[pg 118]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[pg 119] 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.
Albuminoids form a small class of foods, of minor importance, which are similar to proteids in composition, but differ from them in being unable to rebuild the tissues. Gelatin, a constituent of soup and obtained from bones and connective tissue by boiling, is the best known of the albuminoid foods. On account of the nitrogen which they contain, proteids and albuminoids are often classed together as nitrogenous foods.
Carbohydrates.—While the carbohydrates are not so essential to life as are the proteids, they are of very great value in the body. They are composed of carbon, hydrogen, and oxygen, and are obtained mainly from plants. There are several varieties of carbohydrates, but they are similar in composition. All of those used as food to any great extent are starch and certain kinds of sugar.
[pg 120]Starch is the carbohydrate of greatest importance as a food, and it is also the one found in the greatest abundance. All green plants form more or less starch, and many of them store it in their leaves, seeds, or roots (Fig. 60). From these sources it is obtained as food. Glycogen, a substance closely resembling starch, is found in the body of the oyster. It is also formed in the liver and muscles of the higher animals, being prepared from the sugar of the blood, and is stored by them as reserve food (Chapter XI). Glycogen is, on this account, called animal starch. Starch on being eaten is first changed to sugar, after which it may be converted into glycogen in the liver and in the muscles.
