Phenylalanine is one of the amino acids which the body cannot manufacture itself, but must acquire from food. It is abundant in meats and cheese. Phenylalanine is a precursor of tyrosine, and together they lead to the formation of thyroxine or thyroid hormone, and of epinephrine and norepinephrine which is converted into a neurotransmitter, a brain chemical which transmits nerve impulses. This neurotransmitter is used by the brain to manufacture norepinephrine which promotes mental alertness, memory, elevates mood, and suppresses the appetite very effectively.
In one study, 100-500 milligrams of phenylalanine taken every day for two weeks completely eliminated patients depression. These people where depressed from a variety of causes, including drug abuse and schizophrenia and some from no apparent cause, and the amino acid seemed to work especially well for them all.
Along with another amino acid, tryptophan, phenylalanine governs the release of an intestinal hormone called cholecystokinin, known as CCK. This hormone signals the brain to feel satisfied after eating. People given CCK stop eating and feel full sooner. Various studies have shown Phenylalanine's ability to decrease chronic back and dental pain and the pain associated with migraines and menstruation in a non-toxic and non-addictive manner.
Phenylalanine comes in two forms which are mirror images of each other: L-phenylalanine which has a nutritional value, and D-phenylalanine which has painkilling and depression alleviating properties which are attributed to its ability to block the breakdown of enkephalins, the brains natural pain killers. A third form, DL-phenylalanine, is a 50/50 mixture of these two forms. Phenylalanine activity is enhanced by additional Vitamin B 6, especially in studies on depression.
Phenylalanine deficiency can cause bloodshot eyes, cataracts and behavioral changes. Nutritional researchers recommend keeping intake of supplemental forms of phenylalanine to no more than 2.4 grams per day. Overuse of phenylalanine supplements can cause anxiety, headaches and hypertension, and are contraindicated for pregnant woman, those who suffer from anxiety attacks, high blood pressure, PKU, pigmented melanoma, or anyone taking an anti-depressant containing MAO inhibitors.