Phenylalanine is an essential amino acid that carries out many important functions in the human body. The two types of phenylalanine are L-form and D-form.

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.

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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 amino acids.

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.

L-phenylalanine occurs naturally in certain foods, whereas D-phenylalanine is a synthetic variant of L-phenylalanine.

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 study 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

The sections below cover some of the potential health benefits of phenylalanine in more detail.

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.

For the majority of people, phenylalanine consumption does not pose a risk to health. However, people with phenylketonuria (PKU) can become seriously unwell if they consume a diet containing phenylalanine.

PKU is an inherited metabolic condition that affects the body’s ability to break down phenylalanine for conversion into tyrosine. As a result, phenylalanine accumulates in the blood and brain. This can cause damage to the brain and nervous system.

PKU is relatively rare in the United States, affecting around 1 in 15,000 people.

The following may point to the presence of PKU:

  • fairer skin, hair, and eyes than siblings who do not have PKU
  • eczema
  • a musty odor from one’s urine or skin
  • frequent vomiting
  • body spasms or tremors
  • seizures
  • hyperactivity
  • behavioral difficulties, such as frequent temper tantrums and episodes of self-harm

PKU often manifests in early infancy. Children with the condition must avoid eating foods containing high levels of phenylalanine.

However, as an older study from 2011 notes, scientists currently disagree on whether or not such dietary restrictions are necessary for adults.

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

Cognition and mood

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

The 2011 study 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 review 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

An older review from 2007 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.

Phenylalanine is an essential amino acid. This means that the human body is unable to synthesize its own phenylalanine, so people must instead get it from dietary sources. In general, foods that are high in protein are also rich in phenylalanine.

Phenylalanine is involved in the production of the neurotransmitters norepinephrine and dopamine, which are essential for the proper functioning of the brain and nervous system. Phenylalanine also helps with the production of the pigment melanin, which gives color to the skin, hair, and eyes.

Most people can consume phenylalanine without any risk of side effects. However, some people have the rare metabolic condition PKU, which causes the body to be unable to break down phenylalanine. This can lead to harmful levels of phenylalanine in the body. So, people with PKU must be cautious about the amount of phenylalanine they consume.