Food is anything which nourishes the body. Thirteen elements enter into the composition of the body: oxygen, 62½%; carbon, 21½%; hydrogen, 10%; nitrogen, 3%; calcium, phosphorus, potassium, sulphur, chlorine, sodium, magnesium, iron, and fluorine the remaining 3%. Others are found occasionally, but, as their uses are unknown, will not be considered.
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 can 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 gelatine.

The chief office of the carbohydrates is to furnish energy and maintain heat. They contain carbon, hydrogen, and oxygen, and include foods containing starch and sugar. Examples: vegetables, fruits, cereals, sugars, and gums.
The chief office of fats and oils is to store energy and heat to be used as needed, and constitute the adipose tissues of the body. Examples: butter, cream, fat of meat, fish, cereals, nuts, and the berry of the olive-tree.
The chief office of mineral matter is to furnish the necessary salts which are found in all animal and vegetable foods. Examples: sodium chloride (common salt); carbonates, sulphates and phosphates of sodium, potassium, and magnesium; besides calcium phosphates and iron.
Water constitutes about two-thirds the weight of the body, and is in all tissues and fluids; therefore its abundant use is necessary. One of the greatest errors in diet is neglect to take enough water; while it is found in all animal and vegetable food, the amount is insufficient.

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