An essential amino acid , or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms, the nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.  Dietary Reference Intakes: The Essential Guide to Nutrient Requirements . Institute of Medicine's Food and Nutrition Board. usda.gov
|title=Adult amino acid requirements: the case for a major revision in current recommendations |journal=J. Nutr. |volume=124 |issue=8 Suppl |pages=1517S–1523S |year=1994 |pmid=8064412 URL |doi=10.1093/jn/124.suppl_8.1517S </ref> Dietary Reference Intakes: The Essential Guide to Nutrient Requirements . Institute of Medicine's Food and Nutrition Board. usda.gov
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An essential amino acid , or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms, the nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.
|title=Adult amino acid requirements: the case for a major revision in current recommendations |journal=J. Nutr. |volume=124 |issue=8 Suppl |pages=1517S–1523S |year=1994 |pmid=8064412 URL |doi=10.1093/jn/124.suppl_8.1517S
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