Animal Proteins

The albumins in the ordinary diet are derived chiefly from animal foods. The white of egg is a very pure form of this compound, and eggs constitute one of the best sources of nitrogen in a compact and assimilable form, especially convenient as they are readily taken raw.

Casein, the chief protein of milk, represents the main source of this food element for infants and the sick. One quart of milk contains as much protein as six ounces of lean beeksteak, and is more valuable because of its rich supply of mineral matter, to say nothing of the fat and carbohydrate present.

Myosin is the typical protein in muscle tissue. After death it changes to a form called syntonin.

Fibrin and Serum Albumin, found in blood, occur to a slight extent along with myosin in meat.

The only albuminoid which requires consideration here is collagen, a protein found in all connective tissues, including the modified forms, such as cartilage and bone. This substance is changed by boiling to gelatin. This is further altered by the gastric juice to gelatoses and gelatin-peptones, and finally absorbed, but it lacks certain elements found in albumins and other proteins, and consequently cannot be relied upon exclusively as a source of nitrogen for the body. It can replace about two-thirds of the ordinary protein requirement.

Vegetable Proteins

These are chiefly globulins. The main representatives of the albumins in vegetable food are gluten of wheat and legumin of the legumes. A number of alcohol-soluble proteins occur, such as gliadin in wheat and zein in Indian corn. Vegetables contain a large number of nitrogenous substances which are not proteins. Thus while mushrooms contain much nitrogen, little of the latter is in a form which can be absorbed.

Function Of Proteins

Proteins are both body-builders and energy-producers, but are little utilized as fuel when carbohydrates and fats are well represented in the diet. As a fuel, they tend to burn up rapidly, and hence are not economical, and since in health only a small proportion of nitrogen is used from day to day, the part not needed being promptly excreted, it is not advantageous to have the diet largely composed of this food principle. During the periods of rapid growth, in the prenatal, infantile and adolescent states, when the body is forming new tissue at an unusual rate, there must be a liberal nitrogen supply. This is also true in convalescence from wasting disease, and sometimes after excessive physical exertion, when the muscle tissue is actually increasing. In health, when fats and carbohydrates are liberally provided, so that the protein is not required as fuel, a comparatively small amount is needed for repair of tissue. This is especially true in old age.

Nitrogenous Extractives

Substances found in muscle juice, consisting chiefly of creatin, creatinin, and purins (uric acid and related substances), are valuable only as stimulants. They give sapidity to meats and hence are appetizing ; possess the power of stimulating the flow of gastric juice, and by their influence on the nervous system, gently increase the activity of the heart. For this reason, beef tea, beef extracts, etc., are of value to the sick.