<P> (*) Pyrrolysine, sometimes considered the "22nd amino acid", is not used by humans . </P> <P> Eukaryotes can synthesize some of the amino acids from other substrates . Consequently, only a subset of the amino acids used in protein synthesis are essential nutrients . </P> <P> Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years . The following table lists the WHO and United States recommended daily amounts currently in use for essential amino acids in adult humans, together with their standard one - letter abbreviations . </P> <Table> <Tr> <Th> Amino acid (s) </Th> <Th> WHO mg per kg body weight </Th> <Th> WHO mg per 70 kg </Th> <Th> US mg per kg body weight </Th> </Tr> <Tr> <Td> H Histidine </Td> <Td> 10 </Td> <Td> 700 </Td> <Td> 14 </Td> </Tr> <Tr> <Td> I Isoleucine </Td> <Td> 20 </Td> <Td> 1400 </Td> <Td> 19 </Td> </Tr> <Tr> <Td> L Leucine </Td> <Td> 39 </Td> <Td> 2730 </Td> <Td> 42 </Td> </Tr> <Tr> <Td> K Lysine </Td> <Td> 30 </Td> <Td> 2100 </Td> <Td> 38 </Td> </Tr> <Tr> <Td> M Methionine <P> + C Cysteine </P> </Td> <Td> 10.4 + 4.1 (15 total) </Td> <Td> 1050 total </Td> <Td> 19 total </Td> </Tr> <Tr> <Td> F Phenylalanine <P> + Y Tyrosine </P> </Td> <Td> 25 (total) </Td> <Td> 1750 total </Td> <Td> 33 total </Td> </Tr> <Tr> <Td> T Threonine </Td> <Td> 15 </Td> <Td> 1050 </Td> <Td> 20 </Td> </Tr> <Tr> <Td> W Tryptophan </Td> <Td> </Td> <Td> 280 </Td> <Td> 5 </Td> </Tr> <Tr> <Td> V Valine </Td> <Td> 26 </Td> <Td> 1820 </Td> <Td> 24 </Td> </Tr> </Table>

Where are the 9 essential amino acids found