Phenylketonuria, commonly known as PKU, is a genetic condition that affects how the amino acid, phenylalanine, is broken down by the body.

PKU affects around 1 in 10,000 to 15,000 babies in the United States each year and is commonly diagnosed shortly after birth.

Phenylalanine is not something that the body makes naturally. It is derived from foods, such as proteins and some artificial sweeteners.

In this article, we take a look at the symptoms of PKU, the tests that can diagnose it, and what treatments are available.

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PKU is an abnormal gene inherited from one or both parents.

PKU is an inherited, genetic condition that is passed from both parents to their children when the genes are mutated or changed.

Some children inherit the abnormal gene from one parent rather than both. These children are known as PKU carriers and do not have PKU themselves.

PKU is known to be more common in people of Native American and Northern European descent.

Genetic mutations

PKU is a condition caused by a change or mutation in a particular gene. There may be other genetic factors that play a role, however.

People affected by PKU have an alteration or mutation in their PAH gene.

The PAH gene is responsible for providing the body with instructions for producing an enzyme called phenylalanine hydroxylase. This enzyme is vital for converting phenylalanine into other substances used by the body.

When there is a mutation in the PAH gene, the body is unable to process phenylalanine properly. This can make dietary sources of phenylalanine toxic, potentially leading to a buildup and irreversible brain damage in some cases.

PKU can be present in mild or severe versions of the disease. Those with mild forms of PKU are able to process phenylalanine to a small degree. In severe cases of what is referred to as classic PKU, there is very little if any phenylalanine hydroxylase working in the body.

Classic PKU is most worrisome as this form can lead to severe brain damage and other medical conditions if it is not treated.

Although babies affected by PKU may not show symptoms to begin with, these may start to appear by around 6 months.

If left untreated, PKU can cause toxic and harmful levels of phenylalanine within the body. These high levels of phenylalanine can pose serious and irreversible health problems, such as:

  • permanent intellectual disabilities
  • developmental delays
  • behavioral or psychiatric issues, such as hyperactivity with autistic behaviors
  • seizures
  • arm and leg jerking
  • lighter skin and eyes due to abnormal levels of melanin
  • musty body odor
  • skin rashes, such as eczema
  • pregnancy loss in women

If a pregnant woman has PKU that is not under control, her fetus will be exposed to higher than normal levels of phenylalanine in the womb. This can put the unborn child at risk for the following conditions:

  • low birth weights
  • intellectual disability
  • delayed growth
  • heart complications
  • small head size
  • behavioral problems
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Newborns in the U.S. are tested for PKU within the first few days after birth.

In the U.S., newborns have a blood test for PKU screening 48 to 72 hours following birth. Because virtually all babies with PKU are diagnosed by this method, the signs and symptoms of PKU are rarely seen in the U.S.

Pediatricians will discuss the results and the need for additional testing with the parents.

Families with a history of PKU may undergo PKU testing during pregnancy when cells in the placenta or amniotic fluid are examined.

A baby that is being tested for PKU will have a few drops of blood taken from their heel. This sample is then sent to the lab on a paper specific for testing for PKU.

Repeating the test may be recommended if the initial PKU test was done within the first 24 hours following birth, or if the first screening test was found to be abnormal.

Once PKU is diagnosed in an infant, their phenylalanine levels will need to be tested weekly or more frequently until the age of one, if their medical situation requires it.

Twice-monthly testing is usually recommended during childhood or as needed, depending on a doctor’s recommendation. Continued testing into adolescence and adulthood is discussed later in childhood.

A special PKU diet has to be followed to make sure that phenylalanine levels do not get too high. This diet includes a low-phenylalanine formula with specialized protein. In general, phenylalanine is often found in high-protein foods, so including a special low-phenylalanine formula in the diet ensures that people with PKU get enough protein.

The mothers of children with PKU should discuss breast-feeding with their healthcare provider. This is due to the presence of phenylalanine in breast milk.

People with PKU will need to follow a specialized diet throughout their lives. Foods to include in the PKU diet include:

  • fruits
  • vegetables
  • some grains, which may include low-protein cereal, bread, and pasta
  • low phenylalanine foods

Foods to avoid in the PKU diet include those high in phenylalanine, such as:

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People with PKU will have to avoid sources of protein, including meat and dairy products.
  • milk, cheese, ice cream and other dairy products
  • eggs, fish, meat, and poultry
  • nuts
  • beans
  • foods containing aspartame
  • beverages sweetened with products like NutraSweet® and Equal®

A doctor managing a child’s PKU will discuss their personally-tailored diet and dietary needs, as not all children should follow the same diet.

In some cases, medication may be recommended to help treat people with PKU.

Kuvan is one such medication that is sometimes used in mild or particular cases of PKU in combination with a PKU diet. It helps reduce the amount of phenylalanine in the blood. A doctor will discuss this option if it is appropriate in a child’s care plan.

The outlook for people with PKU depends on several factors. These include:

  • age at diagnosis
  • when the child begins PKU treatment
  • how closely the person follows a PKU diet

A well-managed diet is particularly important for pregnant women with PKU. The more closely they stick to a PKU diet, the better the outlook for their child when it is born.

Parents and those affected with PKU should always speak with their doctor or healthcare provider to work out a personal plan of care, as well as what the risk factors are for PKU complications.