L-phenylalanine is an essential amino acid which can be convertedto L-tyrosine by a complex biochemical process which takes place in theliver. L-tyrosine can be converted by neurons in the brain to dopamineand norepinephrine (noradrenaline), hormones which are depleted by stress,overwork and certain drugs. By replenishing norepinephrine in the brain,mental energy levels are enhanced and a feeling of contentment oftenoccurs. Because of the liver conversion necessary for L-phenylalanineto have these effects, L-tyrosine is often faster acting. In addition,the conversion step from L-tyrosine to norepinephrine may be enhancedif the cofactors (vitamins B6 and C) are included.99-107*

Cells in the adrenal medulla synthesize and secrete norepinephrineand epinephrine. Since both norepinephrine and epinephrine can causesmooth muscle (arterial) contraction, care with blood pressure shouldbe taken when supplementing L-phenylalanine or L-tyrosine.*

L-tyrosine is also a precursor of two thyroid hormones. D-phenylalanineis unique among amino acids because the D-form (normally not foundin life) has biological activity that is different from its L-stereoisomer.D, L-phenylalanine is a 50/50 mixture of the two stereoisomers.

Cancer patients should avoid taking L-phenylalanine and L-tyrosine.Certain cancers, such as melanoma, depend on these amino acids to fueltheir growth. Supplemental use of L-phenylalanine and L-tyrosine mayraise or normalize blood pressure. Insomnia may occur from over stimulationif taken too close to bedtime. Individuals with the rare metabolic disorderPhenylketonuria should avoid phenylalanine.