Phenylalanine Benefits, Side Effects

Phenylalanine: Phenylalanine is an indispensable amino acid and the precursor of the amino acid tyrosine. Like tyrosine, phenylalanine is also a forerunner for catecholamines including tyramine, dopamine, epinephrine, and norepinephrine. Catecholamines are neurotransmitters that act as adrenalin-like substances. Interestingly, certain psychotropic drugs (mescaline, morphine, codeine, and papaverine) also have phenylalanine as a constituent. Phenylalanine is highly fixed in the human brain and plasma. Traditional metabolism of phenylalanine requires biopterin, iron, niacin, vitamin B6, copper, and vitamin C.

An ordinary adult ingests 5 g of phenylalanine per day and may optimally need up to 8 g daily. Phenylalanine is highly concentrated in a number of high protein foods, such as meat, shanty cheese, and wheat germ. An increased dietary source of phenylalanine is artificial sweeteners containing aspartame. As a general rule, aspartame should be bypassed by phenylketonurics and pregnant women. When present in sufficiently high levels, phenylalanine can act as a neurotoxin and a metabotoxin. A neurotoxin is a compound that disrupts or attacks neural cells and neural tissue. A metabotoxin is an endogenously produced metabolite that precipitates adverse health effects at chronically tremendous levels. Chronically high levels of phenylalanine are associated with at least five inborn errors of metabolism, incorporating Hartnup disorder, hyperphenylalaninemia due to guanosine triphosphate cyclohydrolase deficiency, phenylketonuria (PKU), tyrosinemia type 2 (or Richner-Hanhart syndrome), and tyrosinemia type III (TYRO3).

 Phenylalanine Benefits, Side Effects

What Is Phenylalanine

Phenylalanine isn’t a health concern for greatest people. However, for people who have the genetic disturbance phenylketonuria (PKU) or certain other health conditions phenylalanine can be a serious strength concern.

Phenylalanine can precipitate intellectual disabilities, brain damage, seizures and other problems in people with PKU. Phenylalanine occurs spontaneously in many protein-rich foods, such as milk, eggs and meat. Phenylalanine is likewise sold as a dietary supplement.

The artificial sweetener aspartame (Equal, NutraSweet), which is added to many medications, diet foods and diet sodas, contains phenylalanine.

Federal regulations require that any drink or food that contains aspartame bear this warning: “Phenylketonurics: Contains phenylalanine.” This warning helps people with PKU avoid products that are a cause of phenylalanine.

If you don’t have PKU, you presumably don’t need to worry about harmful health effects of phenylalanine — with certain important omission. Aspartame in large doses can cause a rapid increase in brain levels of phenylalanine. Because of this, use products with aspartame carefully if you:

  • Take certain medications, such as monoamine oxidase inhibitors, neuroleptics or medications that contain levodopa (Sinemet, Rytary, others)
  • Have the muscle movement disorder tardive dyskinesia
  • Have a sleep disorder, anxiety disorder or other mental health condition; phenylalanine may worsen feelings of anxiety and jitteriness

If you aren’t sure if phenylalanine or aspartame is a concern for you, talk to your doctor. A blood test to determine if you have PKU is available and is now routinely done as part of newborn screening.



Phenylalanine is an amino acid, a “building block” of protein. There are three forms of phenylalanine: D-phenylalanine, L-phenylalanine, and the mix made in the laboratory called DL-phenylalanine. D-phenylalanine is not an essential amino acid. Its role in people is not currently understood. L-phenylalanine is an essential amino acid. It is the only form of phenylalanine found in proteins. Major dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk.

Phenylalanine is used for a skin disease called vitiligo, depression, attention deficit-hyperactivity disorder (ADHD), Parkinson’s disease, multiple sclerosis, pain, acupuncture anesthesia, osteoarthritis, rheumatoid arthritis, weight loss, and alcohol withdrawal symptoms.

Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula C9H11NO2. It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino acid is classified as neutral, and nonpolar because of the inert and hydrophobic nature of the benzyl side chain. The L-isomer is used to biochemically form proteins, coded for by DNA. Phenylalanine is a precursor for tyrosine, the monoamine neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline), and the skin pigment melanin. It is encoded by the codons UUU and UUC.

Phenylalanine is found naturally in the breast milk of mammals. It is used in the manufacture of food and drink products and sold as a nutritional supplement for its reputed analgesic and antidepressant effects. It is a direct precursor to the neuromodulator phenethylamine, a commonly used dietary supplement. As an essential amino acid, phenylalanine is not synthesized de novo in humans and other animals, who must ingest phenylalanine or phenylalanine-containing proteins.

Phenylalanine Dangers

Phenylketonurics have elevated serum plasma levels of phenylalanine up to 400 times normal. High plasma concentrations of phenylalanine influence the blood-brain barrier transport of large neutral amino acids. The high plasma phenylalanine concentrations increase phenylalanine entry into the brain and restrict the entry of other large neutral amino acids (PMID: 19191004). Phenylalanine has been found to interfere with different cerebral enzyme systems. Untreated phenylketonuria (PKU) can lead to intellectual disability, seizures, behavioural problems, and mental disorders. It may also result in a musty smell and lighter skin.

Classic PKU dramatically affects myelination and white matter tracts in untreated infants; this may be one major cause of neurological disorders associated with phenylketonuria. Mild phenylketonuria can act as an unsuspected cause of hyperactivity, learning problems, and other developmental problems in children. It has been recently suggested that PKU may resemble amyloid diseases, such as Alzheimer’s disease and Parkinson’s disease, due to the formation of toxic amyloid-like assemblies of phenylalanine(PMID: 22706200). Phenylalanine also has some potential benefits. Phenylalanine can act as an effective pain reliever.

Its use in premenstrual syndrome and Parkinson’s may enhance the effects of acupuncture and electric transcutaneous nerve stimulation (TENS). Phenylalanine and tyrosine, like L-DOPA, produce a catecholamine-like effect. Phenylalanine is better absorbed than tyrosine and may cause fewer headaches. Low phenylalanine diets have been prescribed for certain cancers with mixed results. For instance, some tumours use more phenylalanine than others (particularly melatonin-producing tumours called melanomas).

What is phenylalanine and is it bad for you?

Phenylalanine isn’t a health concern for most people. However, for people who have the genetic disorder phenylketonuria (PKU) or certain other health conditions phenylalanine can be a serious health concern. Phenylalanine can cause intellectual disabilities, brain damage, seizures and other problems in people with PKU.

What are the benefits of phenylalanine?

Major dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk. Phenylalanine is used for depression, attention deficit-hyperactivity disorder (ADHD), Parkinson’s disease, chronic pain, osteoarthritis, rheumatoid arthritis, alcohol withdrawal symptoms, and a skin disease called vitiligo.

What are the side effects of phenylalanine?

Side effects may include:
  • Heartburn.
  • Fatigue.
  • Nausea.
  • Constipation.
  • Dizziness.
  • Headache.
  • Anxiety and hypomania (a milder form of mania)
  • Sedation.