L-phenylalanine is an essential amino acid. It cannot be synthesised by humans and is instead sourced from diet. It is a neutral, non-polar amino acid and a precursor for tyrosine.

The amino acid tyrosine is classified as non-essential amino acid and is involved in the production of the neurotransmitters norepinephrine (noradrenaline), dopamine, and epinephrine (andrenalin). Tyrosine is also needed to produce the skin pigment melanin.

There are three different forms of phenylalanine: D-phenylalanine, L-phenylalanine, and DL-phenylalanine. L-phenylalanine is the only essential amino acid in the group and found in proteins. The role of D-phenylalanine is not yet clearly understood. DL-phenylalanine is a laboratory formed combination of both D-phenylalanine and L-phenylalanine.

Effects of too much or too little L-phenylalanine

more or lessLike any nutrient, deficiencies or excessive amounts of phenylalanine will affect the body. People deficient in phenylalanine may experience symptoms such as lethargy, confusion, cognitive impairment, poor appetite, and depression. These responses occur due to decreased levels of tyrosine that is needed to produce key proteins and brain chemicals.

Conversely, too much phenylalanine can irreversibly damage the body for suffers of a rare metabolic disorder known as phenylketonuria (PKU). People with PKU lack the enzyme required to process phenylalanine.

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 treatment for vitiligo

White patches that develop on the skin can be a sign of the condition vitiligo. 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.

Treating depression with L-phenylalanine

using L-phenylalanine to treat depressionDue to the relationship between phenylalanine and tyrosine, scientists have been interested to see if phenylalanine can help treat depression. Problems which disrupt normal neurotransmitter signalling are thought to lead to depression.

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 for L-phenylalanine

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.

Food 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. et.al. (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).”