4 BULLETIN 503, U. S. DEPARTMENT OF AGRICULTURE. 
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. 
le 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 physiological 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. 
