What is Phenylketonuria (PKU)? | Nutrition

Phenylketonuria (PKU)

Overview of 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.

Phenylalanine hydroxylase is an enzyme your body uses to convert phenylalanine into tyrosine, which your body needs 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, your body can’t break down phenylalanine. This causes a buildup of phenylalanine in your body.

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 United States, as early screening allows treatment to begin soon after birth. Early diagnosis and treatment can help relieve symptoms of PKU and prevent brain damage.

Alternative names

  • PKU
  • Neonatal phenylketonuria

Symptoms of phenylketonuria

Newborns with PKU initially don’t have any symptoms. However, without treatment, babies usually develop signs of PKU within a few months.

PKU signs and symptoms can be mild or severe and may include:

  • A musty odor in the breath, skin or urine, caused by too much phenylalanine in the body.
  • Neurological problems may include seizures.
  • Skin rashes (eczema).
  • Fair skin and blue eyes, because phenylalanine can’t transform into melanin the pigment.
  • Responsible for hair and skin tone.
  • Abnormally small head (microcephaly).
  • Hyperactivity
  • Intellectual disability
  • Delayed development
  • Behavioural, emotional and social problems.
  • Psychiatric disorders

Causes of phenylketonuria

A defective gene (genetic mutation) causes PKU, which can be mild, moderate or severe. In a person with PKU, this defective gene causes a lack of or deficiency of the enzyme that’s needed to process phenylalanine, an amino acid.

A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts or meat, and even grains such as bread and pasta, or eats aspartame, an artificial sweetener. This buildup of phenylalanine results in damage to nerve cells in the brain.

Diagnosis of phenylketonuria

Newborn blood testing identifies almost all cases of phenylketonuria. All 50 states in the United States require newborns to be screened for PKU. Many other countries also routinely screen infants for PKU.

If you have PKU or a family history of it, your doctor may recommend screening tests before pregnancy or birth. It’s possible to identify PKU carriers through a blood test.

Testing your baby after birth

A PKU test is done a day or two after your baby’s birth. The test is done after your baby is 24 hours old and after your baby has ingested some protein in the diet to ensure accurate results.

  • A nurse or lab technician collects a few drops of blood from your baby’s heel or the bend in your baby’s arm.
  • A laboratory tests the blood sample for certain metabolic disorders, including PKU.
  • If you don’t deliver your baby in a hospital or are discharged soon after birth, you may need to schedule a newborn screening with your paediatrician or family doctor.

If this test indicates your baby may have PKU:

  • Your baby may have additional tests to confirm the diagnosis, including more blood tests and urine tests.
  • You and your baby may have genetic testing to identify gene mutations.

Treatment for phenylketonuria

There is no cure for PKU, but treatment can prevent intellectual incapacity and other health problems. A person with PKU should be treated at a medical centre that specializes in the disorder.

Which foods to avoid

Because the amount of phenylalanine that a person with PKU can safely eat is so low, it’s crucial to avoid all high-protein foods, such as:

  • Milk
  • Eggs
  • Cheese
  • Nuts
  • Soybeans
  • Beans
  • Chicken
  • Beef
  • Pork
  • Fish
  • Potatoes, grains and other vegetables that have protein will likely be limited.

Children and adults also need to avoid certain other foods and beverages, including many diet sodas and other drinks that contain aspartame (NutraSweet, Equal). Aspartame is an artificial sweetener made with phenylalanine.

Some medications may contain aspartame and some vitamins or other supplements may contain amino acids or skim milk powder. Check with your pharmacist about the contents of over-the-counter products or prescription medications.

Medicines

The Food and Drug Administration (FDA) has approved the drug sapropterin (Kuvan) for the treatment of PKU. It works by increasing your tolerance to phenylalanine. The drug is for use in combination with a PKU diet. But it doesn’t work for everyone with PKU.

In approving sapropterin, the FDA directed that studies continue because there are no long-term studies on the drug’s efficacy and long-term safety.

Complications

Severe mental disability occurs if the disorder is not treated. ADHD (attention deficit hyperactivity disorder) seems to be a common problem in those who do not follow a diet very low in phenylalanine.

Prevention

Because PKU is a genetic condition, it can’t be prevented or avoided. Genetic testing is the only way to determine if people carry the defective gene. Some people may carry the gene but not have PKU. These people are called carriers. Both parents must carry one copy of the abnormal gene for the disease to be passed to the baby.

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