PHYSIOLOGICAL CALORIE VALUE 27 



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 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. 



(b) 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 purely 

 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. 



(ii) Certain substances are excreted in combination with protein 

 disintegration products, e.g. hippuric acid. 



(c) Certain substances or their disintegration products seem to 

 be necessary constituents of faeces, e.g. fats (see Chap. XXVII.). 



The energy-value of all excreta must therefore be deducted 

 from the energy intake before an energy balance can be struck. 



C. Measurement of the E.V. of foods by animal calorimetry (a) direct, 



(b) indirect. 



It is obvious as a direct deduction from the first law of energetics 

 that if this law holds in living as well as in non-living matter- 

 energy transformations, the same amount of energy should be 

 evolved from the utilisation of food inside as well as outside the 

 body, provided always that the physical state and chemical 

 end-products are the same in each case. If an animal could be 

 put inside a calorimeter and given a definite amount of food, 



