THE EXPENDITURE OF ENERGY 283 



total energy expended by the body about one-sixth is put 

 out as work and five-sixths as heat. Finally we find that 

 the average total output of energy as work and heat (cal- 

 culated as work) may be taken as about 1,000,000 

 kilogramme-metres. 



We may now consider how far this expenditure is met 

 by the income of energy in food. AVhen a substance is 

 completely burnt, i.e. oxidised, to water and carbonic 

 acid, a certain amount of heat is produced which can be 

 measured. Thus it is possible to determine how much 

 heat is produced by the complete combustion of one 

 gramme of each of the food-stuffs, proteins, fats and 

 carbohydrates. The result obtained is as follows : — 



1 gramme of protein gives 5,700 calories. 



1 „ „ fat „ 9,500 „ 



1 ,, ,, carbohydrate ,, 4,000 ,, 



Kow this must also be the amount of heat produced by 

 the same quantity of each of these food-stuffs during their 

 oxidation in the animal body. In the case of the protein 

 some deduction has to be made because the proteins are 

 not completely oxidised ; the nitrogen they contain leaves 

 the body as urea, which is still capable of undergoing 

 further oxidation to water, nitrogen, and carbonic acid. 

 One gramme of protein gives rise to about \ gramme of 

 urea, and the complete combustion of this amount of 

 urea gives rise to 844 calories. Hence, deducting these 

 from the 5,700 gives us about 4,800 calories, which we 

 may take as being the physiologically available heat of 

 combustion of one gramme of protein. If now we apply 

 these values to the diet given on p. 272 we find that : — 



130 grammes of protein give 624,000 calories, 



50 „ „ fats ,, 475,000 



400 „ „ carbohydrate ,, 1,600,000 „ 



2,699,000 



