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Food is necessary for growth, repair, and energy; therefore the elements composing the body must be found in the food. The thirteen elements named are formed into chemical compounds by the vegetable and animal kingdoms to support the highest order of being, man. All food must undergo chemical change after being taken into the body, before it can be utilized by the body; this is the office of the digestive system.

Food is classified as follows:—

I. Organic 1. Proteid (nitrogenous or albuminous) 2. Carbohydrates (sugar and starch) 3. Fats and oils II. Inorganic 1. Mineral matter 2. Water

The chief office of proteids is to build and repair tissues. They furnish energy, but at greater cost than carbohydrates, fats, and oils. They contain nitrogen, carbon, oxygen, hydrogen, and sulphur or phosphorus, and include all forms of animal foods (excepting fats and glycogen) and some vegetable foods. Examples: milk, cheese, eggs, meat, fish, cereals, peas, beans, and lentils. The principal constituent of proteid food is albumen. Albumen as found in food takes different names, but has the same chemical composition; as, albumen in eggs, fibrin in meat, casein in milk and cheese, vegetable casein or legumen in peas, beans, and lentils; and gluten in wheat. To this same class belongs gelatin.