What Is Phenylketonuria?

Phenylketonuria (PKU) is a rare genetic condition that causes an amino acid called phenylalanine to build up in the body. Amino acids are the building blocks of protein. Phenylalanine is found in all proteins and some artificial sweeteners. Your body uses an enzyme called phenylalanine hydroxylase to convert phenylalanine into tyrosine, a nonessential amino acid. Your body needs tyrosine to create neurotransmitters, such as epinephrine, norepinephrine, and dopamine.

PKU is caused by a defect in the gene that helps create phenylalanine hydroxylase. When this enzyme is missing, the body is unable to break down phenylalanine. This causes a buildup of phenylalanine in the body. Early diagnosis and treatment can help relieve symptoms of PKU and prevent brain damage.

Babies in the United States are screened for PKU shortly after birth. The condition is uncommon in this country, only affecting about 1 in 10,000 to 15,000 newborns each year. The severe signs and symptoms of PKU are rare in the US, as early screening allows treatment to begin soon after birth.



What Are the Symptoms of Phenylketonuria?

PKU symptoms can range from mild to severe. The most severe form of this disorder is known as classic PKU. An infant with classic PKU may appear normal for the first few months of their life. If the baby isn’t treated for PKU during this time, they’ll start to develop the following symptoms:

  • seizures
  • tremors, or trembling and shaking
  • stunted growth
  • hyperactivity
  • skin conditions, such as eczema
  • a musty odor of their breath, skin, or urine

A less severe form of PKU is called variant PKU or non-PKU hyperphenylalaninemia. This occurs when the baby has too much phenylalanine in their body. Infants with this form of the disorder may have only mild symptoms, but they’ll need to follow a special diet to prevent intellectual disabilities.

If PKU isn’t diagnosed at birth and treatment isn’t started quickly, the disorder can cause:

  • irreversible brain damage and intellectual disabilities within the first few months of life
  • behavioral problems and seizures in older children

Once a specific diet and other necessary treatments are started, symptoms start to diminish. People with PKU who properly manage their diet usually don’t show any symptoms.


What Causes Phenylketonuria?

PKU is an inherited condition caused by a defect in the PAH gene. The PAH gene helps create phenylalanine hydroxylase, the enzyme responsible for breaking down phenylalanine. A dangerous buildup of phenylalanine can occur when someone eats high-protein foods, such as eggs and meat. Both parents must pass on a defective version of the PAH gene for their child to inherit the disorder. If just one parent passes on an altered gene, the child won’t have any symptoms, but they’ll be a carrier of the gene.



How Is Phenylketonuria Diagnosed?

Since the 1960s, hospitals in the United States have been routinely screening newborns for PKU by taking a blood sample. A doctor will use a needle or lancet (surgical instrument) to take a few drops of blood from your baby’s heel to test for PKU and other genetic disorders.

The screening test will be performed when the baby is one to two days old and still in the hospital. If you don’t deliver your baby in a hospital, you’ll need to schedule the screening test with your doctor.

Additional tests may be performed to confirm the initial results. They’ll test for the presence of the PAH gene mutation that causes PKU. These tests are often done within six weeks after birth.

If a child or adult shows symptoms of PKU, such as developmental delays, the doctor will order a blood test to confirm the diagnosis. This test will involve taking a sample of blood and analyzing it for the presence of the enzyme needed to break down phenylalanine.


How Is Phenylketonuria Treated?

People with PKU can relieve their symptoms and prevent complications by following a special diet and taking medications.


The main way to treat PKU is to eat a special diet that limits foods containing phenylalanine. Infants with PKU may be fed breast milk. They usually also consume a special formula that’s often known as Lofenalac. When your baby is old enough to eat solid foods, you’ll need to avoid letting them eat foods high in protein. These include:

  • eggs
  • cheese
  • nuts
  • milk
  • beans
  • chicken
  • beef
  • pork
  • fish

To make sure that their body still receives an adequate amount of protein, children with PKU need to consume PKU formula, which contains all the amino acids that the body needs, except phenylalanine. There are also certain low-protein, PKU-friendly foods that can be found at specialty health stores. Children will have to follow the dietary restrictions and consume PKU formula throughout their life to manage their symptoms.

It’s important to note that PKU meal plans vary from person-to-person. People with PKU will need to work closely with a doctor or dietitian to maintain a proper balance of nutrients while limiting their intake of phenylalanine. They’ll also have to monitor their phenylalanine levels by keeping records of the amount of phenylalanine in the foods they eat throughout the day.

Some state legislatures have enacted bills that provide some insurance coverage for the foods and formulas necessary to treat PKU. Check with your state legislature and medical insurance company to find out if this coverage is available for you. If you don’t have medical insurance, you can check with your local health departments to see what options are available to help you afford PKU formula.


The United States Food and Drug Administration (FDA) recently approved sapropterin (Kuvan) for the treatment of PKU. Sapropterin helps lower phenylalanine levels. This medication must be used in combination with a special PKU meal plan. However, it doesn’t work for everyone with PKU. It’s most effective in children with mild cases of PKU.


In Pregnant Women

Pregnancy and Phenylketonuria

Woman with PKU may be at risk for complications if they don’t follow a PKU meal plan during their childbearing years. This includes possible miscarriage. There’s also a chance that the unborn baby will be exposed to high levels of phenylalanine. This can lead to various problems in the baby, including:

  • intellectual disabilities
  • heart defects
  • delayed growth
  • low birth weight
  • an abnormally small head

These signs aren’t immediately noticeable in a newborn, but a doctor will perform tests to check for signs of any abnormalities or medical conditions your child may have.



What Is the Long-Term Outlook for People with Phenylketonuria?

The long-term outlook for people with PKU is very good if a PKU meal plan is followed closely and started shortly after birth. When diagnosis and treatment are delayed, brain damage may occur. This can lead to intellectual disabilities by the child’s first year of life. Untreated PKU can also eventually cause:

  • delayed development
  • behavioral and emotional problems
  • neurological problems, such as tremors and seizures


Can Phenylketonuria Be Prevented?

PKU is a genetic condition, so it can’t be prevented. However, an enzyme assay can be done for people who plan on having children. An enzyme assay is a blood test that can determine whether someone carries the defective gene that causes PKU. The test may also be done during pregnancy to screen unborn babies for PKU.

If you have PKU, you can prevent symptoms by following your PKU meal plan throughout your life.

Article Resources