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(prō′tēn″, prōt′ē-ĭn) [proto- + -in] Any of a class of complex nitrogen-containing compounds synthesized by all living organisms and yielding amino acids when hydrolyzed. Dietary proteins provide the amino acids necessary for the growth and repair of animal tissue.
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COMPOSITION: All amino acids contain carbon, hydrogen, oxygen, and nitrogen; some also contain sulfur. About 20 different amino acids make up human proteins, which may contain other minerals such as iron or copper. A protein consists of from 50 to thousands of amino acids arranged in a specific sequence. The essential amino acids are those the liver cannot synthesize (tryptophan, lysine, methionine, valine, leucine, isoleucine, phenylalanine, threonine, arginine, and histidine); because they cannot be made by the body, they must be consumed as food. A protein containing all of them is called a complete protein. An incomplete protein lacks one or more of the essential amino acids. The nonessential amino acids are synthesized by the liver.
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SOURCES: Milk, eggs, cheese, meat, fish, and some vegetables such as soybeans are the best dietary sources of protein. Proteins are found in both vegetable and animal food sources. Many incomplete proteins are found in vegetables; they contain some but not all of the essential amino acids. A vegetarian diet can compensate for dietary protein deficiencies by combining vegetable groups that complement each other in their basic amino acid groups.
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Principal animal proteins are lactalbumin and lactoglobulin in milk; ovalbumin and ovoglobulin in eggs; serum albumin in serum; myosin and actin in striated muscle tissue; fibrinogen in blood; serum globulin in serum; thyroglobulin in thyroid; globin in blood; thymus histones in thymus; collagen and gelatin in connective tissue; collagen and elastin in connective tissue; and keratin in the epidermis. Chondroprotein is found in tendons and cartilage; mucin and mucoids are found in various secreting glands and animal mucilaginous substances; caseinogen in milk; vitellin in egg yolk; hemoglobin in red blood cells; and lecithoprotein in the blood, brain, and bile.
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FUNCTION: Ingested proteins are a source of amino acids needed to synthesize the body’s own proteins, which are essential for the growth of new tissue or the repair of damaged tissue; proteins are part of all cell membranes. Excess amino acids in the diet may be changed to simple carbohydrates and oxidized to produce adenosine triphosphate and heat; 1 g supplies 4 kcal of heat.
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Infants and children require from 2 to 2.2 g of dietary protein per kilogram of body weight each day for normal health and development. The World Health Organization recommends that healthy adults consume about 0.8 g of protein per kg of body weight daily. The calculation should be made on the basis of ideal body weight rather than the actual weight of the adult or child. High levels of exercise, menstruation, pregnancy, lactation, and convalescence from severe illness require ...