There are some persons, perhaps, who do not realize that beside cane sugar many kinds of carbohydrate occur in our food. Glucose or grape sugar, for example, occurs not only in the fruit indicated by its name, but also in other fruits, in corn, in onions, and in the common vegetables. Glucose is especially suited to act as nourishing food. In keeping with that fact our digestive juices convert most of the sugars we eat, if not all of them, into glucose, which is regularly present in our blood. It is unnecessary to enumerate all or even the more important compounds included in the carbohydrate group; but everyone should know that starch is its chief member, and that after being thoroughly digested starch enters the body as glucose and therefore serves the same purpose as sugar. The value of carbohydrates as a source of heat and energy may be accurately measured, and is technically expressed in terms of a unit, called the calorie. As the energy which our bodies require may be estimated in the same terms, it is possible to determine whether or not our food is equal to our wants. Very naturally the energy requirements of any individual are influenced by his weight and by the work he does. But we may take as a standard the results of an extensive study of American families which indicate that women require four-fifths as much energy-yielding food as men. It also seems safe to conclude that a woman weighing 130 pounds who does her own housework requires food every day having an energy-value of 2,500 calories; smaller women and those who do no work require somewhat less. In a mixed diet the chief source of this energy--and the source from which it is most economically obtained--is the carbohydrates. _Fat_ yields more energy and heat than does carbohydrate, bulk for bulk; but fat is burned by our tissues less readily. We instinctively avoid eating a great deal of this food-stuff; in the course of a day the average person consumes no more than one or two ounces. The natural aversion which many feel toward fat may possibly depend upon the difficulty with which they assimilate it. In colder climates, however, we know fat to be a staple article of diet; and it is not unlikely that the very conditions which make it necessary there explain the unusual tolerance for it. Fat is more than fuel. Deposited in our bodies, beneath the skin for example, it prevents the escape of heat that we generate and protects us against the penetration of cold. This food-stuff, therefore, contributes in several ways toward maintaining the temperature of the body at a constant level.