The word 'protein' is derived from the Greek word protos meaning 'first'. Protein is the basic chemical unit of living organisms and is essential for nutrition, growth and repair. Protein’s contain nitrogen but the nutritive value of protein-rich foods does not depend upon the total nitrogen content but on the constituent- amino acids. Gelatin, for example, is rich in nitrogen but does not contain all the essential amino acids and so is of little nutritive value.
Proteins constitute about 15% of the body weight. The liquid part of blood (plasma) contains over 100 different proteins, each with a specific function or functions.
Amino acids are small units that combine to form a protein molecule. Plants synthesize amino acids with the help of bacteria and fungi from: (i) soil, which supplies the necessary nitrogen and sulphur (ii) water, which provides oxygen and hydrogen; and (iii) atmospheric carbon dioxide, which supplies carbon and oxygen. Animals cannot synthesize amino acids from basic elements but derive them from ingested plants (or animals). Thus, the primary source of all proteins is the vegetable kingdom.
Function of Proteins
Proteins are necessary for the growth and repair of tissues. Protein deficiency occurs due to: (i) low intake because of poverty or loss of appetite; (ii) poor digestion and absorption in disease; (iii) reduced synthesis when the absorbed amino acids are not converted to albumin; and (iv) excessive loss in kidney diseases, cirrhosis of the liver with ascites, chronic dysentery, and profuse expectoration in bronchiectasis or lung abscess. The intestinal epithelium is shed at regular intervals and proteins are required for its regeneration. Unless the mucous membrane of the intestine is healthy, digestion and absorption are impaired.
Bed-rest decreases body protein synthesis when dietary protein is low. This decrease is prevented by higher intake of dietary protein.
When protein deficiency occurs, there is a decrease in plasma albumin. The globulin fraction does not reflect protein deficiency.
Protein deficiency causes Weakness, anemia, swelling (edema), delayed wound healing, and decreased resistance to infection, because antibody formation is reduced. Protein depletion interferes with immune reactions; the resulting false-negative reaction with the tuberculin test becomes positive in two weeks with a high calorie, high-protein diet. Deficient protein intake has also been associated with-a high incidence of toxemia of pregnancy.
Proteins Food Sources
Humans consume many foods that contain proteins or amino acids. One normally need not worry about getting enough protein or amino acids in the typical American diet. Foods from animal sources are typically rich in essential amino acids. These include chicken, fish, eggs, dairy products, beef, and pork. With the increasing emphasis on vegetarian diets, plant sources of protein are gaining in popularity. Such sources include dried beans (black, kidney, northern, red, and white beans), peas, soy, nuts, and seeds.
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