Phenylketonuria (PKU)

What is phenylketonuria?

Phenylketonuria (or PKU) is a rare condition where babies are born unable to break down an amino acid called phenylalanine.

Symptoms of phenylketonuria

Initially, there are no symptoms. But within a few months of birth, depending on the severity of the disease, babies can develop:

  • Smaller than normal head size (called microcephaly).
  • Hyperactivity.
  • A musty or mouse-like odor in urine, breath, or skin.
  • Lighter skin, hair, and eyes than their siblings.
  • Skin disorders such as eczema.
  • Jerking movements of the arms or legs.
  • Tremors.
  • Seizures.
  • Delayed development.
  • Behavioral problems.
  • Psychiatric disorders.
  • Permanent intellectual disability.

How is phenylketonuria diagnosed?

All newborn babies born in the United States are routinely tested for a battery of diseases and conditions. One of those tests is a blood test that, among other things, looks for the presence of phenylalanine hydroxylase, the enzyme needed to process phenylalanine. If that enzyme is missing, more blood and urine tests can confirm the diagnosis of PKU.

Can phenylketonuria be prevented?

Since PKU is a genetic condition — meaning, it’s inherited from the parents — it can’t be prevented. Genetic testing is the only way to determine if people carry the defective gene. Those who carry the gene who don’t have PKU show no signs or symptoms of it. Both parents must carry one copy of the gene for the disease to be passed on. Additionally, tests during pregnancy (chorionic villus sampling or amniocentesis) can screen for PKU.

If you have PKU, you must follow a strict low-phenylalanine diet both before becoming pregnant and during the pregnancy. Buildup of phenylalanine will damage your developing fetus, even if the baby has not inherited the defective gene.

Phenylketonuria treatment

The most important treatment for PKU is a diet that limits foods with phenylalanine. Newborns diagnosed with the disease must use special infant formula, which can be mixed with small amounts of breast milk or regular formula. It’s a delicate balance. Your baby does need some phenylalanine for normal development. But too much can hurt.

Children with PKU will need to follow a low-phenylalanine diet. Phenylalanine is found in most protein foods, including all meats and fish, milk, cheese, eggs, nuts, soy, and beans. It’s also found in other non-protein foods, including certain vegetables and fruit, certain breads, and beer. The sweetener aspartame, which releases phenylalanine when it’s digested, must also be avoided.

The U.S. Food and Drug Administration has approved the drug sapropterin dihydrochloride (Kuvan) for the treatment of PKU. Kuvan can help the body break down phenylalanine. However, the drug only helps some people, and even for those it does help, it does not decrease enough phenylalanine. Those on the drug must still follow a low-phenylalanine diet.

Living with phenylketonuria

Because the diet is so limiting, it’s tough to get all the essential nutrients the body needs. A special daily formula can help ensure those with PKU get proper nutrition. Taking supplements may also be necessary. For example, fish oil can replace some of the long-chain fatty acids (the basic building blocks of lipids, or fats) missing from the standard PKU diet. Those fatty acids can help improve neurological development.

Each person with PKU has a different level of phenylalanine that they can tolerate. If you have PKU, you must work closely with a health care professional to develop your individual diet. Going to the doctor often, and getting regular blood tests helps keep everything in check. You need to eat enough phenylalanine for healthy growth and development. However, you can’t eat so much that it will compromise your health.

The PKU diet is very restrictive. But it must be followed. Those who stay on the diet throughout their lives have better overall physical and mental health than those who don’t. Support from friends and family or a PKU support group can be a lifesaver when facing the challenges that come with this diet.


  • Should I get genetic testing for PKU?
  • If I have PKU what should I do during my pregnancy to protect my baby?
  • How often do I need to bring my baby with PKU in for testing?
  • What determines the severity of PKU?
  • What should I feed my child?
  • What foods should my child stay away from?
  • What happens if my child eats a food he’s not supposed to eat?
  • Will my next child have PKU?