360 PEACTICAL PHYSIOLOGY 



present requirements is laid by for future use. Thus, during summer, 

 hibernating animals store up a large quantity of fat, and this is called 

 upon during the winter sleep to furnish the energy necessary for life. 



In judging whether any diet be efficient the first thing we must deter- 

 mine, therefore, is whether it contains a sufficient amount and a 

 suitable mixture of the nutritive constituents of food. In practice 

 it is found that these facts can be determined by estimating the 

 amount of carbon and of nitrogen which the diet contains. We can 

 find out how much of these two elements is necessary by estimating 

 the amount of them contained in the excreta. 



An ordinary man under ordinary circumstances eliminates about 

 300 grammes of carbon per diem (chiefly as carbon dioxide), and about 

 15 grammes of nitrogen (chiefly as urea, etc., in the urine). Now, the 

 only food-stuff which contains both these elements is protein, and to 

 supply the required amount of nitrogen it would be necessary to give 

 only about 100 grammes of this. Such an amount would, however, 

 only furnish about one-sixth of the necessary amount of carbon. This 

 difficulty could be surmounted by giving about 600 grammes of protein, 

 but then the tissues would be supplied with six times more nitrogen 

 than they required. It is advantageous, therefore, to mix with the 

 protein some food stuff containing an excess of carbon, but no nitrogen. 

 Such a food stuff is fat or carbohydrate. Experience teaches us that of 

 these two the more serviceable is carbohydrate, and for two reasons : 

 firstly, because it is more easily digested ; arid, secondly, because it is 

 cheaper. 



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 abundance 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 



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. 



