Amino acid, essential: An amino acid that cannot be made by humans and so is essential to the human diet. There are 9 essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Quick GuideFat-Burning Foods in Pictures: Blueberries, Green Tea, and More
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Last Editorial Review: 6/14/2012