Phenylalanine is an amino acid, a “building block” of protein. There are three forms of phenylalanine: D-phenylalanine, L-phenylalanine, and the mix made in the laboratory called DL-phenylalanine. D-phenylalanine is not an essential amino acid, and its role in people is not currently understood. L-phenylalanine is an essential amino acid and is the only form of phenylalanine found in proteins. Major dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk.
Phenylalanine is used for depression, attention deficit-hyperactivity disorder (ADHD), Parkinson’s disease, chronic pain, osteoarthritis, rheumatoid arthritis, alcohol withdrawal symptoms, and a skin disease called vitiligo.