Phenylalanine is an amino acid. Like some other amino acids, the human body cannot synthesize it and must obtain it from dietary sources. It is necessary for the production of tyrosine, dopamine, adrenaline and melanin. It is found in both animals and plants including eggs, chicken, liver, beef, milk (including breast milk) and soybeans. it is also found in the artificial sweetener aspartame. Mass produced phenylalanine, which is used in supplements and animal feed is extracted by exploiting the e. coli bacteria. The body requires about 33 milligrams per kilogram of body weight every day.

Phenylketonuria is a genetic disease where the body cannot process phenylalanine due to the lack of a necessary enzyme.Patients with the condition have to regulate their intake of the amino acid.

Phenylalanine at Wikipedia