Foods and the Theory of Digestion : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 9 of 27)

The body is constantly in need of new material. Oxidation, as shown in the preceding chapter, rapidly destroys substances at the cells, and these have to be replaced. Upon this renewal depends the supply of energy. Moreover, there is found to be an actual breaking down of the living material, or protoplasm, in the body. While this does not destroy the cells, as is sometimes erroneously stated, it reduces the quantity of the protoplasm and makes necessary a process of repair, or rebuilding, of the tissues. This also requires new material. Finally, substances, such as water and common salt, are required for the aid which they render 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: proteins (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.

proteins. - The proteins 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 protein, 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 protein which occurs in different grains. Again, certain vegetables, as beans, peas, and peanuts, are rich in a kind of protein which is called legumen.

proteins are compounds of carbon, hydrogen, oxygen, nitrogen, and a small percent of sulfur. Certain ones (the nucleoproteins from grains) also contain phosphorus. All of the proteins are highly complex compounds and form a most important class of nutrients.

Purposes of proteins. - The chief purpose of proteins 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 proteins in its body and then dies. In addition to rebuilding the tissues, proteins may also be oxidized to supply the body with energy.

Albuminoids form a small class of foods, of minor importance, which are similar to proteins 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 albuminoids foods. On account of the nitrogen which they contain, proteins and albuminoids are often classed together as nitrogenous foods.

Carbohydrates. - While the carbohydrates are not so essential to life as are the proteins, 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.

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. 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. 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.

Sugars. - There are several varieties of sugar, but the important ones used as foods fall into one or the other of two classes, known as double sugars (disaccharides) and single sugars. To the first class belong cane sugar, found in sugar cane and beets, milk sugar, found in sweet milk, and maltose, a kind of sugar which is made from starch by the action of malt. The important members of the second class are grape sugar, or dextrose, and fruit sugar, or laevulose, both of which are found in fruits and in honey.

The most important of all sugars, so far as its use in the body is concerned, is dextrose. To this form all the other sugars, and starch also, are converted before they are finally used in the body. The close chemical relation between the different carbohydrates makes such a conversion easily possible.

D.C. Heath and Co. - Publishers
Original copyright 1909