What is Phenylketonuria?

Phenylketonuria, also known as PKU, is a rare inherited disease that causes an amino acid called phenylalanine to build up in the body.

PKU is caused by a defect in the gene that helps make the enzyme needed to break down phenylalanine..

Without the enzyme needed to process phenylalanine, a dangerous buildup can occur if a person with PKU eats protein-containing foods or consumes the artificial sweetener, aspartame. This can eventually lead to serious health problems.

For the rest of their lives, people with PKU – infants, children, and adults – must adhere to a diet that limits phenylalanine, which is mostly found in protein-containing foods.

Babies in the United States and many other countries are screened for PKU soon after birth. Recognizing PKU right away can help prevent major health problems.

Phenylketonuria Symptoms

Newborns with PKU do not initially have any symptoms. However, without treatment, babies often develop symptoms of PKU within a few months.

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

  • Musty odor in the breath, skin, or urine caused by too much phenylalanine in the body
  • Neurological problems that can cause seizures
  • Skin rashes (eczema)
  • Fair skin and blue eyes, because phenylalanine cannot convert to melanin – the pigment responsible for hair and skin tone
  • Abnormally small head (microcephaly)
  • hyperactivity
  • mental disability
  • delayed development
  • Behavioral, emotional and social problems
  • psychological disorders

The severity of PKU depends on its type.

Classic PKU:

The most severe form of the disease is classic PKU. The enzyme needed to convert phenylalanine is missing or severely reduced, resulting in high phenylalanine levels and severe brain damage.

The less severe form of PKU:

In mild or moderate forms, the enzyme retains some functions, so phenylalanine levels are not that high, resulting in a smaller risk of brain damage.
However, most children with the disorder still need a special PKU diet to prevent intellectual disability and other complications.

Pregnancy and PKU

Pregnant women with PKU are at risk for another form of the condition called maternal PKU. If women do not follow the special PKU diet before and during pregnancy, blood phenylalanine levels can rise and harm the developing fetus or cause miscarriage.

Even women with less severe forms of PKU can put their unborn child at risk for not following a PKU diet.

Babies born to mothers with high phenylalanine levels usually do not inherit PKU. But if the mother has high levels of phenylalanine in her blood during pregnancy, it can have serious consequences. Complications at birth can include:

  • low birth weight
  • delayed development
  • facial anomalies
  • abnormally small head
  • Heart defects and other heart problems
  • mental disability
  • behavioral problems

When to See a Doctor

Get medical help if:


If routine newborn screening tests show that your baby may have PKU, your child’s doctor will want to start diet therapy right away to prevent long-term problems.

Women with fertility:

It is especially important for women with a history of PKU to see a doctor and maintain a PKU diet before conception and during pregnancy to reduce the risk of high phenylalanine levels harming their unborn baby.


People with PKU continue to receive lifelong care. Adults with PKU who stop the PKU diet among their teens may see their doctor. Returning to the diet can improve mental function and behavior and slow the damage to the central nervous system that can result from high phenylalanine levels.

Causes of Phenylketonuria

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

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

genetic inheritance

For a child to inherit PKU, both parents must inherit and pass on the defective gene. This pattern of inheritance is called autosomal recessive.

It is possible for one parent to be a carrier, i.e. have the defective gene that causes PKU, but not have the disease. If only one parent has the defective gene, there is no risk of passing PKU to a child, but it is possible for the child to be a carrier.

Most often, PKU is passed on to children by two parents who are carriers of the disease, but they don’t know it.

Risk factors

Risk factors for inheriting PKU include:

Having both parents with a defective gene that causes PKU. Two parents must pass on one copy of the defective gene for their child to develop the condition.
Being of certain ethnicity. The gene defect that causes PKU varies by ethnic group and is less common in African-Americans than in other ethnic groups.

Phenylketonuria Complications

Untreated PKU can lead to complications in infants, children, and adults with the disorder. When mothers with PKU have high blood levels of phenylalanine during pregnancy, they may have fetal birth defects or miscarriage.

Untreated PKU can lead to:

  • Irreversible brain damage and marked intellectual disability beginning in the first few months of life
  • Neurological problems such as seizures and tremors
  • Behavioral, emotional, and social problems in older children and adults
  • Major health and developmental issues

Can Phenylketonuria Be Prevented?

If you have PKU and are considering becoming pregnant:

Follow a low phenylalanine diet. Women with PKU can prevent birth defects by sticking to or returning to a low phenylalanine diet before becoming pregnant. If you have PKU, talk to your doctor before trying to conceive.

Consider getting genetic counseling. If you have PKU, a close relative with PKU, or a child with PKU, you may also benefit from genetic counseling before you become pregnant. A doctor who specializes in medical genetics (genetics) can help you better understand how your PKU runs through your family tree. It can also help you determine your risk of having a child with PKU and help you plan a family.

Mayo Clinic, Phenylketonuria (PKU), 2018.


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  4. Phenylketonuria (PKU). Merck Manual Professional Version. http://www.merckmanuals.com/professional/pediatrics/inherited-disorders-of-metabolism/phenylketonuria-pku. Accessed Oct. 8, 2017.
  5. Bodamer OA. Overview of phenylketonuria. https://www.uptodate.com/contents/search. Accessed Oct. 8, 2017.
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