ANIMAL FOODS AND FEEDING 295 



These are the results which have been obtained in the bomb 

 calorimeter. If, therefore, these food constituents are burned 

 in the animal body in a similar manner as in the calorimeter, 

 then these figures apply directly to that portion of the food that 

 is actually digested. 



Corrected Fuel Values. — In discussing the metabolism of 

 these constituents we showed that in the case of carbohydrates 

 and fats the processes in the body and in the calorimeter are 

 similar ; i.e. the constituents are in both cases completely oxidized 

 to carbon dioxide and water. The figures for these two constit- 

 uents hold therefore for the digestible portion. In the case of 

 proteins, however, katabolism does not yield the simple products 

 of oxidation which are obtained by burning in the calorimeter. 

 Part of the products of the katabolism of proteins are the complex 

 nitrogen compounds excreted in the urine, and these compounds 

 still contain some of the potential energy of the original proteins. 

 It has been found that the amount of energy contained in these 

 excretion products, and thus not liberated when proteins are 

 burned in the body, is equivalent to approximately 1.3 Cal. per 

 gram of protein. The fuel value of proteins, as obtained in the 

 calorimeter, must be reduced therefore by this amount, so that, 



Fuel value (in bomb) of proteins 5.65 Cal. — 1.3 Cal. 

 = 4.35 Cal. = fuel value (in body) or food value 



The corrected values then become, 



I gram digested Carbohydrates yields in body 4.1 Cal. 

 I gram digested Fats yields in body 9.45 Cal. 



I gram digested Proteins yields in body 4.35 Cal. 



Metabolizable or Available Energy. — In man the coefficients 

 of digestibility have been determined on the average mixed diet, 

 and as human beings probably differ less in these coefficients 

 than do different species of animals, it is possible to apply these 

 coefficients not to each food and each individual but once for all 

 to the food values in calories. Doing this, we obtain the follow- 

 ing: 



