CHAPTER III_The nutrition of man

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:

[22] Taken from Landergren: Untersuchungen über die Eiweissumsetzung

des Menschen. Skandinavisches Archiv für Physiologie, Band 14, p.

112; and from A. Magnus-Levy: v. Noorden’s Handbuch der Pathologie

des Stoffwechsels, 1906, p. 312.

+-----------------+-------------+-------------+-------------+

+-----------------+-------------+-------------+-------------+

| 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:

[23] Expressed in this form from Voit’s figures by A. Magnus-Levy.

Loc. cit., p. 311.

+------------------+---------------+----------------+---------------+

+------------------+---------------+----------------+---------------+

|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.

[24] Taken from Johansson, Landergren, Sondén, and Tiegerstedt:

Beiträge zur Kenntniss des Stoffwechsels beim hungernden Menschen.

Skandinavisches Archiv für Physiologie, Band 7, p. 29.

+------+--------------+---------------------+------------------------+

| Day. | Body-weight. +---------+-----------+------------+-----------+

+------+--------------+---------+-----------+------------+-----------+

| 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 |

+------+--------------+---------+-----------+------------+-----------+

[25] The carbon output represents the total carbon of the expired

air, urine, and excrement.

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 | Fat |

| | metabolized. | 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.

+------+---------------+---------------+-------------+

+------+---------------+---------------+-------------+

| 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.”

[26] Leathes: Problems in Animal Metabolism. Philadelphia, 1906, p.

157.

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:

[27] Sivén: Zur Kenntniss des Stoffwechsels beim erwachsenen

Menschen, mit besonderer Berücksichtigung des Eiweissbedarfs.

Skandinavisches Archiv für Physiologie, Band 11, p. 308.

+--------+--------------+-------------+-----------+----------+

+--------+--------------+-------------+-----------+----------+

| 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:

[28] C. Voit: Hermann’s Handbuch der Physiologie des

Gesammtstoffwechsels, Band 6, p. 130.

+-----------------------++-----------------------------+

| 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 |

+-------+-----------------------++--------------+----------------------+

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.

[29] Landergren: Untersuchungen über die Eiweissumsetzung des

Menschen, Skandinavisches Archiv für Physiologie, Band 14, p. 114.

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.

+----+------------------------------------------++---------++------------+

| | Intake. || Output. || |

|Day.+--------+-----+--------+--------+---------++---------++ Proteid |

| |Proteid.| Fat.| Carbo- |Alcohol.|Calories.||Nitrogen ||metabolized.|

| | | |hydrate.| | ||of Urine.|| |

+----+--------+-----+--------+--------+---------++---------++------------+

| | 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:

[30] An experiment by Miura, quoted from A. Magnus-Levy in v.

Noorden’s Handbuch der Pathologie des Stoffwechsels, 1906, p. 331.

+-------+---------------------------------------++---------+----------+

| | Income. || |Balance of|

| +---------+-----+-------------+---------+|Output of| Nitrogen |

| |Nitrogen.| Fat.|Carbohydrate.|Calories.||Nitrogen.| in Body. |

+-------+---------+-----+-------------+---------++---------+----------+

|Av. of | grams |grams| grams | || grams | |

|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:

[31] Max Dapper: Ueber Fleischmast beim Menschen. Inaug. Disser.

Marburg, 1902.

+----+-----+--------------+-------------------+--------+------------------+

|No. | | | Food Composition. | | |

+----+-----+--------------+---------+---------+--------+------------------+

| 1 | 6 |Ordinary mixed| 20.25 | 2930 | +2.18 |+3.2 on 4th day. |

| | | | | | | |

| 2 | 12 |Ditto + 80 | 20.09 | 3250 | +3.32 |+4.75 on 2d day. |

| | | | | | | |

| 3 | 9 |Ditto + 80 | 24.58 | 3400 | +2.55 |+5.98 on 1st 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.”

[32] Zeitschrift für klinische Medizin, Band 44, p. 22.

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]

[33] For an account of the respiration calorimeter and the great

diversity of apparatus accessory thereto, together with a description

of the methods of measurement, analysis, etc., see Publication No.

42, Carnegie Institution of Washington, “A Respiration Calorimeter

with Appliances for the Direct Determination of Oxygen.” By W. O.

Atwater and F. G. Benedict.

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 Measured |

+--------+--------+--------+ from |Material | Material|Respira-+--------+--------+

| (a) | (b) | (c) | Food. |Gained or| Oxidized| tion | | |

| | | | | | d-e | | | |

+--------+--------+--------+--------+---------+---------+--------+--------+--------+

| 2569 | 149 | 103 | 2317 | +229 | 2088 | 2113 | +25 | +1.2 |

+--------+--------+--------+--------+---------+---------+--------+--------+--------+

[34] In the experiment, the body lost 29.16 grams of proteid = 165

calories, but gained fat and glycogen = 393 calories. Hence, there

were 229 calories gained from body material.

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 | Total Calories | Calories per |

| of the | for 24 Hours. | Kilo of |

| Individual. | | Body-weight. |

+-------------+----------------+--------------+

| kilos | | |

| 43.2 | 1333 | 30.9 |

| 48.0 | 1214 | 25.3 |

| 50.0 | 1315 | 25.9 |

| 53.0 | 1527 | 28.8 |

| 55.0 | 1590 | 28.9 |

| 56.5 | 1519 | 26.8 |

| 57.2 | 1560 | 27.3 |

| 58.0 | 1510 | 26.0 |

| 62.5 | 1431 | 22.9 |

| 63.0 | 1418 | 22.5 |

| 63.0 | 1492 | 23.7 |

| 64.0 | 1656? | 25.8 |

| 64.9 | 1475 | 22.7 |

| 65.0 | 1498 | 23.0 |

| 65.0 | 1445 | 22.2 |

| 67.5 | 1608 | 23.8 |

| 67.5 | 1621 | 24.0 |

| 70.0 | 1661 | 23.7 |

| 70.0 | 1620 | 23.1 |

| 71.2 | 1787 | 25.1 |

| 72.6 | 1550 | 21.3 |

| 72.7 | 1657 | 22.8 |

| 73.0 | 1584 | 21.7 |

| 73.0 | 1630 | 22.4 |

| 75.6 | 1670 | 22.1 |

| 82.0 | 1556 | 19.0 |

| 82.7 | 2030? | 24.5 |

| 83.5 | 1670 | 20.0 |

| 88.3 | 2019? | 22.9 |

| 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]

[35] See Armsby: Principles of Animal Nutrition, p. 368.

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]

[36] Taken from Armsby: Principles of Animal Nutrition, p. 383.

+---------+---------------------+------------------+

| 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., CO_{2}/O_{2}. Carbohydrates

(C_{6}H_{12}O_{6}, C_{12}H_{22}O_{11}) all contain hydrogen and oxygen

in the proportion to form water, H_{2}O, and in their oxidation they

need of oxygen only such quantity as will suffice to oxidize the carbon

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.

[37] Atwater and Benedict: Experiments on the Metabolism of Matter

and Energy in the Human Body 1900–1902. Bulletin No. 136, Office of

Experiment Stations, U. S. Department of Agriculture, 1903, p. 141.

HEAT GIVEN OFF BY BODY, INCLUDING FOR WORK EXPERIMENTS THE HEAT

EQUIVALENT OF THE EXTERNAL MUSCULAR WORK.

+----------------+--------+---------------------------------------+--------+

| | Amount +-------------------+-------------------+ Average|

| Experiment. | in 24 | | | 24 |

| | Hours. +---------+---------+---------+---------+ Hours. |

| | | 1 P.M. | 7 P.M. | 1 A.M. | 7 A.M. | |

+----------------+--------+---------+---------+---------+---------+--------+

|Rest experiments| 2262 | 106.3 | 104.4 | 98.3 | 67.9 | 94.3 |

+----------------+--------+---------+---------+---------+---------+--------+

|Work experiments|} | | | | | |

| Heat eliminated|} 4225 | 231.7 | 235.6 | 118.1 | 78.4 | 166.6 |

| | | | | | | |

|Heat equivalent |} | | | | | |

| of external |} 451 | 58.5 | 56.8 | ... | ... | ... |

| muscular work |} | | | | | |

+----------------+--------+---------+---------+---------+---------+--------+

| 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:

[38] Loc. cit., pp. 130 and 131.

+------------------+---------+---------+-----------+---------+----------+

+------------------+---------+---------+-----------+---------+----------+

| 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:

[39] See W. O. Atwater and H. C. Sherman: The effect of severe

and prolonged muscular work on food consumption, digestion, and

metabolism. Bulletin No. 98, Office of Experiment Stations, U. S.

Department of Agriculture.

+------------+-------------+--------------+-------------+

+------------+-------------+--------------+-------------+

| 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. |tion +-----+-----+------+--------+-----+------+------+-----+

| | of | | |Carbo-| | | | In | |

| |Exp. | Pro | Fat.| hy- | Fuel | In | In |Excre-|Loss.|

+----------+-----+-----+-----+------+--------+-----+------+------+-----+

|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.

[40] Atwater and Sherman. Loc. cit., p. 51.

[41] L. Schnyder: Muskelkraft und Gaswechsel. Zeitschrift für

Biologie, Band 33, p. 289.

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]

[42] G. Katzenstein: Ueber die Einwirkung der Muskelthätigkeit auf

den Stoffverbrauch des Menschen. Pflüger’s Archiv für die gesammte

Physiologie, Band 49, p. 330. Also Magnus-Levy: v. Noorden’s Handbuch

der Pathologie der Stoffwechsel, p. 233.

+------------------+----------------------------------------+-----------+

| |Oxygen Consumption in Cubic Centimeters.| |

| +---------+------------------------------+ |

| | Total. +-----------+------------------+ Quotient. |

+------------------+---------+-----------+------------------+-----------+

|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.