Essential amino acid

An essential amino acid, or indispensable amino acid, is an <a href="/facts/Amino_acid/WDxYwBny">amino acid</a> 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 <a href="/facts/Valine/uTMLXBG4">valine</a>, <a href="/facts/Isoleucine/sJW4dxnh">isoleucine</a>, <a href="/facts/Leucine/X7YrkvA5">leucine</a>, <a href="/facts/Methionine/7neydPKo">methionine</a>, <a href="/facts/Phenylalanine/eyojfW7t">phenylalanine</a>, <a href="/facts/Tryptophan/JcUxlxAh">tryptophan</a>, <a href="/facts/Threonine/7zffFQ6W">threonine</a>, <a href="/facts/Histidine/le7781NF">histidine</a>, and <a href="/facts/Lysine/CEetgA9L">lysine</a>.
Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe <a href="/facts/Catabolism/CFWUdphX">catabolic</a> distress. These six are <a href="/facts/Arginine/y49Hdf1e">arginine</a>, <a href="/facts/Cysteine/kbqvUm6d">cysteine</a>, <a href="/facts/Glycine/3nXI1hqW">glycine</a>, <a href="/facts/Glutamine/t94zX4SK">glutamine</a>, <a href="/facts/Proline/8oEC6pNH">proline</a>, and <a href="/facts/Tyrosine/RJhEgE4C">tyrosine</a>. Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body. These six are <a href="/facts/Alanine/fphQEdSc">alanine</a>, <a href="/facts/Aspartic_acid/9DFSFPa1">aspartic acid</a>, <a href="/facts/Asparagine/KYIft5Jd">asparagine</a>, <a href="/facts/Glutamic_acid/7oXbNcvu">glutamic acid</a>, <a href="/facts/Serine/CM6QWruE">serine</a>, and <a href="/facts/Selenocysteine/9ovfzrXW">selenocysteine</a> (considered the 21st amino acid). <a href="/facts/Pyrrolysine/6cEPrKY7">Pyrrolysine</a> (considered the 22nd amino acid), which is proteinogenic only in certain microorganisms, is not used by and therefore non-essential for most organisms, including humans.
The limiting amino acid is the essential amino acid which is furthest from meeting nutritional requirements. This concept is important when determining the selection, number, and amount of foods to consume: Even when total protein and all other essential amino acids are satisfied, if the limiting amino acid is not satisfied, then the meal is considered to be nutritionally limited by that amino acid.

Essential amino acid open-in-new

Essential amino acid