The chemical combination of oxygen with food elements and with the body tissue is known as _oxidation_. It is this chemical action of the oxygen on the food and on the tissues which produces heat and energy, either in muscle, gland, or nerve. This energy, in the muscle, expresses itself in movement; in the gland, in chemical action, and in the nervous system, by activity of brain or nerve centers. The nervous energy is closely allied to electrical force. Nature provides for a reserve of heat and energy, above the immediate needs, by storing a supply of heat-producing material which is utilized whenever the daily supply is insufficient or is lacking. Many hibernating animals store up sufficient fat in summer to provide heat for the entire winter. This fat would not last throughout the winter, however, were the animal active. Many individuals carry sufficient fat to supply all of their needs for months, even though all fat-building elements were omitted from the diet. The fact that more oxygen is required for combustion of fat than of starches and sugars is important for those who wish to call on the fats stored within the body for daily heat and energy and thus reduce in weight. If sufficient starches, sugars, and fats are not consumed in the body to supply the daily heat and energy released by exercise, the body calls on the reserve store in the tissues. If much fat or carbohydrates are consumed in the daily food this will be oxidized before the fat stored in the muscular tissue is called on. The scientific reduction of weight, therefore, lies in the regulation of the daily consumption of starches, sugars, and fats, and the oxidation of more of these substances through an increase in the daily exercise. Deep breathing of pure air should accompany all exercises to supply sufficient oxygen for combustion or oxidation. In warm weather little fat is needed for fuel, and Nature provides fresh green vegetables to replace the root vegetables of the cold weather, which, consisting largely of starches and sugars, are readily converted into heat. In cold weather, especially in high altitudes or latitudes, more fuel foods are required to keep the body warm and more fat is eaten. It must be remembered that anything which creates a greater activity of the tissues, such as muscular exercise, liberates a greater amount of heat. The reverse is also true. A decrease in the amount of muscular movement means a decrease in the liberation of heat. During exercise, a large amount of carbohydrates and fats are released by the movements and oxidized; the liberated heat is carried to all parts of the system and the temperature is raised. Food in the alimentary canal causes an activity in the glands of the digestive organs maintaining their temperature.