What is Phenylalanine?

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  • Written By: B. Schreiber
  • Edited By: Kathryn Hulick
  • Last Modified Date: 07 January 2020
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Phenylalanine is an amino acid, commonly found in food, that is used by the body to form proteins. Phenylalanine is sometimes taken as a dietary supplement. It is thought to have potential to relieve pain and depression, among other things. People with phenylketonuria (PKU) do not have the ability to break down this amino acid, causing it to accumulate and cause brain damage. PKU is a rare inherited genetic disorder.

Like the other 19 or 20 amino acids, phenylalanine is a nitrogen-containing molecule. It is composed of an amine group and a carboxyl group, and an identifying side chain. The side chain in this case is a phenyl group. It is an essential amino acid, meaning that it cannot be produced by the body and must be obtained from the diet.

As a supplement, phenylalanine is often taken in a form that also incorporates a close chemical cousin. Certain supplementary preparations can have complex effects on the body, so medical guidance is recommended. Phenylalanine has been studied in relation to health conditions such as depression, general and specific pain, and Parkinson's disease. Most people are not deficient in phenylalanine because of its widespread presence in healthy diets. High amounts of supplementation could lead to toxicity, and people with PKU should never take phenylalanine supplements.


PKU is an genetic disorder that results from an inability to convert phenylalanine to the amino acid tyrosine. In people who have PKU, phenylalanine builds up in the brain, where it can cause severe damage like mental retardation. In most developed countries, screening for the condition at birth can prevent or greatly reduce the damaging effects of this disorder. This is because the greatest amount of damage occurs in the first days after birth. Although adults seem less vulnerable to nerve damage, lifetime dietary restrictions have been found to be helpful in managing the condition.

Phenylalanine is found in most foods that contain protein, so avoiding it is difficult. People with PKU must follow a strict and often complex diet low in phenylalanine to avoid complications from the disorder. Aid from a doctor and dietitian is an important factor in designing and following such a restricted diet. People with PKU should not consume products artificially sweetened with aspartame, some of which have PKU warning labels. This is because aspartame is made by combining phenylalanine with another amino acid.


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