FOODS: THEIR CLASSIFICATION 433 



of the nutrients proper. Of the three groups of nutrients two, 

 carbohydrates and fats, are exclusively energy yielders. Their 

 function is to be oxidized in the Body and thus to furnish the 

 energy by which the machine does its work. The third nutrient 

 group, the proteins, furnishes all the material by which waste of 

 living tissues is made good, and provides likewise a very con- 

 siderable proportion of the fuel supply of the Body. Because of 

 the twofold function of proteins it is possible for a person or 

 animal to live for a long time upon an exclusively protein diet. 

 Since repair of tissue waste can be made only by proteins, an ani- 

 mal or a man would starve to death upon a protein-free diet, no 

 matter how much of the other food stuffs he might have. For 

 that matter not all proteins are tissue-formers; reference to the 

 classification of proteins in Chap. I shows that only the first 

 two classes, the albumins and globulins, are sufficiently complex 

 to yield all the constituents needed for the formation or repair of 

 living tissues. Albuminoids form a constant part of all flesh food, 

 but they can be used by the Body, in the long run, only as it 'uses 

 carbohydrates and fats, for fuel. 



Carbohydrates. These are mainly of vegetable origin. The 

 most important are starch, found in nearly all vegetable foods, and 

 having the chemical formula (C 6 Hi O 5 ) n ; the dextrins, or gums; 

 and two classes of sugars; double sugars, having the formula 

 CnHaOiii an d represented by cane-sugar, sucrose, and milk-sugar, 

 lactose; and single sugars, having the formula C 6 Hi 2 O 6 , and repre- 

 sented by grape-sugar, dextrose. Glycogen, animal starch, is a 

 constituent of muscle tissue and is eaten as a part of flesh. It and 

 milk-sugar are the only carbohydrates commonly eaten which are 

 of animal origin. Cellulose, a very abundant vegetable carbo- 

 hydrate, is to the human alimentary tract practically indigestible. 



Fats. The most important are stearin, palmatin, and olein, 

 which exist in various proportions in animal fats and vegetable 

 oils; the more fluid containing more olein. Butter contains also a 

 little of a fat named butyrin. Fats are compounds of glycerin and 

 fatty acids, and any such substance which is fusible at the temper- 

 ature of the Body will serve as a food. The stearin of beef and 

 mutton fats is not by itself fusible at the body temperature, but 

 is mixed in those foods with so much olein as to be melted in the 

 alimentary canal. Beeswax, on the other hand, is a fatty body 



