114 VARIOl'S FOOD-PLANTS 



tions, it has been shown that a classification of vegetable 

 foods based upon the manner and degree of their usefulness 

 is at the same time a fairly accurate grouping according 

 to chemical peculiarities, the reason being that their use 

 depends largely upon their composition. Guided by this 

 principle we may now profitabl}' compare vegetable foods 

 with those of animal origin, so as to gain a Ijetter apprecia- 

 tion of their relative value in supplying human needs. A 

 satisfactory understanding of the uses of food-plants clearly 

 involves the stud,v of this larger question. 



41. Food as fuel and building material. Before proceeding 

 to compare vegetable with animal foods certain fundamental 

 facts regarding food in general must be considered. We know 

 that so long as a man is alive and active, the parts of his 

 body are wearing out from daily use, and he is losing heat. 

 If deprived of food, his weight and strength decrease, while, 

 on the other hand, if he is properly fed, nutritive materials 

 become incorporated with the various parts of his body as 

 fast as these wear away, and he finds his strength kept up by 

 a constant supply of encrgj'. Were it possible to conceive 

 of a steam-engine which could derive from the fuel it con- 

 sumes not only heat and power but also material to replace 

 that used up in action, we should have a machine to wliich 

 we might liken tlie human body in its use of food. If we 

 could imagine, furthermore, a small locomotive able to do 

 all this, and also to increase the size of its parts by the addi- 

 tion of extra material, so as to grow into a large locomotive, 

 such a marvclously endowed machine would be very like 

 the body of a child. Thus we see that food answers the 

 double purpose of supplying us with building material and 

 with fuel. But as already intimated in the last cha]iter, 

 proteids, fats, and carbohydrates are not e(iually useful as 

 sources of substance^ and energy. 



As the chief wear in our bodies comes ui)on the nniscles 

 and other parts that are <'omposed largely of nitrogen, and 

 as neither fats nor carbohydrates contain this element, it 

 follows that proteids, being uitrogimuus, must be of the first 

 importance as furnishing building material. This enables 

 us to understand wlij- it is that an animal deprived entirely 



