The loss of potential energy due to the incomplete oxidation of the proteins in the body is approximately 1.3 calories to each gram of protein in food; consequently in calculating the fuel value of protein foods, due allowance must be made for these losses. Allowance must also be made for the incomplete digestion, or losses occurring in the digestion, of the foodstuffs. These losses, as well as the approximate amount of each constituent absorbed, are represented in the following table.[17]
| Lost | Absorbed | |
| Carbohydrates | 2 per cent. | 98 per cent. |
| Fats | 5 per cent. | 95 per cent. |
| Proteins | 8 per cent. | 92 per cent. |
The physiological fuel factors of food, or the amount of heat produced as the result of combustion of 1 gram of organic food material after the above-mentioned losses have been accounted for, may be obtained as follows.[18]
| Carbohydrates | 4.10 × 98% = 4 cal. per gram |
| Fats | 9.45 × 95% = 9 cal. per gram |
| Proteins | 4.35 × 92% = 4 cal. per gram |
EFFECT OF HEAT AND COLD UPON THE FOODSTUFFS
In primeval days, when man led a more natural life, his very existence depended upon his ability to wrest from the earth his 4—9—4; these, then, constitute what are known as the “physiological fuel factors” of carbohydrates, fats, and proteins respectively.
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 × 4 | = | .132 calorie from protein |
| .040 × 9 | = | .360 calorie from fat |
| .050 × 4 | = | .200 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.