Phenylketonuria (PKU)


What Is It?

Phenylketonuria (PKU) is a rare, genetic (inherited) disorder that can cause abnormal mental and physical development if not detected promptly and treated appropriately. Normally, when a person eats protein-containing foods, special chemicals (enzymes) in the body break down (digest) these proteins into amino acids and even smaller parts, which are the body’s building blocks for growth and repair. A person with PKU does not have enough of the specific enzyme that breaks down the amino acid phenylalanine. Therefore, any phenylalanine in the food someone eats cannot be digested properly and collects in the body.

Too much phenylalanine in the body causes problems with the brain and other organs. Damage from a buildup of phenylalanine can begin as early as 3 weeks old and, if undetected and untreated, PKU results in severe mental retardation, hyperactivity and seizures.

PKU affects about one baby in every 10,000 or 15,000 births. To be born with PKU, a baby has to have inherited the PKU gene from both parents. Most often, the parents do not know that they carry the gene.


Newborn babies with PKU do not show any symptoms. At about 3 to 6 months, babies with untreated PKU will begin to be less active, showing little interest in everything around them. By age 1, severe developmental delays can be noted.

Children with PKU have lower levels of melanin, the substance that gives color to hair and skin. That’s because when phenylalanine is broken down, one of its products is used to make melanin. As a result, children with PKU often will have pale skin, blond hair and blue eyes. Dry skin, eczema and a “mousy” odor resulting from the buildup of phenylalanine in hair, skin and urine are also common. Other signs and symptoms may include irritability, muscle stiffness, seizures, a small head (microcephaly), and short stature.


In the United States, newborn babies are screened for PKU routinely within seven to 10 days after birth. Infants must have had a protein meal before being tested for PKU. A very small amount of blood is taken by pricking the baby’s heel. If abnormally high amounts of phenylalanine are found, additional blood tests may be necessary to confirm the diagnosis.

Expected Duration

PKU is an inherited disorder that lasts throughout a person’s lifetime.


PKU occurs when a baby inherits two copies of the specific gene that causes PKU, one from each parent. Each parent usually has only one copy of the gene and therefore doesn’t have PKU. Because the parents don’t know they are carrying the gene, there is nothing they can do to prevent their babies from having the disorder.

Children diagnosed with PKU need to begin dietary restrictions as early as possible after the diagnosis is confirmed, and stay on a special PKU diet for life. Limiting the amount of phenylalanine in their diet and carefully monitoring their growth offers children with PKU the best chance for normal brain development and growth.

A woman who has PKU and is pregnant must strictly control her phenylalanine levels before and during pregnancy to avoid causing damage to her unborn child. High levels of phenylalanine in a pregnant woman can cause her child to have growth retardation, developmental delays, small head size (microcephaly) and other disorders. With careful monitoring and control, women with PKU can give birth to healthy children. A woman with PKU can pass the PKU gene to her child, but the child will not develop PKU unless another copy of the gene is inherited from the father.


The only treatment for PKU is to avoid eating foods that contain phenylalanine. Babies are put on a special formula that does not contain phenylalanine. Phenylalanine is found in most protein-containing foods, so people with PKU are advised to follow a special low-protein diet, avoiding all high-protein foods such as meat, eggs, chicken, fish, milk and cheese, as well as the artificial sweetener aspartame (NutraSweet). Because each person’s protein needs vary over a lifetime, continued medical monitoring is necessary to ensure that people with PKU get the right amount of protein they need to grow and develop properly, but don’t end up with too much phenylalanine in the body at any time.

When To Call A Professional

At your baby’s first checkup with a doctor, make sure that the genetic screening tests that are recommended after birth have been done and are normal. If your baby seems to be less active than you expected, is missing normal developmental milestones, or displays other signs and symptoms of PKU, call your baby’s doctor. Older children who have been diagnosed with PKU may require stricter monitoring if they experience learning disabilities, irritability, hyperactivity or tremors.


The prognosis depends on how early an infant with PKU is diagnosed and then begins the special diet, as well as how strictly and consistently the diet is followed throughout life. Infants with PKU who are identified within the first few days after birth and are put on a strict diet before 3 weeks of age have the best prognosis and usually do not experience severe developmental delay or mental retardation. Dietary restrictions and monitoring of phenylalanine concentrations in the blood need to continue throughout childhood and adulthood to ensure normal brain development and function.

Johns Hopkins patient information

Last revised:

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.