Sugars are crystallisable carbohydrates in which oxygen and hydrogen exist in proportion to form water. There are many varieties, of which the commoner contained in food or used as an adjunct to diet are cane sugar, saccharose or sucrose, grape sugar, dextrose or glucose, levulose or fruit sugar, and sugar of milk or lactose. Inosite, mannite, dextrin, sugar of malt or maltose, honey, a sweet nitrogenous substance called saccharin, and fruit sugar or levulose (diabetin) are also used. Sugar may be derived from the stems of plants, as in the case of the sugar cane, or the palm, from tubers like the beet, from maple-tree sap, and from other vegetable growths.

The sugars present slight differences in their physical properties, such as specific gravity, solubility, and effect upon polarised light. They also differ in sweetness of taste and in digestibility.

As foods, sugars have essentially the same uses as starches (see Farinaceous Foods, p. 137), for all starch must be converted into dextrin or sugar before it can be assimilated. For this very reason, sugars, although they form an excellent class of food, producing force and heat and fattening the body, are not absolutely necessary for the maintenance of health if starches or fats are eaten. Recent experiments in the German, British, and other armies show that a liberal allowance of sugars in the diet tends, during manoeuvres, to maintain strength, lessen hunger and thirst, and lessen the liability to heat exhaustion.

They possess additional properties, in that they have a more agreeable flavour than starches, are more satisfying to the palate, and they have antiseptic and preservative power. Hence sugars and sirups are extensively employed to preserve fruits either in solution or in dried form, like "candied" cherries, ginger, etc.

When taken for food, sugar is quickly soluble, and on this account taxes the digestive organs but little. Cane sugar, however, needs to be converted into grape sugar before it can be absorbed and assimilated, and grape sugar, which needs no change, is therefore sometimes spoken of as a predigested carbohydrate.

Between seven and eight million tons of sugar are consumed each year in the world at large. The English-speaking nations are the largest consumers. In 1895 the per capita consumption in England was 86 pounds as against 30 pounds in Germany, France, and Holland, and 7 pounds in Italy, Greece, and Turkey (Mary Hinman Abel). Cane sugar was originally used exclusively in preparation of medicines, not as a food.

Many persons acquire an inordinate fondness for sugar, and continued overindulgence in this food is very sure to give rise to flatulent dyspepsia, constipation, and disorders of assimilation and nutrition. It may even cause functional glycosuria. (See Diabetes).

Sugar is very fattening. In the West Indies the negroes always grow fat in the sugar season, when they chew the cane in the fields.

Sugars are emphatically force producers. Chauveau and Kauf-mann have demonstrated that during muscular activity the consumption of sugar in the body is increased fourfold. If one pound of sugar were burned so as to utilise all the heat, it would raise five gallons of water from the freezing to the boiling point (A. C. True).

Harley found experimentally that the muscle-energy producing effect of sugar is so great that two hundred grammes (seven ounces) added to a small meal increased the total amount of work done from 6 to 30 per cent, and that when sugar was added to a large meal it increased this total from 8 to 16 per cent.