26 LIBERATION OF ENERGY 



each of the following carbohydrates has the accompanying value 

 in calories : 



Average values have therefore been adopted and accepted as 

 standard. E.g. 



Carbohydrate . . . .4-1 Calories. 

 Fat 9-3 „ 



Protein (Physical value) . . 5-3 ,, 



Protein (Physiological value) . 4-1 ,, 



Of course, the discerning student will understand that, except 

 in rare and restricted feeding experiments which have a special 

 end in view, pure carbohydrate, fat and protein are not exhibited. 

 Determinations are made of the energy- value of actual foods. 

 This gives opportunity for the display of some ingenuity on the 

 part of the investigator, since some of the commoner articles of 

 diet do not readily lend themselves to combustion and are not 

 easily dried. Nevertheless, extended experiments are being con- 

 ducted by physiologists, in which, as part of the routine, the total 

 energy- value of the daily diet is determined. 



The energy-value of the diet does not necessarily represent the 

 energy used by the organism. 



{a) Some energy-carrying substances cannot be digested, and 

 therefore are excreted unchanged in chemical composition and 

 energy content, e.g. cellulose. 



(6) Other constituents of the diet may undergo some chemical 

 alteration, but may be excreted not fully deprived of their energy. 



(i) Proteins are not completely oxidised in the body. Their 

 end-products are urea and allied substances. 



Because of the difference in the end-products there is a physio- 

 logical calorie value for proteins different from their piu'cly 

 physical value. Rubner determined this physiological value by 

 deducting from the absolute value, the heat value of nitrogenous 

 end-products in faeces and urine with their heats of solution. 

 He arrived at the figure 4-015. The accepted average value is 

 4-1 Calories per gram of protein. 



