840 THE POTENTIAL ENERGY OF FOOD. [BOOK n. 



share contributed by the large mass of carbohydrates is only 

 slightly greater than that belonging to the much smaller quantity 

 of fat. 



If we translate the units of heat into units of work, the 

 2,360,000 gramme-degree, or 2,360 kilogramme-degree calories 

 will give us almost exactly one-million kilogramme-meters. 



If we take another diet of a different character proposed by 

 another physiologist, Voit, namely one containing proteids 118 gr., 

 fats 56 gr., carbohydrates 500 gr., we find that this gives a total 

 energy of 3,063,000 calories. If we take the various diets which 

 have been determined statistically, we shall find that, though they 

 differ from each other in the proportions of the food-stuffs, they do 

 not differ very widely as regards total energy from that of Voit. 

 Of course if we were to examine in this way the daily food of 

 persons living under very different conditions, some rich, some 

 poor, some doing a large amount of daily work, others a very little, 

 and so on, we should find the total available energy of that daily 

 food varying very considerably ; in some cases it falls below 

 2,000,000 cal, in some cases it rises above 5,000,000 ; the latter 

 however probably need a large additional correction for the un- 

 utilized portion. But taking what we may call the average man, 

 living in average circumstances and doing an average amount of 

 work, we may probably consider his daily income of available 

 energy to be in round numbers 3,000,000 calories or about a million 

 and a quarter kilogramme-meters of work. 



The Expenditure. 



528. There are two ways only in which energy is set free 

 from the body: mechanical labour and heat. The body loses 

 energy in producing muscular work, as in locomotion and in 

 other kinds of labour, in the movements of the air in respiration 

 and speech, and, though to a hardly recognizable extent, in the 

 movements of the air or contiguous bodies by the pulsations of 

 the vascular system. The body loses energy in the form of heat 

 by conduction and radiation, by respiration and perspiration, and 

 by the warming of the urine and faeces ; under the head of heat we 

 include the energy which is spent in the evaporation of the sweat. 

 All the internal work of the body, all the mechanical labour of the 

 internal muscular mechanisms with their accompanying friction, 

 all the molecular labour of the nervous and other tissues, is con- 

 verted into heat before it leaves the body. The most intense 

 mental action, unaccompanied by any muscular manifestations, the 

 most energetic action of the heart or of the bowels, with the slight 

 exceptions mentioned above, the busiest activity of the secreting 

 or metabolic tissues, all these end simply in augmenting the ex- 

 penditure in the form of heat. 



