210 PRACTICAL PHYSIOLOGY 



When muscular work is performed the excretion of carbon rises, 

 whereas that of nitrogen is scarcely affected at all, so that in such cases 

 the diet should contain an excess of carbon. 



Another method of determining how much food will be required, is 

 to estimate how much energy must be liberated in order to meet 

 the needs of the organism. We can do this by placing the person in a 

 respiration-calorimeter in which all the actual heat which leaves the 

 body is collected and measured. By adding this result to the thermal 

 equivalent of the muscular work which he meanwhile performs, we 

 obtain the total amount of energy eliminated. This result is expressed 

 in calories, a kilo-calorie being the amount of heat necessary to raise 

 the temperature of one kilo of water through one degree centigrade. 

 In this way it is found that about 3,500 kilo-calories are necessary 

 per diem for an adult doing ordinary work. 1 



Having determined how much energy will be required, we must now 

 find out how much food must be supplied to yield it. We can deter- 

 mine the caloric value of the various food-stuffs by burning them in a 

 chemical calorimeter. Since the end products of the combustion of 

 fats and carbohydrates (viz., C0 2 and H 2 0) are the same in the body 

 as in vitro, their physical caloric values are the same as their physio- 

 logical, viz., 4*1 for carbohydrates and 9*3 for fats. A very important 

 end product of the metabolism of proteid is, however, urea which still 

 contains some potential energy, so that it has a physical beat-value 

 of its own. In order to find the physiological heat value of proteid, 

 therefore, we must deduct from its physical value the physical value 

 of the amount of urea arising from it. By this means it has been 

 shown that the physiological heat value of proteid is practically the 

 same as that of carbohydrate, viz., 4-1. 



By an examination of the diets taken ly various classes of people, averages 

 of the relative amounts of the various classes of food stuffs have been 

 obtained. Such a table for a man doing an ordinary amount of work 

 is the following : 



Proteid, - 125 grammes. 2 



Carbohydrate, 500 grammes. 



Fat, ---50 grammes. 



1 In physical chemistry the unit of heat chosen is one thousand times smaller 

 than the physiological Calorie, it being in this case the amount of heat necessary 

 to raise the temperature of one gramme of water through one degree centigrade. 

 The small calorie is written with a small " c," the large one with a capital " C." 

 The heat unit can be transformed into units of work by multiplying by 425 '5, a unit 

 of work being expressed as the amount of force necessary to raise a weight of one 

 gramme to a height of one metre a gramme metre or of one kilogramme to the 

 same height, a kilogramme metre. 



2 Dry weight. 



