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What is phenylalanine?

L-phenylalanine is an essential amino acid. This means our bodies cannot produce it, so we need to include it in our diet. It is a neutral, non-polar amino acid and it is particularly important because it is a precursor for tyrosine.

Find out more about essential amino acids

The amino acid tyrosine is classified as a non-essential amino acid. Tyrosine has a number of uses in the body. It has a key role in producing the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). Tyrosine is also crucial in producing the skin pigment called melanin.

There are three different forms of phenylalanine:

  • L-phenylalanine. This is the only essential amino acid in the group and it is present in proteins.
  • D-phenylalanine. Scientists do not fully understand the role of this form yet.
  • DL-phenylalanine. This is a chemical that has been created in a laboratory and it is a combination of both D-phenylalanine and L-phenylalanine.

Phenylalanine excess and deficiency

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Like any nutrient, too much or too little phenylalanine will affect the body. People deficient in phenylalanine may experience symptoms such as lethargy, confusion, cognitive impairment, poor appetite, and depression. These symptoms arise because of the reduction in tyrosine in the body, as tyrosine has a key role in producing proteins and brain chemicals.

Conversely, too much phenylalanine can particularly affect people with phenylketonuria (PKU). This is a rare metabolic disorder, and in these cases excess phenylalanine can irreversibly damage the body. People with PKU lack the enzyme which processes phenylalanine.

Additionally, excess phenylalanine causes intellectual disability if not identified and treated within the first three weeks of life. Left untreated, older children exhibit autistic behaviours combined with hyperactivity.

Phenylalanine health benefits


Vitiligo is a skin condition characterised by the development of white patches. This is the result of a lack of pigmentation in the skin. When the cells that help form skin pigments (melanocytes) are destroyed these white patches develop.

The exact cause of this condition is unknown. However, studies show that oral and topical administration of phenylalanine together with UVA exposure can help to treat vitiligo1.


Due to the relationship between phenylalanine and tyrosine, it is no surprise that scientists are exploring whether phenylalanine can help treat depression. Problems which disrupt normal neurotransmitter signalling are thought to lead to depression.

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While this is a very complex issue, increasing phenylalanine concentrations may help to boost the production of specific neurotransmitters. This may then assist to treat depression.

There have been a couple of studies that have investigated the effects of phenylalanine supplementation on patients diagnosed with depression. A study published in 1984 showed that combining phenylalanine supplementation with a substituted phenethylamine antidepressant (Selegiline) helped to reduce depression symptoms2.

In another more recent study low levels of phenylalanine and tyrosine negatively impacted people who had previously suffered from depression3.

However, both these studies were poorly controlled and the results cannot be considered statistically significant. More research is necessary to determine how effective phenylalanine supplementation may be in the treatment of depression, as well as the potential side effects.

Other potential uses

There have been investigations into using phenylalanine to treat alcoholism, multiple sclerosis, Parkinson’s disease, acupuncture anaesthesia, attention deficit-hyperactivity disorder (ADHA), arthritis, and pain relief. However there have been no significant findings to date.

Dietary sources of L-phenylalanine

Almost all protein foods contain phenylalanine. Poultry, beef, fish, pork, yogurt, milk, cheese, eggs, nuts, seeds and soy products are rich in phenylalanine. The artificial sweetener aspartame is also enriched with phenylalanine.

Phenylalanine dietary supplements

Phenylalanine is available in dietary supplements. However, it is very important to check with a medical practitioner before taking supplements with this amino acid. Certain medications and health conditions may cause adverse side effects.


  1. Antoniou, C. (1989). Vitiligo therapy with oral and topical phenylalanine with UVA exposure. International Journal of Dermatology. Volume 28, Issue 8, (pp. 545-7).
  2. W. Birkmayer, W. et al. (1984). L-Deprenyl Plus L-Phenylalanine in the Treatment of Depression. The Journal of Neural Transmission. Volume 59, Issue 1, (pp. 81-7).
  3. Roise, J. et al. (2005). The Subjective and Cognitive Effects of Acute Phenylalanine and Tyrosine Depletion in Patients Recovered from Depression. Neuropsychopharmacology. Volume 30, Issue 4. (pp. 775-85).