Nitrogenous substances are those which contain nitrogen, an element which is essential to the life of every cell.

Classification

In older classifications, the term Protein is used to include all food substances containing nitrogen, this class being further subdivided into (1) Proteids, those proteins which can alone maintain the nitrogen equilibrium of the body; (2) Albuminoids or Gelatinoids, those which contain nitrogen in a form which can replace other proteins only to a limited extent; (3) Extractives, containing nitrogen in a form unable to make good the body loss of nitrogen. These extractives are properly food accessories.

Proteins are now defined as compounds of carbon, hydrogen, oxygen, nitrogen, sulphur, and sometimes phosphorus, which contain nitrogen in a form which the body can use. They are variously classified, but a simple division is one into (1) Simple Proteins, (2) Compound Proteins, and (3) Albuminoids. Examples of simple proteins are albumins, globulins, and certain alcohol-soluble proteins found in cereals, as the gliadin of wheat; of compound proteins, the nucleo-protein of organs consisting chiefly of cells, as pancreas or sweetbread, and the haemoglobin of the blood; of albuminoids, the familiar product, gelatin, formed from the collagen of connective tissue ;

Source

The proteins of the diet are derived both from the animal and vegetable kingdoms. The principal animal proteins are obtained from meats, fish, eggs, and milk. Meat and fish proteins are derived principally from the muscles; egg proteins from both the white and yolk; milk proteins chiefly from the casein, which forms the main constituent of cheese and curds. Animal foods are much richer in protein than plant foods. The only substances of vegetable origin which can compare with animal nutrients in this respect are the legumes (peas, beans, lentils, peanuts, etc.), and certain nuts, such as almonds, pine nuts, cocoa beans (the source of cocoa and chocolate). The entire grain of some cereals possesses a high percentage of protein; this is particularly true of so-called hard wheat, and in lesser degree of oats, Indian corn, rye, and barley. As a rule, flour and meal are about half as rich in protein as the dried grains from which they are prepared, though oatmeal and Indian meal lose but little through milling.

Animal foods are usually digested with ease, and require less cooking and less mastication to insure good utilization; in the form of meat, they are liked because of their high flavor and stimulating properties. In vegetable foods, on account of the presence of cellulose, some of the protein is likely to escape digestion and absorption. For the invalid or convalescent, certain animal foods are preferred as a source of protein, as being more concentrated, and easier of digestion. Of ordinary vegetable food rich in protein, bread, either stale and dry, or thoroughly toasted, is perhaps the most suitable for invalids. Starchy roots and tubers (as potatoes), and green vegetables and fruits, though valuable for other dietetic reasons, are very poor in protein.