4 BULLETIN 503, U. S. DEPARTMENT OF AGEICULTURE. 



form of true protein. As regards carbohydrates, various sugars, 

 pectose bodies, and, in some cases, pentosans, very generally con- 

 stitute the reserve material which the plants store up instead of 

 the starch, which is the principal carbohydrate in potatoes, sweet 

 potatoes, etc. These facts are brought out clearly in figure 1, which 

 shows the composition of common root vegetables in comparison 

 with bread, and in figure 2, which makes a similar comparison 

 between root vegetables, bread, and milk as sources of energy to the 

 body. As a class these succulent roots are characterized by very 

 marked flavors and odors, the flavor being due in part to the sugar 

 and plant acids, and in part to the small amounts of volatile oils and 

 similar substances which they contain and to which the odors are 

 mainly due. Thus the peculiar flavors of turnips, radishes, onions, 

 etc., are due chiefly to sulphur compounds. 



It is not enough to consider protein and energy value in discussing 

 food values. Mineral substances must be taken into account also, 

 since they are essential for body growth and maintenance and for 

 other phj^siological purposes. The need for iron in making red 

 blood (hemoglobin) and the need for lime in making bone are well- 

 known examples of the necessity for mineral substances. Work done 

 in recent years has emphasized another important reason for sup- 

 plying mineral matters in the diet, and from vegetable as well as 

 animal food materials. It is now an accepted fact that the body 

 performs its functions best when the tissues and fluids are either 

 neutral or slightly alkaline and that different classes of food mate- 

 rials, after they have been digested, leave the tissues and fluids of 

 the body in different condition, some alkaline, some acid, and may, 

 therefore, be spoken of as potentially acid or potentially alkaline. 

 Many vegetables and fruits, owing to the presence of citric and 

 other similar acids, are not alkaline when eaten but are potentially 

 alkaline, because these acids leave behind an alkaline salt after 

 being burned in the body. Foods rich in protein, such as meat, 

 poultry, fish, and eggs, are potentially acid, because the sulphur and 

 phosphorus which they contain are not completely burned but are 

 partially left behind in the form of so-called fixed acids. It is 

 to neutralize such acid residue that the potash and other salts of 

 alkaline property supplied by fruits and roots and other vegetables 

 are so valuable. Expressed in everyday terms, the results of labora- 

 tory experiments show that when the diet contains such foods as 

 meat, eggs, and fish, a generous supply of vegetable foods should 

 be supplied also — an ample justification of the old household cus- 

 tom of serving potatoes, turnips, beets, and other vegetables in 

 abundance with meat. 



