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Amino acids are the building blocks of protein.
Twenty amino acids are needed to build the various proteins used in the growth, repair, and maintenance of body tissues. The classification of an amino acid as essential or nonessential does not reflect its importance, because all twenty amino acids are necessary for health. Instead, this classification system simply reflects whether or not the body is capable of manufacturing a particular amino acid. The essential amino acids are isoleucine , leucine , lysine , methionine , phenylalanine , threonine, tryptophan , and valine . Another amino acid, histidine , is considered semiessential because the body does not always require dietary sources for it.