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.