protein /prō″tē·in, prō″tēn/ [Gk, proteios, first rank] , any of a large group of naturally occurring complex organic nitrogenous compounds. Each is composed of large combinations of amino acids (usually 50 or more) containing the elements carbon, hydrogen, nitrogen, oxygen, and occasionally sulfur, phosphorus, iron, iodine, or other essential constituents of living cells. Twenty-two amino acids have been identified as vital for proper growth, development, and maintenance of health. The body can synthesize 13 of these, the nonessential amino acids, whereas the remaining 9 must be obtained from dietary sources and are termed essential. Protein is the major source of building material for muscles, blood, skin, hair, nails, and the internal organs. It is necessary for the formation of many hormones, enzymes, and antibodies and may act as a source of energy. Rich dietary sources are meat, poultry, fish, eggs, milk, and cheese, which are classified as complete proteins because they contain the nine essential amino acids. Nuts and legumes, including navy beans, chickpeas, soybeans, and split peas, are also good sources but are incomplete proteins because they do not contain all the essential amino acids in adequate amounts. Protein deficiency causes abnormal growth and tissue development in children, leading to kwashiorkor, whereas in adults it results in lack of vigor and stamina, weakness, mental depression, poor resistance to infection, impaired healing of wounds, and slow recovery from disease. Excessive intake of protein may in some conditions result in fluid imbalance.
