ELEMENTARY PHYSIOLOGICAL CHEMISTRY 209 



We may, therefore, divide the proximate principles of foods into two 

 groups, viz.: 



Tissue formers Proteid, inorganic salts, water. 



Combustion material Proteid, fats, carbohydrates. 



It will be observed that, of the organic food stuffs, proteid may 

 functionate in either capacity, and hence, that a diet containing it 

 alone can serve as an efficient food. That this is so, is proved by 

 the fact that the Indians of the Pampas live entirely on flesh. 

 Without proteid it is impossible to maintain life, but if more than is 

 necessary for the repair of the broken-down tissues be supplied, the 

 excess may serve as fuel. 



The most serviceable combustion material seems, however, to be 

 carbohydrate ; next comes proteid, and the least available is fat, this 

 latter being pre-eminently the form in which the excess of food over 

 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 see to, 

 therefore, is that it contains a sufficient amount and a suitable mixture 

 of the nutritive constituents 1 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 determine 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 C0 2 in expired air), and 

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

 Now, the only food-stuff which contains both these elements is proteid, 

 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 proteid, 

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

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

 proteid 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. 



1 These nutritive constituents are sometimes called the proximate principles of 

 food, because, consisting as they do of carbon, hydrogen, oxygen, and nitrogen 

 combined more or less into highly complex bodies, they are really elementary 

 constituents of the organism. 



O 



