<P> An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized de novo (from scratch) by the organism, and thus must be supplied in its diet . The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine . </P> <P> 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 catabolic distress . These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine . Five amino acids are dispensable in humans, meaning they can be synthesized in sufficient quantities in the body . These five are alanine, aspartic acid, asparagine, glutamic acid and serine . </P> <Table> <Tr> <Th> Essential </Th> <Th> Conditionally essential </Th> <Th> Non-essential </Th> </Tr> <Tr> <Td> Histidine (H) </Td> <Td> Arginine (R) </Td> <Td> Alanine (A) </Td> </Tr> <Tr> <Td> Isoleucine (I) </Td> <Td> Cysteine (C) </Td> <Td> Aspartic acid (D) </Td> </Tr> <Tr> <Td> Leucine (L) </Td> <Td> Glutamine (Q) </Td> <Td> Asparagine (N) </Td> </Tr> <Tr> <Td> Lysine (K) </Td> <Td> Glycine (G) </Td> <Td> Glutamic acid (E) </Td> </Tr> <Tr> <Td> Methionine (M) </Td> <Td> Proline (P) </Td> <Td> Serine (S) </Td> </Tr> <Tr> <Td> Phenylalanine (F) </Td> <Td> Tyrosine (Y) </Td> <Td> Selenocysteine (U) </Td> </Tr> <Tr> <Td> Threonine (T) </Td> <Td> </Td> <Td> Pyrrolysine * (O) </Td> </Tr> <Tr> <Td> Tryptophan (W) </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Valine (V) </Td> <Td> </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Essential </Th> <Th> Conditionally essential </Th> <Th> Non-essential </Th> </Tr>

The number of essential amino acid found in man
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