These are rich in protein which contains nitrogen as a distinguishing element. Proteins also contain carbon, hydrogen and oxygen. Most proteins contain sulphur and some other elements. Chief among the protein foods are:

Fruits--olives, avocados.
Nuts--all kinds except chestnuts, cocoanuts and acorns.
Grains--all kinds.
Legumes--beans, peas, lentils, peanuts.
Eggs--all varieties.
Lean meats of all kinds--including fish, poultry, etc.

Classification Of Proteins

The plant and animal proteins most commonly met with in physiology are collectively designated "native proteins." This is a more or less arbitrary classification for which there seems to be little need. Formerly there existed considerable confusion in naming and classifying proteins and we still meet some differences of usage in the literature of the subject, but the tendency is more and more to follow the recommendations of the joint committee of the American Physiological Society and the American Society of Biological Chemists made in 1907. The following classification follows this recommendation in general:

1. Simple Proteins: These are defined as proteins that yield only amino acids or their derivatives on digestion or hydrolysis. This definition, however, is faulty as many of these proteins have been shown to have some carbohydrate material in their composition. Among these are:

A. Albumens: egg albumen, lactalbumen, serum albumen are of animal origin. Albumens of plant origin are legumelin of peas and leucosin of wheat.

B. Globulins: egg globulin, lactoglobulin, serum globulin, fibrinogen of the blood, myosin (muscle globulin) are of animal origin. Of plant origin are legumin of peas, tuberin of potatoes, edestin of wheat and seeds, and excelsin of the Brazil nut.

D. Albuminoids, or Scleroproteins: These are found in the connective tissues of the body. Among these are collagen which forms the ground substance of bone and cartilage and in the white fibrous or inelastic connective tissue (tendons, aponeuroses, ligaments, dura mater, pericardium, fascia); elastin found in the yellow (elastic) connective tissues in the walls of the blood vessels (especially arteries), and of the air tubes of the lungs; keratin, found in the outer layer of the skin and in nails, hair, feathers, hooves, etc.

E. Glutelins: Glutenin of wheat is an example of the glutelins.

F. Prolamines: Gliadin of wheat, zein of corn, and hordein of barley are examples.

2. Compound (complex or conjugated) Proteins: These are composed of a simple protein united with some other substance and are named according to the character of the other substance, as:

A. Chromoproteins: a simple protein united with a pigment--hemoglobin is an example.

B. Nucleoproteins: One or more simple proteins united with nucleic acid found chiefly in the nuclei of the cells, but also in the germ of wheat and in the thymus gland.

C. Phosphoproteins: Proteins containing phosphorus, as ovo-vitelin (vitelin of egg yolk), casein or caseinogen of milk.

D. Glycoproteins: A protein united with carbohydrate as mucin in saliva and mucus.

E. Lecithoproteins, or Lecithans: a protein united with lecithin, a compound of fat containing phosphorus and nitrogen and found in the brain, seminal fluid, and in many plants.

3. Derived Proteins: Proteins produced from the previously named proteins in various ways, but chiefly by means of digestion or hydrolysis, that is, by the action of digestive enzymes and by acids and alkalies. These are:

A. Coagulated protein, formed by heat.

B. Acid Metaproteins formed by the action of acids.

C. Alkali Metaproteins formed by the action of alkalis.

D. Casein, Fibrin.

E. Secondary Derivatives formed in the process of digestion--peptoses or albumoses, peptones and peptoids or peptids.