MUSCLE 257 



carbohydrates are burned completely to CO2 and HgO, and 

 therefore yield the same amount of energy as in the 

 calorimeter. 



On the other hand, the proteins are not completely burned 

 in the body, the process of combustion stopping at the form- 

 ation of urea, CONgH^. Something like 3 grm. of protein 

 yield 1 grm. of urea. 



In the case of fats and carbohydrates where the com- 

 bustion in the body is the same as in the calorimeter, this 

 method is at once applicable to determine the energy which 

 muscle can liberate. The physical and iDhysiological avail- 

 ahility is the same. 



1 grm. fat— 93 Calories. 



1 grm. carbohydrate— 41 Calories. 



But in the case of proteins it is necessary to determine — 

 (1) The energy evolved in complete combustion in the 

 calorimeter. (2) The energy evolved by the combustion of 

 the urea formed, and to subtract the latter from the former. 



Complete combustion of 1 grm. yields about 5 '6 Calories — 

 the 'physical energy equivalent. The combustion of the urea 

 formed from 1 grm. of protein yields about 1"3 Calories. 

 Hence about 4 '3 Calories are available in the body — the 

 ])hysiological energy equivalent. Variations occur according 

 to the nature of the protein used, and generally it is accepted 

 that — 1 grm. protein yields in the body 41 Calories. 



5. The Determination of the Amount of Proteins, Fats, and 

 Carbohydrates oxidised in the Body. — To determine the amounts 

 of each of these proximate principles oxidised in the body, 

 it is necessary to investigate — 



A. Proteins. — The amount of nitrogen excreted gives 

 a measure of the protein used since protein contains 1 6 per 

 cent, of nitrogen. Hence the nitrogen excreted x6'2o = 

 amount of protein used. 



Proteins contain nearly 3| times as much carbon as 

 nitrogen, so for each grm. of nitrogen from protein, 3*4 

 grm. of carbon are excreted (fig. 132). 



B. Fats and Carbohydrates — All carbon excreted, above 

 that derived from proteins, must come from fats and carbo- 

 hydrates (fig. 132). In the fasting animal it comes from 



17 



