THE ENERGY OF THE BODY. 493 



We have seen (421) that 1 grm. of proteid gives rise in the body to J 

 grm. urea. Hence, to obtain the energy of 1 grm. proteid material avail- 

 able for the economy, we must deduct from its potential energy one-third 

 the potential energy of 1 grm. urea that is, in round numbers, 700 or 800 

 calories. This will give us 5000 700, or 5500 800, that is, 4300 or 4700 

 calories, according as we take the lower or higher data ; or we may take as 

 a mean 4500 calories. The data, then, so far, are as follows : 



1 gramme proteid 4500 calories. 



1 gramme fat 9000 u 



1 gramme carbohydrate 4000 " 



The average diet of an average man that is, the average amount of 

 each food-stuff respectively taken daily may be determined experimentally 

 or statistically. Thus, a man may determine by a series of trials the diet 

 on which, while neither losing or gaining weight and maintaining " nitro- 

 genous equilibrium " ( 436), he enjoys good health. Or an average may 

 be struck of a large number of diets used by various people. We shall 

 have something to say of this latter statistical method when we come to 

 speak of diet. For the present purpose we may use one arrived at experi- 

 mentally, which we will speak of as Ranke's diet, since it was determined 

 by a physiologist of that name from observations on himself. It was com- 

 posed of 1000 grms. proteid, 100 grms. fat, 240 grms. carbohydrate. Such 

 a diet would give 



100 grammes proteid (4500) 450,000 calories. 



100 grammes fat (9000) 900,000 " 



240 grammes carbohydrate (4000) 960,000 " 



2,310,000 " 



If we translate the units of heat into units of work, the 2,310,000 gramme- 

 degree, or 2310 kilogramme-degree calories will give us about 980,000, or, 

 in round numbers, somewhere about one million kilogramme-metres. 



We may, in passing, call attention to the fact that the proteids supply a 

 relatively small part of the total energy, and that the share contributed by 

 the large mass of carbohydrates is not much greater than that belonging to 

 the much smaller quantity of fat. In the average diet obtained by the sta- 

 tistical method, in which the data are largely drawn from public institutions, 

 the (cheaper) carbohydrates are still further increased at the expense of the 

 (dearer) fats, a change which may tend to reduce somewhat the total energy ; 

 but this does not materially affect the broad results just given. 



The Expenditure. 



442. There are two ways only in which energy is set free from the 

 body : mechanical labor and heat. The body loses energy in producing 

 muscular work, as in locomotion and in other kinds of labor, in the move- 

 ments of the air in respiration and speech, and, though to a hardly recog- 

 nizable 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 feces. All the internal work of the body, all 

 the mechanical labor of the internal muscular mechanisms with their accom- 

 panying friction, all the molecular labor of the nervous and other tissues, 

 is converted into heat before it leaves the body. The most intense mental 



