



AVAILABILITY AND FUEL VALUE OF FOOD MATERIALS. 103 
furnished by the 39 grams of vegetable protein, or 5.65 
calories per gram. Both the animal and the vegetable protein 
average approximately 5.65 calories per gram, which is there- 
fore the average heat of combustion of the total protein of the 
diet. 
In a similar manner we may compute the average heat of 
combustion of the fats and carbohydrates in mixed diet. Out 
of every 100 grams of total carbohydrates 95 are obtained from 
the vegetable food materials, with a total heat of combustion 
of 392.8 calories. ‘The heat of combustion per gram therefore 
amounts to (392.8 + 95 =) 4.135 calories, or approximately 
4.15 calories per gram. ‘The animal carbohydrates with a heat 
of combustion of 3.90 calories per gram furnish (3.90 * 5 =) 
19.5 calories of energy, making the total heat of combustion of 
roo grams of carbohydrates in mixed diet (392.8 + 19.5 =) 
412.3 calories, or approximately 4.15 calories per gram. 
The factors for total animal food, total vegetable food and 
total food in the remaining columns of Table ro are obtained in 
a manner similar to that just described. 
In what has been said regarding the available energy of the 
different nutrients in different classes of food materials refer- 
ence was made to the fuel value per gram of total protein, fats, 
or carbohydrates. The figures in the last column of Table ro 
give the factors representing these fuel values. It may fre- 
quently occur, however, that corresponding factors are needed 
for the fuel value of one gram of available protein, fats, or 
carbohydrates. The potential energy of one gram of available 
protein is assumed to be the same as that of one gram of total 
protein. In other words, it is assumed that the energy of one 
gram of available is the same as that of one gram of unavail- 
able protein. While this may not be strictly true, there is 
little or no experimental evidence warranting any other suppo- 
sition. It follows, therefore, that the fuel value of one gram 
of available protein is its heat of combustion less 1.25 calories 
—the energy lost in the urine. If, as has been assumed, none 
of the potential energy of its available fats and carbohydrates 
is lost in the organic matter of the urine, then the fuel value 
of one gram of available fats or carbohydrates will be the same 
as its heat of combustion, or as the heat of combustion of one 
gram of corresponding material of total food. These factors 
