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 | Fuel Value. |
| | Combustion. | |
+---------------------------+-------------+-------------+
| | | |
| | 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's[7] commonly quoted estimates for the fuel value of the
nutrients of mixed diet are,--for protein and carbohydrate