~Determination of Fuel Value of Food.~--In determining the amount of heat produced by a given amount of food, it is first essential to reduce the amount to grams (for example, 1 lb. equals 480 grams): first, because the gram is a unit of weight commonly used in dietetic calculations; second, because the fuel factors are based on the amount of heat produced by the burning of one gram of organic foodstuffs. Knowing the composition of food, that is the number of hundredths of protein, carbohydrate and fat it contains, it is a simple matter to estimate its fuel value by multiplying the amount of each contained in one gram by its physiological fuel factor 4.4.9. Thus if the composition of a food is 3-3/10% protein, 4% fat and 5% carbohydrate, one gram would contain .033 gram of protein, .04 gram of fat and 0.5 gram of carbohydrate. Hence one gram of milk would produce .033 x 4 = .132 calorie from protein .04 x 9 = .36 calorie from fat .05 x 4 = .20 calorie from carbohydrate or .692 calorie in all But it is not necessary to estimate the fuel value of so small a quantity as one gram, and, since the value of protein, carbohydrates and fats is always the same it is more satisfactory to estimate the amount of the organic constituents contained in the entire given quantity of food, rather than stopping to figure out the fuel value of the small quantity. This is done by multiplying the entire number of grams of food given by the amount of protein, fat and carbohydrate contained in one gram, then multiplying these results by the physiological fuel factor of each. Thus 100 grams of milk would yield 100 x .033 = 3.3 x 4 = 13.2 calories from protein 100 x .04 = 4.0 x 9 = 36.0 calories from fat 100 x .05 = 5.0 x 4 = 20.0 calories from carbohydrates or a total of 69.2 calories from 100 grams of milk. ~The Standard or 100 Calorie Portion.~--Just as it has been more convenient to estimate a larger rather than a smaller quantity of food material, so it is frequently more desirable to estimate a hundred calories, rather than one calorie. This is especially useful when dietaries of high caloric (fuel) value are to be estimated, or dietaries in which foods of like composition and fuel value are to be interchangeable. In such cases it is a simple matter to select the desired number of 100 calorie portions of those foods which are to make up the dietary.[19]