Amino acids are classified as either essential or non essential essential amino acids cannot be synthesised in the body and must be obtained from the diet whereas non essential amino acids whilst still essential for health can be synthesised in the body and are therefore not classified as 39essential39 phenylalaine is an essential amino acid and is the precursor for the amino acid tyrosine like tyrosine it is the precursor for the neurotransmitters called catecholamines adrenali