Please Note: People with a condition known as phenylketonuria (PKU), in which phenylalanine cannot be broken down, should not take any form of phenylalanine supplements. Nor should phenylalanine supplements be taken in conjunction with prescription anti-depressants, particularly monoamine oxidase (MAO) inhibitors.
PKU is a rare metabolic disorder that occurs in people who are missing an enzyme that the body requires to use phenylalanine. This causes high levels of phenylalanine to build up and if not treated before 3 weeks of age, can cause severe, irreversible intellectual disability. Individuals with PKU must consume a diet that avoids phenylalanine and take tyrosine supplements to have optimum brain development and growth.
Phenylalanine is found naturally in the breast milk of mammals. It is used in nutritional supplements for its reputed analgesic and antidepressant effects. L-phenylalanine is an essential amino acid and is the only form of phenylalanine found in proteins. Major dietary sources of L-phenylalanine include beef, poultry, pork, fish, milk, yogurt, eggs, cheese, soy products (including soy protein isolate, soybean flour, and tofu), and certain nuts and seeds.