Proteins when burned in the calorimeter give off their carbon as carbon dioxide, their hydrogen as water, and their nitrogen as nitrogen gas.* Thus the nitrogen contributes nothing to and takes nothing from the heat of combustion; and the latter is dependent here, as in the case of carbohydrates and fats, upon the amount of carbon and hydrogen present and the extent to which they are already combined with oxygen. A little additional heat is obtained by the burning of the small amount of sulphur present in the protein.

The relation between the elementary composition and heat of combustion will be made clearer by the following table, which includes a number of typical compounds found in the food or formed in the body.

* As a matter of fact a small part of the nitrogen is oxidized to nitric acid in the bomb calorimeter, but this is determined and its heat of formation subtracted, so that the final results are as stated above.

Heats Of Combustion And Approximate Elementary Composition Of Typical Compounds

Since the energy used in the body is obtained from the oxidation of the same kinds of compounds which exist in food, i.e. from carbohydrates, fats, and proteins (or their cleavage products), we can estimate the amount of energy transformed in the body if we know the amount of each kind of foodstuff oxidized. Account must, however, be taken of the completeness of the oxidation in each case.

When undergoing complete oxidation in the bomb calorimeter the foodstuffs yield the following average heats of combustion:

In the body carbohydrates and fats are oxidized to the same products as in the calorimeter and so yield the same amounts of heat. Protein, however, which burns in the bomb to carbon dioxide, water, and nitrogen, yields in the body no free nitrogen, but urea and other organic nitrogen compounds which are eliminated as end products. These organic nitrogenous end products are combustible; they represent a less complete oxidation of protein in the body than takes place in the bomb. The loss of potential energy calculated on the assumption that all nitrogen left the body as urea would be about 0.9 Calorie per gram of protein, but on account of the elimination of other substances of higher heat of combustion (creatinine, uric acid, etc.), the actual loss in the form of combustible end products is considerably greater and averages about 1.3 Calories for each gram of protein broken down in the body.

Hence, when the body burns material which it has previously absorbed, it obtains:

In calculating the fuel value of the food, however, allowance must be made for the fact that a part of each of the materials is lost in digestion.*

The approximate averages on a mixed diet are:

The approximate physiological fuel values of the food constituents are then:

Carbohydrates 4.1 X 98% = 4. Calories per gram. Fats 9.45 X 95% =9. Calories per gram.

Protein 4.35 X 92% =4. Calories per gram.

The figures given by Rubner as representing the fuel values of food constituents are as follows:

Carbohydrates 4.1 Fats 9.3

Protein 4.1

* The expression "lost in digestion" is here used in the sense explained in Chapter IV (Enzymes And Digestion).

These were derived from experiments with dogs fed on meat, starch, sugar, etc., and therefore do not allow for so much loss in digestion as has been found to occur with men living on ordinary mixed diet.