FOODS: THEIR CLASSIFICATION 429 



so that we shall have also to consider foods in their relationship 

 to growth. 



Classes of Foods. We are aware that the materials which com- 

 pose our meals include indigestible substances as well as true foods. 

 These indigestible materials serve, as we shall see, an important 

 function through the bulk they impart to the food; it would be 

 extremely difficult to maintain the Body in health upon a diet 

 from which they were excluded; we may borrow for them an ex- 

 pressive term used by feeders of cattle for bulky stuffs of little 

 nutritive value, and designate them as roughage. 



The true foods fall into two classes, energy yielders and non- 

 energy yielders. The latter class includes all the inorganic con- 

 stituents of the diet, such as water and the various salts; and a 

 number of organic substances which serve definite purposes not 

 involving the liberation of energy by them. These non-energy 

 yielders are commonly classed as accessories of the diet, to signify 

 their subordinate relation to the energy yielding food. We have 

 to recognize, however, that some of the so-called accessories are 

 necessary to health. These we may call the essential accessories. 

 They include water, the various salts, and a group of organic sub- 

 stances to be described in detail in a later paragraph, known as 

 the vitamines. The other accessories of the diet, chiefly organic, 

 may be designated as occasional accessories. Among these are 

 included the special substances which give flavor to the food, and 

 by making it palatable aid in its digestion. All drugs, including 

 the essential principles of tea, coffee, and cocoa, fall also into this 

 class, as do the substances classed as condiments, pepper, mustard, 

 etc. 



The organic constituents of our food not included among the 

 group of accessory articles of diet belong chemically to one or 

 other of three great subdivisions. They are either carbohydrates, 

 fats, or proteins. The entire supply of energy for the Body, and 

 its repair and maintenance to great extent are derived from these 

 three classes of food stuffs. Because of their prime importance 

 they are usually set apart from the other foods as nutrients proper. 



Occurrence of Nutrients in Food. The articles which in com- 

 mon language we call foods are, in most cases, mixtures of several 

 nutrients with inorganic and organic accessory substances and 

 with roughage. Bread, for example, contains water, salts, gluten 



