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nder in the general work of the body. Since these are constantly being lost in one way or another, they also must be replaced. These different needs of the body for new materials are supplied through *The Foods.*--Foods are substances that, on being taken into the healthy body, are of assistance in carrying on its work. This definition properly includes oxygen, but the term is usually limited to substances introduced through the digestive organs. As suggested above, foods serve at least three purposes: 1. They, with oxygen, supply the body with energy. 2. They provide materials for rebuilding the tissues. 3. They supply materials that aid directly or indirectly in the general work of the body. *The Simple Foods, or Nutrients.*--From the great variety of things that are eaten, it might appear that many different kinds of substances are suitable for food. When our various animal and vegetable foods are analyzed, however, they are found to be similar in composition and to contain only some five or six kinds of materials that are essentially different. While certain foods may contain only a single one of these, most of the foods are mixtures of two or more. These few common materials which, in different proportions, form the different things that are eaten, are variously referred to as simple foods, food-stuffs, and _nutrients_, the last name being the one generally preferred. The different classes of nutrients are as follows: Nutrients: Proteids (Albuminoids) Carbohydrates Fats Mineral salts Water It is now necessary to become somewhat familiar with the different nutrients and the purposes which they serve in the body. *Proteids.*--The proteids are obtained in part from the animal and in part from the plant kingdom, there being several varieties. A well-known variety, called _albumin_, is found in the white of eggs and in the plasma of the blood, while the muscles contain an abundance of another variety, known as _myosin_. Cheese consists largely of a kind of proteid, called _casein_, which is also present in milk, but in a more diluted form. If a mouthful of wheat is chewed for some time, most of it is dissolved and swallowed, but there remains in the mouth a sticky, gum-like substance. This is _gluten_, a form of proteid which occurs in different grains. Again, certain vegetables, as beans, peas, and peanut
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