445
DIETETICS people of different classes eat known amounts of different food materials, and the amounts digested are determined by weighings and analyses of the food and excreta. Suppose the diet to consist of bread and milk. The larger part of the nutrients will be digested, absorbed, taken into the circulation, and utilized. A small proportion, however, will pass through the intestines undigested, and along with this undigested residue there will also be excreted a certain amount of residuum, mainly from the digestive juices. That is to say, these two forms of residues, the undigested portions of the food and the metabolic products, are not oxidized, but are excreted together. To find what proportion of the diet is actually available to the. body for the two great purposes of building of tissue and yielding of energy, we must take the total quantity of nutrients, and subtract the amounts given off unconsumed. Thus the handling of the bread and milk within the body, and getting it into condition to be metabolized in the system, involves the rejection by the intestines of a certain amount of material unavailable for the general purposes of nutrition. This brings out the difference between digestibility and availability. The former applies to the amounts actually digested in the alimentary canal, the latter to the amounts actually available for use. The difference is practically the amount used to digest the food and make it available. This subject is comparatively new, and although experimental inquiries have been actively pushed since 1880, and especially since 1890, the results are still insufficient for exact statements regarding the availability1 of different kinds of food material. The following estimates are based upon the results of nearly 300 experiments, made in Europe and in the United States, with persons, chiefly men, living upon different diets. The figures are subject to revision as data accumulate. While they are not to be taken as exact measures of the availability of every kind of food oi each given class, they probably represent fairly well the average availability of these classes of food materials under ordinary circumstances. Coefficients of Availability and Fuel-Value of Nutrients. Fuel-Value per gram of Total Nutrients.
Coefficients of Availability. Kind of Food. o& Meats and fish Eggs . Dairy products Animal food (of mixed diet) Cereals . Legumes (dried) Sugars . Starches Vegetables Fruits . Vegetable food (of mixed diet) Total food (of mixed diet) .
Per c. Per c. Per c. 98 95 97 98 95 97 98 95 97
Cals. 4-25 4-35 4-25
Cals. 9-00 9'00 8-80
Cals. 3-80 3-80 3'80
97 85 78
95 90 90
4-25 3-70 3-20
8-95 8'35 8-35
83 85
90 90
98 98 97 98 98 95 90
23-
8-35 8-35
344-05 344-00 SffiO
85
90
97
3-
8-35
4-00
92
95
97
4-
8-90
4-00
4. Fuel- Value of Food.—The fuel-value of a particular food material depends on the proportions of available nutrients which can be oxidized in the body. It is commonly assumed that the available fats and carbohydrates are completely oxidized. Their fuel-value is the same as the heats of combustion of the available nutrients. The oxidation of the nitrogenous compounds,—proteids and nonproteids,—however, is not complete, considerable portions of 1 Atwater and Bryant, Connecticut, Agricultural Experiment Station, 1899.
the nitrogen being excreted in the kidneys in urea,., creatinin, and other organic compounds. In determining the quantities of energy derived from the oxidation of the nitrogenous material in the body, therefore, allowance must be made for the energy lost to the body in these latter compounds. It will be seen that the term fuel-value is here used to represent the potential energy of the material actually oxidized, or capable of being oxidized, in the body. It thus represents the actually available energy, and might be called the physical fuel-value in distinction from the physiological fuel-value, which depends upon the ways the energy, made kinetic, is used by the body. This distinction corresponds to the one above made between the amounts of available nutrients and the extent to which they are actually utilized by the body. The estimates of Rubner2 commonly quoted for the fuel-value of the nutrients of mixed diet are as follow :—protein, 4'1; fats, 9,3;. and carbohydrates, 4‘1 calories per gram. These factors were, however, originally, intended to represent the average heat of combustion of the different nutrients in the ordinary mixed diet, allowance being made for the energy lost in the incomplete oxidation of the nitrogenous compounds of the urine. Later estimates have been made by Atwater and associates,1 using data much more extensive than were available to Rubner, and including results ot a. considerable amount of experimenting in the United States. The factors for the different nutrients represent in each case the amount of energy which the body is capable of obtaining from onegram of the particular nutrient as it exists in the food as eaten in. ordinary mixed diet. Estimates of Heats of Combustion and Fuel- Value of Nutrients in Ordinary Mixed Diet. Nutrients.
Heat of Combustion.
Fuel-Value.
One gram of protein . One gram of fats One gram of carbohydrates
Calories. 5-65 9-40 4-10
Calories. 4-0 8'9 4-0
Thus, averaging together the fats in ordinary mixed diet, one gram burned in the calorimeter would yield 9 "40 calories of energy in the form of heat. Of this gram 95 per cent, is reckoned as available for oxidation in the body, and the part so oxidized would yield' energy, in the form of heat, muscular work, &c., equivalent to(9-40 x -95 =) 8'9 calories. In like manner the heat of combustion of average protein is estimated at 5‘65 calories per gram. Of this, 92 per cent, is estimated as available, and its potential energy wmuld be (5'75 x -92 =) 5'2 calories. But part of this escapes oxidation, and is excreted by the kidneys. Allowing 1 "2 calories* for the potential energy of this portion, that actually oxidized will yield (5'2 - 1‘2 =) 4'0 calories, which is taken as the fuel-valueof the protein. Similarly, 97 per cent, of the total energy of carbohydrates is assumed to be available for use in the body. Nutrients 8 of the same class, but from different food materials, vary both in. availability and in heat of combustion, and hence in fuel-value.1 The above estimates seem to represent the best averages available8 (1900), but are subject to revision. 1 5. Composition of Food Materials. — Different food 9 materials vary widely in their1 proportions of nutrients,, water, and refuse. In general, the animal foods have themost protein and fats, while the5 vegetable foods are rich in the carbohydrates—starch and0 sugar. The chief nutrient of lean meats and fish is protein. Cheese has a large quantity of protein because it contains the most of the proteids of the milk. Among the vegetable foods, beans and peas have a high proportion of protein. The proportion in oatmeal is also large. In wheat it is moderate, and in maize meal and rice it is rather small. The materials with the highest fuel-value are those with the most fat, because the fuel-value of the fat is, weight for weight,, two and one-fourth times as great as that of either sugar, starch, or protein. Hence fat pork and butter lead the other materials in fuel-value. The fat meats in general 2
Ztschr. Biol., 21, 1885, p. 377.
