Definition: Amino acids are the chemical building blocks or sub-units of protein. There are 20 amino acids that make up protein and nine that are essential, which means that the body cannot make them and they must be ingested in the diet. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Amino acid
By Paul Rogers, About.com Guide
Updated February 13, 2007
About.com Health's Disease and Condition content is reviewed by our Medical Review Board
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