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.