Proteins form the fundamental structure of cells and are the most abundant of all organic compounds in the body. Most proteins are found in muscle, with the remainder in other cells, blood, body fluids, and body secretions. Enzymes and many hormones are proteins. Proteins are composed of amino acids and have molecular weights of a few thousand to a few hundred thousand. More than 20 common amino acids form the building blocks for proteins (Table 27.7). Of these, nine are considered essential and must be provided by the diet. Although the nonessential amino acids are also required for normal protein synthesis, the body can synthesize them from other amino acids.
Complete proteins are those that can supply all of the essential amino acids in amounts sufficient to support normal growth and body maintenance. Examples are eggs, poultry, and fish. The proteins in most vegetables and grains are called incomplete proteins because they do not provide all of the essential amino acids in amounts sufficient to sustain normal growth and body maintenance. Vegetarians need to eat a variety of vegetables and soy protein to avoid amino acid deficiencies.
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