*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 peanuts, are rich in a kind of proteid which is called _legumen_. Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small per cent of sulphur. Certain ones (the nucleo-proteids from grains) also contain phosphorus. All of the proteids are highly complex compounds and form a most important class of nutrients. *Purposes of Proteids.*—The chief purpose of proteids in the body is to rebuild the tissues. Not only do they supply all of the main elements in the tissues, but they are of such a nature chemically that they are readily built into the protoplasm. They are absolutely essential to life, no other nutrients being able to take their place. An animal deprived of them exhausts the proteids in its body and then dies. In addition to rebuilding the tissues, proteids may also be oxidized to supply the body with energy.