Dietetics

Dietetics, the science of diet, i.e. the food and nutrition of man in health and disease (see Nutrition). This article deals mainly with that part of the subject which has to do with the composition and nutritive values of foods and their adaptation to the use of people in health. The principal topics considered are: (1) Food and its functions; (2) Metabolism of matter and energy; (3) Composition of food materials; (4) Digestibility of food; (5) Fuel value of food; (6) Food consumption; (7) Quantities of nutrients needed; (8) Hygienic economy of food; (9) Pecuniary economy of food.

1. Food and its Functions.—For practical purposes, food may be defined as that which, when taken into the body, may be utilized for the formation and repair of body tissue, and the production of energy. More specifically, food meets the requirements of the body in several ways. It is used for the formation of the tissues and fluids of the body, and for the restoration of losses of substance due to bodily activity. The potential energy of the food is converted into heat or muscular work or other forms of energy. In being thus utilized, food protects body substance or previously acquired nutritive material from consumption. When the amount of food taken into the body is in excess of immediate needs, the surplus may be stored for future consumption.

Ordinary food materials, such as meat, fish, eggs, vegetables, &c., consist of inedible materials, or refuse, e.g. bone of meat and fish, shell of eggs, rind and seed of vegetables; and edible material, as flesh of meat and fish, white and yolk of eggs, wheat flour, &c. The edible material is by no means a simple substance, but consists of water, and some or all of the compounds variously designated as food stuffs, proximate principles, nutritive ingredients or nutrients, which are classified as protein, fats, carbohydrates and mineral matters. These have various functions in the nourishment of the body.

The refuse commonly contains compounds similar to those in the food from which it is derived, but since it cannot be eaten, it is usually considered as a non-nutrient. It is of importance chiefly in a consideration of the pecuniary economy of food. Water is also considered as a non-nutrient, because although it is a constituent of all the tissues and fluids of the body, the body may obtain the water it needs from that drunk; hence, that contained in the food materials is of no special significance as a nutrient.

Mineral matters, such as sulphates, chlorides, phosphates and carbonates of sodium, potassium, calcium, &c., are found in different combinations and quantities in most food materials. These are used by the body in the formation of the various tissues, especially the skeletal and protective tissues, in digestion, and in metabolic processes within the body. They yield little or no energy, unless perhaps the very small amount involved in their chemical transformation.

Protein1 is a term used to designate the whole group of nitrogenous compounds of food except the nitrogenous fats. It includes the albuminoids, as albumin of egg-white, and of blood serum, myosin of meat (muscle), casein of milk, globulin of blood and of egg yolk, fibrin of blood, gluten of flour; the gelatinoids, as gelatin and allied substances of connective tissue, collagen of tendon, ossein of bone and the so-called extractives (e.g. creatin) of meats; and the amids (e.g. asparagin) and allied compounds of vegetables and fruits.

The albuminoids and gelatinoids, classed together as proteids, are the most important constituents of food, because they alone can supply the nitrogenous material necessary for the formation of the body tissues. For this purpose, the albuminoids are most valuable. Both groups of compounds, however, supply the body with energy, and the gelatinoids in being thus utilized protect the albuminoids from consumption for this purpose. When their supply in the food is in excess of the needs of the body, the surplus proteids may be converted into body fat and stored.

The so-called extractives, which are the principal constituents of meat extract, beef tea and the like, act principally as stimulants and appetizers. It has been believed that they serve neither to build tissue nor to yield energy, but recent investigations2 indicate that creatin may be metabolized in the body.

The fats of food include both the animal fats and the vegetable oils. The carbohydrates include such compounds as starches, sugars and the fibre of plants or cellulose, though the latter has but little value as food for man. The more important function of both these classes of nutrients is to supply energy to the body to meet its requirements above that which it may obtain from the proteids. It is not improbable that the atoms of their molecules as well as those from the proteids are built up into the protoplasmic substance of the tissues. In this sense, these nutrients may be considered as being utilized also for the formation of tissue; but they are rather the accessory ingredients, whereas the proteids are the essential ingredients for this purpose. The fats in the food in excess of the body requirements may be stored as body fat, and the surplus carbohydrates may also be converted into fat and stored.

To a certain extent, then, the nutrients of the food may substitute each other. All may be incorporated into the protoplasmic structure of body tissue, though only the proteids can supply the essential nitrogenous ingredients; and apart from the portion of the proteid material that is indispensable for this purpose, all the nutrients are used as a source of energy. If the supply of energy in the food is not sufficient, the body will use its own proteid and fat for this purpose. The gelatinoids, fats and carbohydrates in being utilized for energy protect the body proteids from consumption. The fat stored in the body from the excess of food is a reserve of energy material, on which the body may draw when the quantity of energy in the food is insufficient for its immediate needs.

What compounds are especially concerned in intellectual activity is not known. The belief that fish is especially rich in phosphorus and valuable as a brain food has no foundation in observed fact.

2. Metabolism of Matter and Energy.—The processes of nutrition thus consist largely of the transformation of food into body material and the conversion of the potential energy of both food and body material into the kinetic energy of heat and muscular work and other forms of energy. These various processes are generally designated by the term metabolism. The metabolism of matter in the body is governed largely by the needs of the body for energy. The science of nutrition, of which the present subject forms a part, is based on the principle that the transformations of matter and energy in the body occur in accordance with the laws of the conservation of matter and of energy. That the body can neither create nor destroy matter has long been universally accepted. It would seem that the transformation of energy must likewise be governed by the law of the conservation of energy; indeed there is every reason a priori to believe that it must; but the experimental difficulties in the way of absolute demonstration of the principle are considerable. For such demonstration it is necessary to prove that the income and expenditure of energy are equal. Apparatus and methods of inquiry devised in recent years, however, afford means for a comparison of the amounts of both matter and energy received and expended by the body, and from the results obtained in a large amount of such research, it seems probable that the law obtains in the living organism in general.

The first attempt at such demonstration was made by M. Rubner3 in 1894, experimenting with dogs doing no external muscular work. The income of energy (as heat) was computed, but the heat eliminated was measured. In the average of eight experiments continuing forty-five days, the two quantities agreed within 0.47%, thus demonstrating what it was desired to prove—that the heat given off by the body came solely from the oxidation of food within it. Results in accordance with these were reported by Studenski4 in 1897, and by Laulanie5 in 1898.

The most extensive and complete data yet available on the subject have been obtained by W. O. Atwater, F. G. Benedict and associates6 in experiments with men in the respiration calorimeter, in which a subject may remain for several consecutive days and nights. These experiments involve actual weighing and analyses of the food and drink, and of the gaseous, liquid and solid excretory products; determinations of potential energy (heat of oxidation) of the oxidizable material received and given off by the body (including estimation of the energy of the material gained or lost by the body); and measurements of the amounts of energy expended as heat and as external muscular work. By October 1906 eighty-eight experiments with fifteen different subjects had been completed. The separate experiments continued from two to thirteen days, making a total of over 270 days. In some cases the subjects were at rest; in others they performed varying amounts of external muscular work on an apparatus by means of which the amount of work done was measured. In some cases they fasted, and in others they received diets generally not far from sufficient to maintain nitrogen, and usually carbon, equilibrium in the body. In these experiments the amount of energy expended by the body as heat and as external muscular work measured in terms of heat agreed on the average very closely with the amount of heat that would be produced by the oxidation of all the matter metabolized in the body. The variations for individual days, and in the average for individual experiments as well, were in some cases appreciable, amounting to as much as 6%, which is not strange in view of the uncertainties in physiological experimenting; but in the average of all the experiments the energy of the expenditure was above 99.9% of the energy of the income,—an agreement within one part in 1000. While these results do not absolutely prove the application of the law of the conservation of energy in the human body, they certainly approximate very closely to such demonstration. It is of course possible that energy may have given off from the body in other forms than heat and external muscular work. It is conceivable, for example, that intellectual activity may involve the transformation of physical energy, and that the energy involved may be eliminated in some form now unknown. But if the body did give off energy which was not measured in these experiments, the quantity must have been extremely small. It seems fair to infer from the results obtained that the metabolism of energy in the body occurred in conformity with the law of the conservation of energy.

3. Composition of Food Materials.—The composition of food is determined by chemical analyses, the results of which are conventionally expressed in terms of the nutritive ingredients previously described. As a result of an enormous amount of such investigation in recent years, the kinds and proportions of nutrients in our common sorts of food are well known. Average values for percentage composition of some ordinary food materials are shown in Table I. (Table I. also includes figures for fuel value.)

Table I.—Percentage Composition of some Common Food Materials.

Food Material.	Refuse.	Water.	Protein.	Fat.	Carbo- hydrates.	Mineral Matter.	Fuel Value per ℔
	%	%	%	%	%	%	Calories.
Beef, fresh (medium fat)—
Chuck	16.3	52.6	15.5	15.0	· ·	0.8	910
Loin	13.3	52.5	16.1	17.5	· ·	0.9	1025
Ribs	20.8	43.8	13.9	21.2	· ·	0.7	1135
Round	7.2	60.7	19.0	12.8	· ·	1.0	890
Shoulder	16.4	56.8	16.4	9.8	· ·	0.9	715
Beef, dried and smoked	4.7	53.7	26.4	6.9	· ·	8.9	790
Veal—
Leg	14.2	60.1	15.5	7.9	· ·	0.9	625
Loin	16.5	57.6	16.6	9.0	· ·	0.9	685
Breast	21.3	52.0	15.4	11.0	· ·	0.8	745
Mutton—
Leg	18.4	51.2	15.1	14.7	· ·	0.8	890
Loin	16.0	42.0	13.5	28.3	· ·	0.7	1415
Flank	9.9	39.0	13.8	36.9	· ·	0.6	1770
Pork—
Loin	19.7	41.8	13.4	24.2	· ·	0.8	1245
Ham, fresh	10.7	48.0	13.5	25.9	· ·	0.8	1320
Ham, smoked and salted	13.6	34.8	14.2	33.4	· ·	4.2	1635
Fat, salt	· ·	7.9	1.9	86.2	· ·	3.9	3555
Bacon	7.7	17.4	9.1	62.2	· ·	4.1	2715
Lard, refined	· ·	· ·	· ·	100.0	· ·	· ·	4100
Chicken	25.9	47.1	13.7	12.3	· ·	0.7	765
Turkey	22.7	42.4	16.1	18.4	· ·	0.8	1060
Goose	17.6	38.5	13.4	29.8	· ·	0.7	1475
Eggs	11.2	65.5	13.1	9.3	· ·	0.9	635
Cod, fresh	29.9	58.5	11.1	0.2	· ·	0.8	220
Cod, salted	24.9	40.2	16.0	0.4	· ·	18.5	325
Mackerel, fresh	44.7	40.4	10.2	4.2	· ·	0.7	370
Herring, smoked	44.4	19.2	20.5	8.8	· ·	7.4	755
Salmon, tinned	· ·	63.5	21.8	12.1	· ·	2.6	915
Oysters, shelled	· ·	88.3	6.0	1.3	3.3	1.1	225
Butter	· ·	11.0	1.0	85.0	· ·	3.0	3410
Cheese	· ·	34.2	25.9	33.7	2.4	3.8	1885
Milk, whole	· ·	87.0	3.3	4.0	5.0	0.7	310
Milk, skimmed	· ·	90.5	3.4	0.3	5.1	0.7	165
Oatmeal	· ·	7.7	16.7	7.3	66.2	2.1	1800
Corn (maize) meal	· ·	12.5	9.2	1.9	75.4	1.0	1635
Rye flour	· ·	12.9	6.8	0.9	78.7	0.7	1620
Buckwheat flour	· ·	13.6	6.4	1.2	77.9	0.9	1605
Rice	· ·	12.3	8.0	0.3	79.0	0.4	1620
Wheat flour, white	· ·	12.0	11.4	1.0	75.1	0.5	1635
Wheat flour, graham	· ·	11.3	13.3	2.2	71.4	1.8	1645
Wheat, breakfast food	· ·	9.6	12.1	1.8	75.2	1.3	1680
Wheat bread, white	· ·	35.3	9.2	1.3	53.1	1.1	1200
Wheat bread, graham	· ·	35.7	8.9	1.8	52.1	1.5	1195
Rye bread	· ·	35.7	9.0	0.6	53.2	1.5	1170
Biscuit (crackers)	· ·	6.8	9.7	12.1	69.7	1.7	1925
Macaroni	· ·	10.3	13.4	0.9	74.1	1.3	1645
Sugar	· ·	· ·	· ·	· ·	100.0	· ·	1750
Starch (corn starch)	· ·	· ·	· ·	· ·	90.0	· ·	1680
Beans, dried	· ·	12.6	22.5	1.8	59.6	3.5	1520
Peas, dried	· ·	9.5	24.6	1.0	62.0	2.9	1565
Beets	20.0	70.0	1.3	0.1	7.7	0.9	160
Cabbage	50.0	4.2	0.7	0.2	4.5	0.4	100
Potatoes	20.0	62.6	1.8	0.1	14.7	0.8	295
Sweet potatoes	20.0	55.2	1.4	0.6	21.9	0.9	440
Tomatoes	· ·	94.3	0.9	0.4	3.9	0.5	100
Apples	25.0	63.3	0.3	0.3	10.8	0.3	190
Bananas	35.0	48.9	0.8	0.4	14.3	0.6	260
Grapes	25.0	58.0	1.0	1.2	14.4	0.4	295
Strawberries	5.0	85.9	0.9	0.6	7.0	0.6	150
Almonds	45.0	2.7	11.5	30.2	9.5	1.1	1515
Brazil nuts	49.6	2.6	8.6	33.7	3.5	2.0	1485
Chestnuts	16.0	37.8	5.2	4.5	35.4	1.1	915
Walnuts	58.1	1.0	6.9	26.6	6.8	0.6	1250

It will be observed that different kinds of food materials vary widely in their proportions of nutrients. In general the animal foods contain the most protein and fats, and vegetable foods are rich in carbohydrates. The chief nutrient of lean meat and fish is protein; but in medium fat meats the proportion of fat is as large as that of protein, and in the fatter meats it is larger. Cheese is rich in both protein and fat. Among the vegetable foods, dried beans and peas are especially rich in protein. The proportion in oatmeal is also fairly large, in wheat it is moderate, and in maize meal and rice it is rather small. Oats contain more oil than any of the common cereals, but in none of them is the proportion especially large. The most abundant nutrient in all the cereals is starch, which comprises from two-thirds to three-fourths or more of their total nutritive substance. Cotton-seed is rich in edible oil, and so are olives. Some of the nuts contain fairly large proportions of both protein and fat. The nutrient of potatoes is starch, present in fair proportion. Fruits contain considerable carbohydrates, chiefly sugar. Green vegetables are not of much account as sources of any of the nutrients or energy.

Similar food materials from different sources may also differ considerably in composition. This is especially true of meats. Thus, the leaner portions from a fat animal may contain nearly as much fat as the fatter portions from a lean animal. The data here presented are largely those for American food products, but the available analyses of English food materials indicate that the latter differ but little from the former in composition. The analyses of meats produced in Europe imply that they commonly contain somewhat less fat and more water, and often more protein, than American meats. The meats of English production compare with the American more than with the European meats. Similar vegetable foods from the different countries do not differ so much in composition.

4. Digestibility or Availability of Food Materials.—The value of any food material for nutriment depends not merely upon the kinds and amounts of nutrients it contains, but also upon the ease and convenience with which the nutrients may be digested, and especially upon the proportion of the nutrients that will be actually digested and absorbed. Thus, two foods may contain equal amounts of the same nutrient, but the one most easily digested will really be of most value to the body, because less effort is necessary to utilize it. Considerable study of this factor is being made, and much valuable information is accumulating, but it is of more especial importance in cases of disordered digestion.

The digestibility of food in the sense of thoroughness of digestion, however, is of particular importance in the present discussion. Only that portion of the food that is digested and absorbed is available to the body for the building of tissue and the production of energy. Not all the food eaten is thus actually digested; undigested material is excreted in the faeces. The thoroughness of digestion is determined experimentally by weighing and analysing the food eaten and the faeces pertaining to it. The difference between the corresponding ingredients of the two is commonly considered to represent the amounts of the ingredients digested. Expressed in percentages, these are called coefficients of digestibility. See Table II.

Table II.—Coefficients of Digestibility (or Availability) of Nutrients in Different Classes of Food Materials.

Kind of Food.	Protein.	Fat.	Carbohydrates.
	%	%	%
Meats	98	98	· ·
Fish	96	97	· ·
Poultry	96	97	· ·
Eggs	97	98	· ·
Dairy products	97	96	98
Total animal food of mixed diet	97	97	98
Potatoes	73	· ·	98
Beets, carrots, &c.	72	· ·	97
Cabbage, lettuce, &c.	· ·	· ·	83
Legumes	78	90	95
Oatmeal	78	90	97
Corn meal	80	· ·	99
Wheat meals without bran	83	· ·	93
Wheat meals with bran	75	· ·	92
White bread	88	· ·	98
Entire wheat bread	82	· ·	94
Graham bread	76	· ·	90
Rice	76	· ·	91
Fruits and nuts	80	86	96
Sugars and starches	· ·	· ·	98
Total vegetable food of mixed diet	85	90	97
Total food of mixed diet	92	95	97

Such a method is not strictly accurate, because the faeces do not consist entirely of undigested food but contain in addition to this the so-called metabolic products, which include the residuum of digestive juices not resorbed, fragments of intestinal epithelium, &c. Since there is as yet no satisfactory method of separating these constituents of the excreta, the actual digestibility of the food is not determined. It has been suggested that since these materials must originally come from food, they represent, when expressed in terms of food ingredients, the cost of digestion; hence that the values determined as above explained represent the portion of food available to the body for the building of tissue and the yielding of energy, and what is commonly designated as digestibility should be called availability. Other writers retain the term “digestibility,” but express the results as “apparent digestibility,” until more knowledge regarding the metabolic products of the excreta is available and the actual digestibility may be ascertained.

Experimental inquiry of this nature has been very active in recent years, especially in Europe, the United States and Japan; and the results of considerably over 1000 digestion experiments with single foods or combinations of food materials are available. These were mostly with men, but some were with women and with children. The larger part of these have been taken into account in the following estimations of the digestibility of the nutrients in different classes of food materials. The figures here shown are subject to revision as experimental data accumulate. They are not to be taken as exact measures of the digestibility (or availability) of every kind of food in each given class, but they probably represent fairly well the average digestibility of the classes of food materials as ordinarily utilized in the mixed diet.

5. Fuel Value of Food.—The potential energy of food is commonly measured as the amount of heat evolved when the food is completely oxidized. In the laboratory this is determined by burning the food in oxygen in a calorimeter. The results, which are known as the heat of combustion of the food, are expressed in calories, one calory being the amount of heat necessary to raise the temperature of one kilogram of water one degree centigrade. But it is to be observed that this unit is employed simply from convenience, and without implication as to what extent the energy of food is converted into heat in the body. The unit employed in the measurement of some other form of energy might be used instead, as, for example, the foot-ton, which represents the amount of energy necessary to raise one ton through one foot.

Table III.—Estimates of Heats of Combustion and of Fuel Value of Nutrients in Ordinary Mixed Diet.

Nutrients.	Heat of Combustion.	Fuel Value.
	Calories.	Calories.
One gram of protein	5.65	4.05
One gram of fats	9.40	8.93
One gram of carbohydrates	4.15	4.03

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’s7 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. Atwater8 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.

Table V.—Standards for Dietaries. Available Nutrients and Energy per Man per Day.

	Protein.	Fat.	Carbo- hydrates.	Fuel Value.
Voit’s Standards.	Grams.9	Grams.	Grams.	Calories.
Man at hard work	133	95	437	3270
Man at moderate work	109	53	485	2965
Atwater’s Standards.
Man at very hard muscular work	161	· ·10	· ·10	5500
Man at hard muscular work	138	· ·	· ·	4150
Man at moderately active muscular work	115	· ·	· ·	3400
Man at light to moderate muscular work	103	· ·	· ·	3050
Man at “sedentary” or woman at moderately active work	92	· ·	· ·	2700
Woman at light muscular work, or man without muscular exercise	83	· ·	· ·	2450

8. Hygienic Economy of Food.—For people in good health, there are two important rules to be observed in the regulation of the diet. One is to choose the foods that “agree” with them, and to avoid those which they cannot digest and assimilate without harm; and the other is to use such sorts and quantities of foods as will supply the kinds and amounts of nutrients needed by the body and yet to avoid burdening it with superfluous material to be disposed of at the cost of health and strength.

As for the first-mentioned rule, it is practically impossible to give information that may be of more than general application. There are people who, because of some individual peculiarity, cannot use foods which for people in general are wholesome and nutritious. Some persons cannot endure milk, others suffer if they eat eggs, others have to eschew certain kinds of meat, or are made uncomfortable by fruit; but such cases are exceptions. Very little is known regarding the cause of these conditions. It is possible that in the metabolic processes to which the ingredients of the food are subjected in the body, or even during digestion before the substances are actually taken into the body, compounds may be formed that are in one way or another injurious. Whatever the cause may be, it is literally true in this sense that “what is one man’s meat is another man’s poison,” and each must learn for himself what foods “agree” with him and what ones do not. But for the great majority of people in health, suitable combinations of the ordinary sorts of wholesome food materials make a healthful diet. On the other hand, some foods are of particular value at times, aside from their use for nourishment. Fruits and green vegetables often benefit people greatly, not as nutriment merely, for they may have very little actual nutritive material, but because of fruit or vegetable acids or other substances which they contain, and which sometimes serve a most useful purpose.

Table VI.—Amounts of Nutrients and Energy Furnished for One Shilling in Food Materials at Ordinary Prices.

Food Materials as Purchased.	Prices per ℔	One Shilling will buy
		Total Food materials.	Available Nutrients.			Fuel Value.
			Protein.	Fat.	Carbo- hydrates.
	s. d.	℔	℔	℔	℔	Calories.
Beef, round	0 10	1.20	.22	.14	· ·	1,155
	0 8½	1.41	.26	.17	· ·	1,235
	0 5	2.40	.44	.29	· ·	2,105
Beef, sirloin	0 10	1.20	.19	.20	· ·	1,225
	0 9	1.33	.21	.22	· ·	1,360
	0 8	1.50	· ·	· ·	· ·	· ·
	0 5	2.40	· ·	· ·	· ·	· ·
Beef, rib	0 9	1.33	.19	.19	· ·	1,200
	0 7½	1.60	· ·	· ·	· ·	· ·
	0 4½	2.67	· ·	· ·	· ·	· ·
Mutton, leg	0 9	1.33	.20	.20	· ·	1,245
	0 5	2.40	.37	.35	· ·	2,245
Pork, spare-rib	0 9	1.33	.17	.31	· ·	1,645
	0 7	1.71	.22	.39	· ·	2,110
Pork, salt, fat	0 7	1.71	.03	1.40	· ·	6,025
	0 5	2.40	.04	1.97	· ·	8,460
Pork, smoked ham	0 8	1.50	.20	.48	· ·	2,435
	0 4½	2.67	.36	.85	· ·	4,330
Fresh cod	0 4	3.00	.34	.01	· ·	710
	0 3	4.00	.45	.01	· ·	945
Salt cod	0 3½	3.43	.54	.07	· ·	1,370
	0 10	1.20	.07	.01	.04	275
Milk, whole, 4d. a qt.	0 2	6.00	.19	.23	.30	1,915
”  3d. a qt.	0 1½	8.00	.26	.30	.40	2,550
”  2d. a qt.	0 1	12.00	.38	.46	.60	3,825
Milk, skimmed, 2d. a qt.	0 1	12.00	.40	.03	.61	2,085
Butter	1 6	.67	.01	.54	· ·	2,320
	1 3	.80	.01	.64	· ·	2,770
	1 0	1.00	.01	.81	· ·	3,460
Margarine	0 4	3.00	· ·	2.37	· ·	10,080
Eggs, 2s. a dozen	1 4	.75	.10	.07	· ·	475
” 1½s. a dozen	1 0	1.00	.13	.09	· ·	635
” 1s. a dozen	0 8	1.50	.19	.13	· ·	950
Cheese	0 8	1.50	.38	.48	.04	2,865
	0 7	1.71	.43	.55	.04	3,265
	0 5	2.40	.60	.77	.06	4,585
Wheat bread	0 11⁄8	10.67	.76	.13	5.57	12,421
Wheat flour	0 13⁄5	7.64	.67	.07	5.63	12,110
	0 1½	8.16	.72	.07	6.01	12,935
Oatmeal	0 12⁄5	8.39	1.11	.54	5.54	14,835
	0 1½	8.16	1.08	.53	5.39	14,430
Rice	0 1¾	6.86	.45	.02	5.27	10,795
Potatoes	0 02⁄3	18.00	.25	.02	2.70	5,605
	0 0½	24.00	.34	.02	3.60	7,470
Beans	0 2	6.00	1.05	.10	3.47	8,960
Sugar	1 ¾	6.86	· ·	· ·	6.86	12,760

The proper observance of the second rule mentioned requires information regarding the demands of the body for food under different circumstances. To supply this information is one purpose of the effort to determine the so-called dietary standards mentioned above. It should be observed, however, that these are generally more applicable to the proper feeding of a group or class of people as a whole than for particular individuals in this class. The needs of individuals will vary largely from the average in accordance with the activity and individuality. Moreover, it is neither necessary nor desirable for the individual to follow any standard exactly from day to day. It is requisite only that the average supply shall be sufficient to meet the demands of the body during a given period.

The cooking of food and other modes of preparing it for consumption have much to do with its nutritive value. Many materials which, owing to their mechanical condition or to some other cause, are not particularly desirable food materials in their natural state, are quite nutritious when cooked or otherwise prepared for consumption. It is also a matter of common experience that well-cooked food is wholesome and appetizing, whereas the same material poorly prepared is unpalatable. There are three chief purposes of cooking; the first is to change the mechanical condition of the food. Heating changes the structure of many food materials very materially, so that they may be more easily chewed and brought into a condition in which the digestive juices can act upon them more freely, and in this way probably influencing the ease and thoroughness of digestion. The second is to make the food more appetizing by improving the appearance or flavour or both. Food which is attractive to the eye and pleasing to the palate quickens the flow of saliva and other digestive juices and thus aids digestion. The third is to kill, by heat, disease germs, parasites or other dangerous organisms that may be contained in food. This is often a very important matter and applies to both animal and vegetable foods. Scrupulous neatness should always be observed in storing, handling and serving food. If ever cleanliness is desirable it must be in the things we eat, and every care should be taken to ensure it for the sake of health as well as of decency. Cleanliness in this connexion means not only absence of visible dirt, but freedom from undesirable bacteria and other minute organisms and from worms and other parasites. If food, raw or cooked, is kept in dirty places, peddled from dirty carts, prepared in dirty rooms and in dirty dishes, or exposed to foul air, disease germs and other offensive and dangerous substances may easily enter it.

9. Pecuniary Economy of Food.—Statistics of economy and of cost of living in Great Britain, Germany and the United States show that at least half, and commonly more, of the income of wage-earners and other people in moderate circumstances is expended for subsistence. The relatively large cost of food, and the important influence of diet upon health and strength, make a more widespread understanding of the subject of dietetics very desirable. The maxim that “the best is the cheapest” does not apply to food. The “best” food, in the sense of that which is the finest in appearance and flavour and which is sold at the highest price, is not generally the most economical.

The price of food is not regulated largely by its value for nutriment. Its agreeableness to the palate or to the buyer’s fancy is a large factor in determining the current demand and market price. There is no more nutriment in an ounce of protein or fat from the tender-loin of beef than from the round or shoulder. The protein of animal food has, however, some advantage over that of vegetable foods in that it is more thoroughly, and perhaps more easily, digested, for which reason it would be economical to pay somewhat more for the same quantity of nutritive material in the animal food. Furthermore, animal foods such as meats, fish and the like, gratify the palate as most vegetable foods do not. For persons in good health, foods in which the nutrients are the most expensive are like costly articles of adornment. People who can well afford them may be justified in buying them, but they are not economical. The most economical food is that which is at the same time most healthful and cheapest.

The variations in the cost of the actual nutriment in different food materials may be illustrated by comparison of the amounts of nutrients obtained for a given sum in the materials as bought at ordinary market prices. This is done in Table VI., which shows the amounts of available nutrients contained in the quantities of different food materials that may be purchased for one shilling at prices common in England.

When proper attention is given to the needs of the body for food and the relation between cost and nutritive value of food materials, it will be found that with care in the purchase and skill in the preparation of food, considerable control may be had over the expensiveness of a palatable, nutritious and healthful diet.

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