What is phenylalanine?What to know about phenylalanine

What is phenylalanine?

Phenylalanine is an essential amino acid that carries out many important functions in the human body.

This article describes what phenylalanine is, including the different types, and lists some dietary sources of it.

It also outlines some of the potential benefits, risks, and side effects of consuming this amino acid and provides information on the optimum amount of phenylalanine for different people.

Phenylalanine is a type of amino acid. Amino acids are molecules that combine to form proteins.

Phenylalanine is an essential amino acid in humans, meaning that the body cannot synthesize its own phenylalanine. Instead, humans must get phenylalanine from the foods they eat.

Phenylalanine is a component in several important proteins and enzymes. For example, the body can convert phenylalanine into tyrosine, which it then uses to synthesize the neurotransmitters dopamine and norepinephrine.

Phenylalanine also plays a role in the production of other important am


There are two types of phenylalanine: L-form and D-form. These different forms have different roles in the body. The L-form becomes incorporated into proteins, whereas the D-form acts as a pain reliever.


Scientists consider phenylalanine to be an essential amino acid. This means that the human body is unable to produce its own phenylalanine. Instead, a person must get phenylalanine from dietary sources.

One 2020 studyTrusted Source explains that because phenylalanine is a component of many natural proteins, high protein foods are good sources of this essential amino acid.

Some examples of high protein foods include:

Foods and substances that are low in phenylalanine include:

  • fruits
  • vegetables
  • fats
  • sugars
  • starches
  • Neurological health benefits

    The body uses phenylalanine to produce the neurotransmitter norepinephrine. This neurotransmitter has many important functions, including:

    • helping signals travel between the brain and the body’s nerve cells
    • maintaining an awake or alert state
    • improving memory
    • reducing hunger pains

    Norepinephrine also has antidepressant properties. For this reason, some scientists believe that L-phenylalanine could help treat depression.

  • Skin benefits

    Some evidence suggests that phenylalanine may improve the skin condition vitiligo, in which areas of the skin become depigmented. Phenylalanine stimulates the production of melanin, which is the natural pigment that gives skin its color.

  • Side effects

    Side effects of phenylalanine consumption are only a concern for people with PKU. Some examples of complications relating to PKU include the following.Side

  • Cognition and mood

    In people with PKU, phenylalanine may cause cognition- and mood-related side effects.

    The 2011 studyTrusted Source investigated the effects of phenylalanine consumption in adults with PKU. In this double-blind, placebo-controlled trial, one group of adults with PKU took a phenylalanine supplement for 4 weeks, while a separate group of adults with PKU took a placebo for the same amount of time.

    At the end of the trial, those in the placebo group reported better moods and scored higher on tests of sustained attention than those receiving the phenylalanine supplement.

    Pregnancy and giving birth

    One 2015 reviewTrusted Source notes that females with PKU have a higher likelihood of having a child who also has PKU. Healthcare professionals refer to this as maternal PKU syndrome.

    According to the review, babies with maternal PKU syndrome are more likely to have the following health problems:

    • heart defects
    • microcephaly
    • cognitive impairments
    • intrauterine growth restriction

    Optimum amount

    An older review from 2007Trusted Source suggests that the minimum daily dosage of phenylalanine is around 9.1 milligrams per kilogram (mg/kg) of body weight. According to the review, the maximum daily dosage of phenylalanine is 38–48 mg/kg of body weight.

    People with PKU need to consume much less phenylalanine than people who do not have this condition. People with PKU should talk with a doctor or dietitian for further advice regarding their phenylalanine intake.

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