They are divided into two classes, Fats and Carbohydrates. The carbohydrates embrace the sugars and starches and include such substances as the starches of vegetables and grains (notably corn, rice, wheat, and the root vegetables), and the sugar of milk, of fruits, vegetables, and the sap of trees. Their chief office is to create energy. They are almost entirely absent from meat and eggs, the animal having converted them into fats.
Carbohydrates are easily digested.
Fat
Fat is the most concentrated form of fuel and is readily oxidized. It is almost pure carbon, hence less chemical work is required to convert it into fuel, but more oxygen is needed. A pound of fat has about three times as much fuel value as a pound of wheat flour, which consists largely of starch.
Fat forms about fifteen per cent of the weight of the normal body, and it has about twice the fuel value of carbohydrates.
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.