The simple sugars that are most important in human nutrition are dextrose (grape sugar), levulose (fruit sugar), and galactose (from milk sugar). These are hexose (six carbon) sugars. The pentoses are also simple sugars; but, as we shall see, they scarcely occur in nature, being obtained chiefly by splitting up certain gums.

68. Dextrose

An important simple sugar is dextrose or grape sugar, or what is known in the market as glucose. Excepting in the hands of the chemist, it is seldom seen as crystals, although these appear in the "candying" of honey and-raisins. Its commercial forms are as a constituent of molasses and the sirups. Dextrose is found in practically the same plants that contain saccharose, such as Sorghum, maize, and the fruits. So far as known, it is always associated with some other sugar. On account of its difficult crystallization and a lower degree of sweetness, it is less valuable for commercial purposes than cane sugar. That which appears in the market is largely made from starch by the use of an acid, and it is often utilized for adulterating the more costly saccharose. Many seem to regard glucose as a substance deleterious to health, but in consideration of the fact that, in digestion, starch and most other sugars are reduced to this compound before entering the circulation of the animal, this view does not seem to be sustained. In fact, there is a lack of evidence to show the ill effect of glucose either upon man or animals.

69. Levulose

Another simple sugar is levulose or fruit sugar, the composition of which is identical with dextrose, but which has a different chemical constitution. It accompanies dextrose, and is found in some fruits in considerable quantities, and especially in honey. It is as sweet as cane sugar, but does not form crystals with the same readiness.

70. Galactose

This is obtained by a cleavage of milk sugar (see later) into this sugar and dextrose. It may also be obtained from certain gums.

71. The Pentoses

There are several pentoses, none of which occur in nature, but which are prepared by chemical methods from the gums. Thus, from gum arabic containing arabin, arabinose may be obtained, and froip zylin (wood gum) zylose may be prepared. Certain of these sugars have been isolated from animal compounds. They also have been found to appear in human urine. They are of great importance in the nutrition of herbivorous animals, but appear in human food only to a limited extent.