tyrosine are regarded by some authors (Boisen, Hvelplund, &
Weisbjerg, 2000) as semi-essential amino acids as they are synthesised
from methionine and phenylalanine, respectively. Therefore,
the total amino acid requirements should include the sum of
methionine + cystine (sulphur-containing amino acids) and phenylalanine
+ tyrosine (aromatic amino acids). Fulfilling the requirements
as above, equivalent to the summed quantities alone, may
not be sufficient because methionine and phenylalanine cannot be
synthesised from cystine and tyrosine, respectively (Boisen et al.,
2000).