By Biplab Das 

What is Phenylalanine?

Phenylalanine is one of the essential amino acids. Animals and humans cannot synthesize it. But plants and microorganisms can produce it. Therefore, humans and animals have to obtain phenylalanine from diet. Phenylalanine is available in three chemical forms: L-phenylalanine, the natural form of phenylalanine found in proteins throughout the body, D-phenylalanine, a mirror image of L-phenylalanine that is synthesized artificially, and DL-phenylalanine, a combination of the previous two forms. L-phenylalanine is required for protein synthesis. D-phenylalanine, however, does not participate in protein biosynthesis although it is found in proteins in small amounts. 

L-phenylalanine can be converted into L-tyrosine, a non-essential amino acid, which is in turn converted into L-DOPA. L-DOPA is a precursor for dopamine, norepinephrine (noradrenalin), and epinephrine (adrenaline).

Health Benefits of Phenylalanine 

Preliminary evidences suggest that D-phenylalanine may help reduce chronic pain associated with certain health conditions by stimulating nerve pathways in the brain that control pain. One animal study suggested that D-phenylalanine may improve rigidity, walking disabilities, speech difficulties, and depression associated with Parkinson's disease. Furthermore, research shows that combining L-phenylalanine (oral and topical) with UVA radiation for people with vitiligo (a condition characterized by irregular depigmentation or white patches of skin) may lead to some darkening or repigmentation of the whitened areas, particularly on the face. Other beneficial effects of phenylalanine include improvement in mood by enhancing the production of brain chemicals such as dopamine and norepinephrine. But, further research is needed to confirm this benefit.

Deficiency Symptoms of Phenylalanine 

Symptoms of phenylalanine deficiency include confusion, lack of energy, decreased alertness, decreased memory, and diminished appetite.

Side effects of Phenylalanine

DL-phenylalanine may cause anxiety, jitteriness, and hyperactivity in children. Children with Attention Deficit Hyperactivity Disorder (ADHD) may need to avoid foods containing aspartame, which is made from phenylalanine. Children with phenylketonuria (PKU), a metabolic disorder,  lack an enzyme called phenylalanine hydroxylase needed to breakdown phenylalanine and then to eliminate it via the urine. This means for children with PKU, phenylalanine cannot be eliminated and as a result it builds up in their blood stream. Phenylalanine doses in excess of 5,000 mg a day may be toxic and can cause nerve damage. High quantities of DL-phenylalanine may also cause mild side effects such as nausea, heartburn and headaches.

Dietary Sources of Phenylalanine

Dietary Sources of Phenylalanine

L-phenylalanine is found in most foods that contain protein such as beef, poultry, pork, fish, milk, yogurt, eggs, cheese, soy products (including soy protein isolate, soybean flour, and tofu), and certain nuts and seeds. The artificial sweetener aspartame is also high in phenylalanine.

D-phenylalanine is synthesized in the laboratory and is not found naturally in food.

Table: List of select foods  containing  phenylalanine  (ordered from high to low content, grams/100 grams of food portion)

Data source: USDA National Nutrient Database for Standard Reference, Release 19 (2006
 

References:
1) The Chemistry of Amino Acids, University of Arizona, Dept. of Biology.
2) Complimentary and Alternative Medicine Index: Supplements - Phenylalanine, University of Maryland .
3) Amino Acids and Protein, University of Cinncinati.