CHEMISTRY OF DIGESTION AND NUTRITION. 215 



the other food-stuffs is, as we shall see, more or less accessory. ^TftTinay be 

 worth while to recall here the familiar fact that in respect to the nutritive 

 importance of proteids there is a wide difference between animal and vegetable 

 life. "What is said above applies, of course, only to animals. Plants can, 

 and for the most part do, build up their living protoplasm upon diets con- 

 taining no proteid. With some exceptions which need not be mentioned here, 

 the food-stuffs of the great group of chlorophyll-containing plants, outside of 

 oxygen, consist of water, CO 2 , and salts, the nitrogen being found in the last- 

 mentioned constituent. 



Albuminoids. Gelatin, such as is found in soups or is used in the form of 

 table-gelatin, is a familiar example of the albuminoids. They are not found 

 to any important extent in our raw foods, and they do not therefore usually 

 appear in the analyses given of the composition of foods. An examination of 

 the composition and properties of these bodies (see section on The Chemistry 

 of the Body) shows that they resemble closely the proteids. Unlike the fats 

 and carbohydrates, they contain nitrogen, and they are evidently of complex 

 structure. Nevertheless, they cannot be used in place of proteids to build 

 protoplasm. They are important foods without doubt, but their value is similar 

 in a general way to that of the non-nitrogenous foods, fats and carbohydrates, 

 rather than to the so-called " nitrogenous foods," the proteids. 



Carbohydrates. We include among carbohydrates the starches, sugars, 

 gums, and the like (see Chemical section) ; they contain no nitrogen. Their 

 physiological value lies in the fact that they are destroyed in the body and a 

 certain amount of energy is thereby liberated. The energy of muscular work 

 and of the heat of the body comes largely from the destruction or oxidation 

 of carbohydrates. Inasmuch as we are continually giving off energy from 

 the body, chiefly in the form of muscular work and heat, it follows that 

 material for the production of this energy must be taken in the food. Carbo- 

 hydrates form perhaps the easiest and cheapest source of this energy. They 

 constitute the bulk of our ordinary diet. 



Fats. In the group of fats we include not only what is ordinarily under- 

 stood by the term, but also the oils, animal and vegetable, which form such a 

 common part of our food. Fats contain no nitrogen (see Chemical section). 

 Their use in the body is substantially the same as that of the carbohydrates. 

 Weight for weight, they are more valuable than the carbohydrates as sources 

 of energy, but the latter are cheaper, more easily digested, and more easily 

 destroyed in the body. For these reasons we find that under most conditions 

 fats are a subsidiary article of food as compared with the carbohydrates. From 

 the standpoint of the physiologist, fats are of special interest because the 

 animal body stores up its reserve of food material mainly in that form. The 

 history of the origin of the fats of the body is one of the most interesting 

 parts of the subject of nutrition, and it will be discussed at some length in its 

 proper place. 



As has been said, our ordinary foods are mixtures of some or all of the 

 food-stuffs, together with such things as flavors or condiments, whose nutritive 



