Essential amino acid needed for the normal growth of infants and children. It is also needed for normal protein use all through life. Precursor to tyrosine which is used to manufacture certain hormones (epinephrine, norepinephrine, dopa, dopamine) which are important for the transmission of nerve impulses. As neurotransmitters, these substances are believed to influence mood, appetite control and memory. It is found in large amounts in milk, eggs, and other common foods.