Phenylketonuria (PKU) is an autosomal recessive disorder due to the deficiency of the enzyme phenylalanine hydroxylase. This leads to the failure in converting phenylalanine to tyrosine. As a result, phenylalanine accumulates in the body. All newborns in the United States are routinely tested for PKU.
If diagnosed with phenylketonuria, a PKU diet is essential to manage the condition and minimize the buildup of phenylalanine. A PKU diet focuses on foods that are low in phenylalanine.
Protein-rich foods are a major source of phenylalanine, so avoiding them is an important part of the PKU diet. Of course, the body still requires protein, which is where specially prepared medical foods come in.
A low-phenylalanine PKU diet will not cure the illness, but it will help manage symptoms. Currently, there is no cure for the condition, but it can be controlled by proper diet and supplements.
What is the phenylketonuria diet?
Individuals suffering from phenylketonuria should ingest a low-protein diet.
Food items that should be avoided or taken in moderation are:
- Milk and cheese
- Chicken, beef, and pork
- Foods and drinks that contain artificial sweeteners, such as aspartame
Because individuals with phenylketonuria ingest low-protein food, they end up having deficiencies of other important nutrients. Therefore, to make up for this deficit, a phenylketonuria formula is advised to them.
What is the phenylketonuria formula?
A phenylketonuria formula is a mixture of various kinds of amino acids, except the amino acid phenylalanine, which is in other proteins. This formula contains carbohydrates, fats, vitamins, and minerals to meet the nutritional requirements of the individual.
A unique ingredient called glycomacropeptide is present in some of the formulations.
What are the clinical features of phenylketonuria?
A child with phenylketonuria may have the following:
- Developmental delay
- Mental retardation
- Blue eyes
- Aggressive behavior
- Blond hair
- Mousy odor to urine
What is the cause of phenylketonuria?
The genetic predisposition of phenylketonuria involves a mutation in the PAH gene.
The function of the PAH gene is to instruct the cell for the synthesis of an enzyme called phenylalanine hydroxylase. This enzyme helps in the metabolic breakdown of the amino acid phenylalanine to other important compounds in the body.
If there is a mutation of this gene, phenylalanine from the diet is not utilized properly, and it accumulates in the body to toxic levels. The neurons present in the brain are especially sensitive to phenylalanine levels and excessive amounts of it can cause brain damage.
Inheritance of phenylketonuria
This disorder is inherited in an autosomal recessive pattern. This implies that both copies of the gene in each cell have a mutation.
The parents of an individual with an autosomal recessive disorder carry one mutated gene each; this means that they just have the trait of the disease and do not typically show signs and symptoms of the disease.
What are the types of phenylketonuria?
There are four variants of phenylketonuria, including:
- Hyperphenylalaninemia: The level of phenylalanine is just above normal.
- Mild phenylketonuria: The level of phenylalanine in the blood is mildly increased.
- Moderate or variant phenylketonuria: The level of phenylalanine in the blood is neither too low nor too high.
- Classic phenylketonuria: The blood level of phenylalanine is extremely high. This is the most severe form of the disorder. It occurs when the enzyme phenylalanine hydroxylase is severely deficient or absent in the body. People with this variant of phenylketonuria suffer from severe brain damage and other serious health problems.
How is phenylketonuria diagnosed?
Newborns should be screened for phenylketonuria.
A phenylketonuria test is done after 24 hours of birth. The test is usually done after the baby has ingested some proteins in the diet to ensure adequate results.
- A sample of blood is collected from the baby's heel or arms by a nurse or a lab technician.
- The sample is then sent to the laboratory to check for metabolic disorders involving phenylketonuria.
If the test results come positive for phenylketonuria, then:
- Additional tests are done to confirm the diagnosis, which includes further blood tests and urine analysis tests.
- Genetic testing of both the baby and the parents is done to identify gene mutations.
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Rush ET. Phenylketonuria (PKU). Medscape. https://emedicine.medscape.com/article/947781-overview
Genetic and Rare Diseases Information Center. Phenylketonuria. https://rarediseases.info.nih.gov/diseases/7383/phenylketonuria
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