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What is phenylalanine?

Phenylalanine is an essential amino acid [1]. In foods, methionine is incorporated into proteins.

Phenylalanine abbreviation (symbol): Phe

Phenylalanine Functions in the Human Body

Phenylalanine is [1]:

  • A building block of proteins
  • A glucogenic amino acid — it can be converted into glucose [9]
  • A ketogenic amino acid — it can be converted into ketones [9]
  • A precursor of:
    • The amino acid tyrosine, which can be further converted to thyroxine (thyroid hormone)
    • Melanin (a pigment in the skin, hair and iris)
    • The hormone epinephrine or adrenaline
    • A stimulating neurotransmitter norepinephrine or noradrenaline
    • An inhibitory neurotransmitter dopamine

Foods High in Phenylalanine

  • ANIMAL FOODS: meat, fish, cheese [2]
  • PLANT FOODS: beans, wheat germ, lentils, seeds, nuts, baker’s yeast, spirulina [2]

Foods low in phenylalanine: cereals (except wheat germ), fruits, vegetables [2]

Phenylalanine Supplements

Oral phenylalanine supplements without prescription (over-the-counter) are available [3]:

  • L-phenylalanine is found in protein foods and body proteins. As a supplement it is produced utilizing E. coli bacteria.
  • D-phenylalanine is a synthetic form.
  • DL-phenylalanine (DLPA) contains a mixture of L- and D-phenylalanine.

Phenylalanine Health Benefits

L-phenylalanine supplements in combination with sun exposure are POSSIBLY EFFECTIVE for treating vitiligo (a patchy skin depigmentation) [3,4].

Phenylalanine is POSSIBLY INEFFECTIVE [3,4] in reducing appetite [5], treatment of attention deficit hyperactivity disorder (ADHD) and relieving chronic pain.

There is INSUFFICIENT EVIDENCE [3,4] about the effectiveness of phenylalanine in the prevention or treatment of alcohol withdrawal symptoms, arthritis, depression, multiple sclerosis, Parkinson’s disease, phenylketonuria [8] or phenylalanine deficiency with tyrosinemia, or as an anesthetic in acupuncture.

Phenylalanine Safety: Side Effects, Toxicity

L-phenylalanine supplements in recommended doses are POSSIBLY SAFE [4]. The safety of D-phenylalanine is not known [4].

Side effects of L-phenylalanine supplements may include nausea, heartburn and headache; DL-phenylalanine can cause anxiety and hyperactivity in children [3].

Phenylalanine in doses exceeding 5 g/day may cause nerve damage [3].

Who should not take phenylalanine supplements?

  • Pregnant and breastfeeding women
  • Patients with schizophrenia
  • Patients with phenylketonuria, alkaptonuria and other conditions with high blood phenylalanine levels [8]
  • Children with ADHD should not take DL- or D-phenylalanine
  • Patients who take isocarboxaside, phenelzine, tranylcypromine sulfate, levodopa or antipsichotics, such as chlorpromazine, haloperidol, valproic acid or carbamazepine
  • Reference: [3,4]

Phenylalanine and Pregnancy

Pregnant women with high blood phenylalanine levels should limit foods high in phenylalanine (animal foods) and phenylalanine supplements, because high blood phenylalanine levels during pregnancy have been associated with an increased risk of birth defects [4].

Phenylalanine-Drug Interactions

  • Phenylalanine supplements taken together with MAO inhibitors (antidepressants), such as phenelzine, isocarboxaside and tranylcypromine sulfate, may result in severe increase in blood pressure (hypertensive crisis), which may lead to heart attack or stroke [3].
  • Phenylalanine supplements may decrease the effectiveness of levodopa (a drug for Parkinson’s disease) and baclofen (muscle relaxant) [3].
  • Phenylalanine supplements may worsen involuntary movements, a side effect of antipsichotics, such as chlorpromazine, haloperidol, valproic acid, carbamazepine [3,4].


Phenylketonuria is a rare hereditary (autosomal recessive) disorder in which the amino acid phenylalanine cannot be metabolized so it accumulates in the blood and may affect the brain. Additionally, synthesis of the skin pigment melanin from phenylalanine is impaired, so babies with phenylketonuria often have light-colored skin and hair and blue eyes [6]. Other symptoms, which develop later in infancy, include small head size, seizures, skin rashes (eczema), unusual position of hands, tremor and spasms in arms and legs, musty odor of the breath, skin and urine. Complications may include mental retardation and attention deficit hyperactivity disorder (ADHD).

In the U.S. and many other countries, the blood test for phenylalanine is routinely done in all babies a day or two after birth.

Treatment includes a life-long low-protein diet with no animal foods and severe restriction of other high-protein foods and the sweetener aspartame, which contains phenylalanine [6]. Breastfeeding is usually allowed. Special formulas low in phenylanaline are available. With a strict diet, the prognosis is good.


Alkaptonuria (the Greek alkapton = homogentisic acid) is a rare inherited (autosomal recessive) disorder in which the amino acids phenylalanine and tyrosine cannot be properly metabolized, which results in the accumulation of homogentisic acid in the skin, cartilage and other tissues and in the urine [7].

The affected infants may have no symptoms or may have brownish darkening of the urine after exposure to air (brown or black stains in diapers); after age of 30, grey discoloration of the ear, dark spots in the eye sclera and cornea and complications, such as arthritis, heart valve disorders, coronary heart disease or kidney stones can appear. Diagnosis is made by an urine test.

A low-phenylalanine and low-tyrosine diet and high doses of vitamin C may reduce the risk of arthritis.

Frequently Asked Questions

1. Is phenylalanine in chewing gum or soda bad for you?

Phenylalanine is an amino acid that is a part of an artificial sweetener aspartame, which is often used in sugar-free gum or in diet soda. Individuals with a rare genetic disorder phenylketonuria need to avoid phenylalanine, so this is why you can see warnings like: “Contains phenylalanine” on the chewing gum and soda products.

  1. L-phenylalanine  PubChem
  2. List of foods high and low in phenylalanine  US Department of Agriculture
  3. Phenylalanine  University of Maryland
  4. Phenylalanine uses, side effects  WebMD
  5. Pohle-Karuza JR et al, 2008, Effects of l-phenylalanine on energy intake in overweight and obese women: Interactions with dietary restraint status  ScienceDirect
  6. Phenylketonuria  Genetics Home Reference
  7. Alkaptonuria  Genetics Home Reference
  8. van Vliet D et al, 2014, Single amino acid supplementation in aminoacidopathies: a systematic review  PubMed Central
  9. Amino acid  ChemPep

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