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Phenylketonuria (PKU)

Medically reviewed by: Michael F. Cellucci, MD

What Is Phenylketonuria (PKU)?

Phenylketonuria (fen-ul-kee-tuh-NUR-ee-uh), or PKU, is a metabolic disorder that some babies are born with. It's caused by a defect in the enzyme that breaks down the amino acid phenylalanine.

Newborn babies in the United States have their blood tested for PKU as part of newborn screening. This lets doctors start treatment, usually a special diet, right away to help prevent problems.

What Are the Signs & Symptoms of PKU?

The body uses amino acids to build proteins. Phenylalanine (fen-ul-AL-uh-neen) is needed for normal growth in infants and children and for normal protein production. But if too much builds up in the body, the brain is affected. Phenylketonuria that's not treated can lead to developmental delays and permanent intellectual disability.

Phenylalanine also affects melanin, the pigment responsible for hair color and skin color. So kids with PKU often have fair skin, light hair, and blue eyes.

A child with PKU may also have:

  • seizures
  • growth problems
  • behavioral problems
  • skin rashes
  • a musty odor to the breath, skin, or urine (pee) from too much phenylalanine in the body

What Causes PKU?

Phenylalanine is one of the eight amino acids that we get only from food sources. Usually, the body breaks down phenylalanine with an enzyme called phenylalanine hydroxylase (PAH) to make proteins.

People with PKU are born with an inherited defect in the gene that controls the production of PAH. So their bodies can't break down phenylalanine properly.

How Is PKU Diagnosed?

All newborn babies in the United States have their blood tested for signs of PKU as part of newborn screening.

How Is Phenylketonuria Treated?

Treatment for PKU involves following a strict diet that is low in phenylalanine. Babies with PKU need to be on a special formula as soon as possible.

Children and adults with PKU should eat a low-protein diet. They should avoid high-protein foods, like milk, dairy, meats, eggs, nuts, soy, and beans. A person with PKU also should avoid the artificial sweetener aspartame, which contains phenylalanine. Special formulas may be needed at any age for the person to get the right amount of calories and essential nutrients.

The special diet should start as soon as PKU is diagnosed and continue for the rest of the person's life. There are medicines that can help some people with milder forms of PKU, but medical nutrition therapy is still the main treatment for the condition.

What Else Should I Know?

PKU is treatable when found early. Doctors will closely watch kids who have it. People with PKU must strictly follow the right diet for the rest of their lives. This can prevent the problems caused by too much phenylalanine.

If your child has phenylketonuria, work with the health care team to help keep phenylalanine levels in an acceptable range. Your doctor will check the level of phenylalanine in your child's blood regularly.

Because PKU is a genetic condition, you may want to speak to a genetic counselor about testing other family members and how PKU runs in families. You also can find more information and support online at:

Medically reviewed by: Michael F. Cellucci, MD
Date reviewed: July 2022