The monosaccharides are all soluble, crystallizable, diffusible substances, unaffected by digestive enzymes, and if not attacked by bacteria in the digestive tract, they are absorbed and enter the blood current unchanged. All of the three hexoses described below are susceptible to alcoholic fermentation, and are utilized for the production of glycogen in the animal body and the maintenance of the normal glucose content of the blood. A few of the leading facts regarding the occurrence in food and the nutritive relations of individual monosaccharides are given below.

Glucose (d.glucose, dextrose, grape sugar, starch sugar, diabetic sugar) is widely distributed in nature, occurring in the blood of all animals in small quantity (usually about 0.1 per cent) and more abundantly in fruits and plant juices, where it is usually associated with fructose and sucrose. It is especially abundant in grapes, of which it often constitutes 20 per cent or more of the weight of the fresh fruit and considerably more than half of the solid matter. Sweet corn, onions, and unripe potatoes are among the common vegetables containing considerable amounts of glucose.

Glucose is also obtained from many other carbohydrates by hydrolysis either by acids or by enzymes, and thus becomes the principal form in which the carbohydrate of the food enters into the animal economy. In the healthy animal body the glucose of the blood is constantly being burned and replaced. In diabetes the body loses to a greater or less degree the power to burn glucose, which then accumulates in excessive amount in the blood, from which it escapes through the kidneys. A temporary and usually unimportant loss of glucose in the urine may occur as the result of feeding large quantities at a time. This condition is known as alimentary glycosuria. Ordinarily any surplus of glucose absorbed from the digestive tract is converted into glycogen which, as described beyond, is readily reconvertible into glucose. Thus, while other carbohydrates occur in food in greater quantity, glucose occupies a very prominent place, partly because it is more widely distributed than any other carbohydrate, being a normal constituent of both plants and animals, and partly because it is the form in which most of the carbohydrate material of the food comes ultimately into the service of the body tissues (Chapter V (The Fate Of The Foodstuffs In Metabolism Carbohydrates. Oxidation Of Carbohydrate)). It is estimated that over half the energy manifested in the human body is derived from the oxidation of glucose.

It is not to be inferred from the foregoing statement that the body obtains the energy of the glucose by oxidizing it directly as such. The aldehydic properties of glucose make it susceptible to direct oxidation; but, as the elaborate researches of Nef have shown, the glucose molecule in alkaline solution breaks up to form simpler substances of 2, 3, and 4 carbon atoms which are more readily oxidizable than glucose itself. There is strong evidence (Chapter V (The Fate Of The Foodstuffs In Metabolism Carbohydrates. Oxidation Of Carbohydrate)) that in the body tissues glucose is broken into 3-carbon molecules, which latter readily undergo oxidation.

Fructose (d.fructose, fruit sugar, levulose) occurs with more or less glucose in plant juices, in fruits, and especially in honey, of which it constitutes about one half the solid matter. It results in equal quantity with glucose from the hydrolysis of cane sugar and in smaller proportion from some other less common sugars. Fructose may occur in normal blood, but probably only in insignificant amounts. It serves, like glucose, for the production of glycogen; and the fructose which enters the body either through being eaten as such or as the result of the digestion of cane sugar is mainly changed to glycogen on reaching the liver, so that it does not enter largely into the blood of the general circulation. Glucose and fructose are partially convertible, either one into the other, under the influence of very dilute alkalies. It is not surprising, therefore, that fructose should be converted in the liver into glycogen, which on hydrolysis yields glucose.

Galactose is not found free in nature, but results from the hydrolysis of milk sugar, either by acids or by digestive enzymes, and appears to have the same power as glucose and fructose to promote the formation of glycogen in the animal body. Anhydrides of galactose, known as galactans, occur quite widely distributed in plants; and galactosides, which are compounds containing galactose in chemical combination with radicles of other than carbohydrate nature, are found in the animal body, notably as constituents of the brain and nerve tissues.