Carbohydrates and fats are each composed of carbon, hydrogen, and oxygen, the difference being that there is less oxygen in fat, hence, more oxygen from the air is required for combustion of fat than for carbohydrates. One pound of starch requires one and one-fifth pounds of oxygen for perfect combustion, while one pound of suet requires three pounds of oxygen. For this reason the Eskimo, who depends largely upon the fats for body heat and energy, must have plenty of fresh air. One ounce of fat yields two and one-half times as much energy as an ounce of sugar or starch. If sufficient fat is not consumed, or is not formed from the carbohydrates (starches and sugars), a certain portion of the protein of the body is converted into fat and used in energy. When the food supply is short, or much energy is called for, the surplus supply of carbohydrates is first used, and, if the carbohydrates are not sufficient, the proteins are used. Of the proteins, the gelatinoids are used first, and next the albuminoids, or tissue builders. If the demand, either in mental or physical energy, exceeds the daily supply for a length of time, the body becomes lean. In warm weather little fat is needed for fuel and nature provides fresh, green vegetables to replace the root vegetables of the winter, 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 in winter. Those who store up an abundance of fat suffer most from a rise in temperature, because combustion not only creates heat, but heat also aids combustion. Fats are not digested in the stomach. The connective tissue about the fat is digested here, and the fat is passed on into the small intestine, where it is acted upon by the carbonates and by lipase, one of the enzymes of the intestinal juices. These first change the fat into an emulsion and then into a form of soap and glycerine. In this saponified form, it is in condition to be absorbed into the circulation and carried out to the tissues, where it is assimilated and used in energy; a similar chemical change is produced in the conversion of oil into soap. Common examples of fats are butter, cream, the fat of meats and of nuts, and the oil of grains and seeds,—notably the cocoanut, olive, and, of the grains, oatmeal.