The amount of energy which a given quantity of food will produce on complete oxidation outside the body, however, is greater than that which the body will actually derive from it. In the first place, as previously shown, part of the food will not be digested and absorbed. In the second place, the nitrogenous compounds absorbed are not completely oxidized in the body, the residuum being excreted in the urine as urea and other bodies that are capable of further oxidation in the calorimeter. The total heat of combustion of the food eaten must therefore be diminished by the heat of combustion of the oxidizable material rejected by the body, to find what amount of energy is actually available to the organism for the production of work and heat. The amount thus determined is commonly known as the fuel value of food.
Rubner’s[7] commonly quoted estimates for the fuel value of the nutrients of mixed diet are,—for protein and carbohydrates 4.1, and for fats 9.3 calories per gram. According to the method of deduction, however, these factors were more applicable to digested than to total nutrients. Atwater[8] and associates have deduced, from data much more extensive than those available to Rubner, factors for total nutrients somewhat lower than these, as shown in Table III. These estimates seem to represent the best average factors at present available, but are subject to revision as knowledge is extended.
Table IV.—Quantities of Available Nutrients and Energy in Daily Food Consumption of Persons in Different Circumstances.
| Number of Studies. | Nutrients and Energy per Man per Day. | ||||
| Protein. | Fat. | Carbo- hydrates. | Fuel Value. | ||
| Persons with Active Work. | Grams. | Grams. | Grams. | Calories. | |
| English royal engineers | 1 | 132 | 79 | 612 | 3835 |
| Prussian machinists | 1 | 129 | 107 | 657 | 4265 |
| Swedish mechanics | 5 | 174 | 105 | 693 | 4590 |
| Bavarian lumbermen | 3 | 120 | 277 | 702 | 6015 |
| American lumbermen | 5 | 155 | 327 | 804 | 6745 |
| Japanese rice cleaner | 1 | 103 | 11 | 917 | 4415 |
| Japanese jinrikshaw runner | 1 | 137 | 22 | 1010 | 5050 |
| Chinese farm labourers in California | 1 | 132 | 90 | 621 | 3980 |
| American athletes | 19 | 178 | 192 | 525 | 4740 |
| American working-men’s families | 13 | 156 | 226 | 694 | 5650 |
| Persons with Ordinary Work. | |||||
| Bavarian mechanics | 11 | 112 | 32 | 553 | 3060 |
| Bavarian farm labourers | 5 | 126 | 52 | 526 | 3200 |
| Russian peasants | .. | 119 | 31 | 571 | 3155 |
| Prussian prisoners | 1 | 117 | 28 | 620 | 3320 |
| Swedish mechanics | 6 | 123 | 75 | 507 | 3325 |
| American working-men’s families | 69 | 105 | 135 | 426 | 3480 |
| Persons with Light Work. | |||||
| American artisans’ families | 21 | 93 | 107 | 358 | 2880 |
| English tailors (prisoners) | 1 | 121 | 37 | 509 | 2970 |
| German shoemakers | 1 | 99 | 73 | 367 | 2629 |
| Japanese prisoners | 1 | 43 | 6 | 444 | 2110 |
| Professional and Business Men. | |||||
| Japanese professional men | 13 | 75 | 15 | 408 | 2190 |
| Japanese students | 8 | 85 | 18 | 537 | 2800 |
| Japanese military cadets | 11 | 98 | 20 | 611 | 3185 |
| German physicians | 2 | 121 | 90 | 317 | 2685 |
| Swedish medical students | 5 | 117 | 108 | 291 | 2725 |
| Danish physicians | 1 | 124 | 133 | 242 | 2790 |
| American professional and business men and students | 51 | 98 | 125 | 411 | 3285 |
| Persons with Little or no Exercise. | |||||
| Prussian prisoners | 2 | 90 | 27 | 427 | 2400 |
| Japanese prisoners | 1 | 36 | 6 | 360 | 1725 |
| Inmates of home for aged—Germany | 1 | 85 | 43 | 322 | 2097 |
| Inmates of hospitals for insane—America | 49 | 80 | 86 | 353 | 2590 |
| Persons in Destitute Circumstances. | |||||
| Prussian working people | 13 | 63 | 43 | 372 | 2215 |
| Italian mechanics | 5 | 70 | 36 | 384 | 2225 |
| American working-men’s families | 11 | 69 | 75 | 263 | 2085 |
The heats of combustion of all the fats in an ordinary mixed diet would average about 9.40 calories per gram, but as only 95% of the fat would be available to the body, the fuel value per gram would be (9.40 × 0.95 =) 8.93 calories. Similarly, the average heat of combustion of carbohydrates of the diet would be about 4.15 calories per gram, and as 97% of the total quantity is available to the body, the fuel value per gram would be 4.03. (It is commonly assumed that the resorbed fats and carbohydrates are completely oxidized in the body.) The heats of combustion of all the kinds of protein in the diet would average about 5.65 calories per gram. Since about 92% of the total protein would be available to the body, the potential energy of the available protein would be equivalent to (5.65 × 0.92 =) 5.20 calories; but as the available protein is not completely oxidized allowance must be made for the potential energy of the incompletely oxidized residue. This is estimated as equivalent to 1.15 calories for the 0.92 gram of available protein; hence, the fuel value of the total protein is (5.20 − 1.15 =) 4.05 calories per gram. Nutrients of the same class, but from different food materials, vary both in digestibility and in heat of combustion, and hence in fuel value. These factors are therefore not so applicable to the nutrients of the separate articles in a diet as to those of the diet as a whole.
6. Food Consumption.—Much information regarding the food consumption of people in various circumstances in different parts of the world has accumulated during the past twenty years, as a result of studies of actual dietaries in England, Germany, Italy, Russia, Sweden and elsewhere in Europe, in Japan and other oriental countries, and especially in the United States. These studies commonly consist in ascertaining the kinds, amounts and composition of the different food materials consumed by a group of persons during a given period and the number of meals taken by each member of the group, and computing the quantities of the different nutrients in the food on the basis of one man for one day. When the members of the group are of different age, sex, occupation, &c., account must be taken of the effect of these factors on consumption in estimating the value “per man.” Men as a rule eat more than women under similar conditions, women more than children, and persons at active work more than those at sedentary occupation. The navvy, for example, who is constantly using up more nutritive material or body tissue to supply the energy required for his muscular work needs more protein and energy in his food than a bookkeeper who sits at his desk all day.
In making allowance for these differences, the various individuals are commonly compared with a man at moderately active muscular work, who is taken as unity. A man at hard muscular work is reckoned at 1.2 times such an individual; a man with light muscular work or a boy 15-16 years old, .9; a man at sedentary occupation, woman at moderately active muscular work, boy 13-14 or girl 15-16 years old, .8; woman at light work, boy 12 or girl 13-14 years old, .7; boy 10-11 or girl 10-12 years old, .6; child 6-9 years old, .5; child 2-5 years old, .4; child under 2 years, .3. These factors are by no means absolute or final, but are based in part upon experimental data and in part upon arbitrary assumption.
The total number of dietary studies on record is very large, but not all of them are complete enough to furnish reliable data. Upwards of 1000 are sufficiently accurate to be included in statistical averages of food consumed by people in different circumstances, nearly half of which have been made in the United States in the past decade. The number of persons in the individual studies has ranged from one to several hundred. Some typical results are shown in Table IV.
7. Quantities of Nutrients needed.—For the proper nourishment of the body, the important problem is how much protein, fats and carbohydrates, or more simply, what amounts of protein and potential energy are needed under varying circumstances, to build and repair muscular and other tissues and to supply energy for muscular work, heat and other forms of energy. The answer to the problem is sought in the data obtained in dietary studies with considerable numbers of people, and in metabolism experiments with individuals in which the income and expenditure of the body are measured. From the information thus derived, different investigators have proposed so-called dietary standards, such as are shown in the table below, but unfortunately the experimental data are still insufficient for entirely trustworthy figures of this sort; hence the term “standard” as here used is misleading. The figures given are not to be considered as exact and final as that would suggest; they are merely tentative estimates of the average daily amounts of nutrients and energy required. (It is to be especially noted that these are available nutrients and fuel value rather than total nutrients and energy.) Some of the values proposed by other investigators are slightly larger than these, and others are decidedly smaller, but these are the ones that have hitherto been most commonly accepted in Europe and America.