in rest, with or without food. In the experiment with “extra severe muscular work,” the results are different simply because here the subject worked sixteen hours, necessitating a portion of the work being done at night-time. Finally, it should be mentioned that the differences in output of carbon dioxide in these experiments are somewhat greater than in many experiments of this type, although all show the same general characteristics. This may be explained, as stated by the authors from whom the data are taken, “by the fact that J. C. W. was a larger and heavier man than any of the others; that the differences in diet were wider, and that the amounts of external muscular work were larger in these experiments than in those with the other subjects.”
If we pass from experiments of this type, conducted in a calorimeter, to those cases where competitive trials of endurance are held by trained athletes, i. e., where external muscular activity is pushed to the extreme limit, we then see even more strikingly displayed the effect of work in increasing the energy exchange of the body. One of the best illustrations of this type of experiment is to be found in the observations made in connection with the six-day bicycle race held in New York City, at the Madison Square Garden, in December, 1898.[39] The observations in question were made upon three of the athletes, one of whom withdrew early in the fourth day, while the others continued until the close of the race—142 consecutive hours—winning the first and fourth places, respectively. The following table gives the computation of energy of the material metabolized, exclusive of body-fat lost:
| Subject. | Duration of Experiment. | Total Energy Metabolized. | Average per Day. |
|---|---|---|---|
| days | calories | calories | |
| Miller | 6 | 28917 | 4820 |
| Albert | 6 | 36441 | 6074 |
| Pilkington | 3 | 13301 | 4464 |
Miller, the winner of the race, who averaged a daily energy exchange of 4820 calories, rode 2007 miles during the week, and finished the race without physical or mental weakness resulting from the fatigue and strain. During the first five days, he rode about 21 hours a day and slept only 1 hour. Albert, who weighed a few pounds less than Miller, covered 1822 miles in 109 hours, with an average daily exchange of 6074 calories. We may add a table (on the following page) showing the balance of income and outgo of nitrogen in these three subjects, as being of general interest in this connection. The figures given are averages per day.
Subject. | Duration | Income in Food. | Nitrogen. | ||||||
|---|---|---|---|---|---|---|---|---|---|
Proteid. | Fat. | Carbo- | Fuel | In | In | In | Loss. | ||
days | grams | grams | grams | calories | grams | grams | grams | grams | |
| Miller | 6 | 169 | 181 | 585 | 4770 | 29.4 | 36.2 | 1.8 | 8.6 |
| Albert | 6 | 179 | 198 | 559 | 6095 | 29.1 | 33.7 | 2.5 | 7.1 |
| Pilkington | 3 | 211 | 178 | 509 | 4610 | 36.0 | 38.9 | 2.2 | 5.1 |
The special significance of these data, as bearing upon the topic under discussion, is that apparently all three of the subjects were drawing in a measure upon their body material. As stated by Atwater and Sherman, Pilkington lost per day 5.1 grams of nitrogen; that is to say, the total nitrogen excreted exceeded the total nitrogen of the food by 5.1 grams per day, corresponding to 33 grams of proteid, which must have been drawn from the supply in the body. If we assume that lean flesh contains 25 per cent of proteid, this would mean about 4 3/4 ounces per day. The other two subjects, Miller and Albert, lost from the body per day 8.6 grams and 7.1 grams respectively of nitrogen, which would imply a loss of about 54 grams and 44 grams of body proteid respectively, or 8 ounces and 6 1/4 ounces of lean flesh per day. It is evident, therefore, that none of the three subjects consumed sufficient food to avoid loss of body proteid, under the existing conditions of muscular activity. Indeed, it may be noted in Miller’s case that the average fuel value of the food per day was 4770 calories, while the average expenditure of energy per day was 4820 calories. We should naturally expect, however, that any small deficiency in fuel value would be made good by a call upon body fat. “Why the body should use its own substance under such circumstances is a question which at present cannot be satisfactorily answered. The fact that such was the case, each of the contestants who finished the race consuming during the period body protein equivalent to 2 or 3 pounds of lean flesh, and that no injury resulted therefrom, would seem to indicate that these men had stores of protein which could be metabolized to aid in meeting the demands put upon the body by the severe exertion, without robbing any of the working parts, and at the same time relieving the system of a part of the labor of digestion. Possibly, the ability to carry such a store of available protein is one of the factors which make for physical endurance.”[40] This possibility we shall have occasion to discuss in another connection. At present, the facts presented are to be accepted as accentuating the general law that the energy exchange of the body, everything else being equal, is increased proportionally to increase in the extent of external muscular activity. It may be noted that Albert, who did considerably less work than Miller, showed a much larger exchange of energy than the latter athlete. This, however, is to be connected with the fact that his fuel intake was 1300 calories larger per day than Miller’s; in other words, the conditions were not equal. This fact also calls to mind the observations of Schnyder,[41] who, studying the relationship between muscular activity and the production of carbon dioxide, maintained that the quantity of this excretory product formed depends less upon the amount of work accomplished than upon the intensity of the exertion; efficiency in muscular work varying greatly with the condition of the subject, and his familiarity with the particular task involved.
From what has been said, it is obvious that oxygen consumption, as well as output of carbon dioxide, must vary enormously with variations in the muscular activity of the body. The one important factor influencing the quantities of oxygen and carbon dioxide exchanged in the lungs, i. e., the extent of the respiratory interchange, is muscular activity; and since, as we have seen, carbonaceous material is the substance mainly oxidized in muscle work, it follows, as carbon dioxide is excreted principally through the lungs, that the respiratory interchange becomes in good measure an indicator of the extent of chemical decomposition incidental to external work. If we recall that man, on an average, at each inspiration draws in about 500 cubic centimeters of air (30 cubic inches), and that for the 24 hours he averages 15 breaths a minute, it is easy to see that in one minute the average man will inspire 7.5 litres of air, or 450 litres an hour, with a total of 10,800 litres for the entire day, which is equivalent to about 380 cubic feet. This would be a volume of air just filling a room 7 1/3 feet in length, width, and height. Inspired air loses to the body 4.78 volumes per cent of oxygen, while expired air contains an excess of 4.34 volumes per cent of carbon dioxide. In muscular work, respiration is increased in frequency and in depth. The volume of air exchanged in the lungs during severe labor may be increased sevenfold, while oxygen consumption and carbon dioxide excretion are frequently increased 7–10 times. The following figures, being values for one minute, show the effect on oxygen consumption of walking on a level and climbing, the subject being a man of 55.5 kilos body-weight:[42]
| Form of Work. | Oxygen Consumption in Cubic Centimeters. | Respiratory | ||
|---|---|---|---|---|
Total. | After Deducting Value | |||
Total. | For Each Kilo | |||
| Standing at rest | 263.75 | . . . . | . . . . | 0.801 |
| Walking on a level | 763.00 | 499.25 | 8.990 | 0.805 |
| Climbing | 1253.20 | 989.45 | 17.819 | 0.801 |
Remembering that these figures represent the oxygen consumption for only one minute of time, it is easy to see the striking effect of moderate and vigorous exercise on respiratory interchange. Simply walking along a level suffices to increase the consumption of oxygen threefold over what occurs when the body stands at rest. When the more vigorous exercise attendant on lifting the body up a steep incline is attempted, most striking is the great increase in the amount of oxygen consumed. We thus see another forcible illustration of the influence of muscular activity upon the exchange of matter in the body, and a further confirmation of the statement, so many times made, that oxidation—especially the oxidation of fats and carbohydrates by which large quantities of heat are set free, easily convertible into mechanical energy—is a primary factor in the metabolic processes, by which the machinery of the living man is able to work so efficiently.
Finally, we cannot avoid the conclusion that the outgoings of the body, in the form of matter and energy, are subject to great variation, incidental to the degree of activity of the day or hour. The ordinary vicissitudes of life, bringing days of physical inaction, followed perhaps by periods of unusual activity; changes in climatic conditions, with their influence upon heat production in the body; alterations in the character and amount of the daily dietary, etc.,—all seemingly combine as natural obstacles to the maintenance of a true nutritive balance. Outgo, however, must be met by adequate amounts of proper intake if there is to be an approach toward a balance of nutrition. In some way the normal, healthy man does maintain, approximately at least, a condition of balance; not necessarily for every hour or for every day, but the intake and outgo if measured for a definite period, not too short, say for a week or two, will be found to approach each other very closely. Body equilibrium and approximate nitrogen balance may be reasonably looked for, as well as a balance of total energy, in the case of a healthy man leading a life which conforms to ordinary physiological requirements. The man who, on the other hand, consciously or unconsciously, continues an intake way beyond the outgo, whose daily income of nitrogen and total fuel value far exceeds the requirements of his body, obviously lives with an accumulating plus balance, which ordinarily shows itself in increasing body-weight and with a storing away of fat.
Equally conspicuous is the effect of an inadequate income of proper nutriment; a food supply which persistently fails to furnish the available nitrogen and total energy value called for by the body under the conditions prevailing, will inevitably result in a minus balance, which, if continued too long, must of necessity tax the body’s store to the danger limit. At the same time, the well-nourished individual, without being unduly burdened by a bulky store of energy-containing material, is always supplied with a sufficient surplus to meet all rational demands, when from any cause the intake fails, for brief periods of time, to be commensurate with the needs of the body. It is reasonable to believe, however, that in the maintenance of good health, and the preservation of a high degree of efficiency, the body should be kept in a condition approaching a true nutritive balance.