Phenylketonuria (PKU)

Phenylketonuria is a hereditary metabolic disorder. Children with PKU are born without the enzyme needed to break down phenylalanine. Phenylalanine is an amino acid (the building blocks of proteins) in many foods and drinks. Normally, your body breaks down and gets rid of extra phenylalanine. Children with PKU can't make the enzyme that is needed to break down phenylalanine to get rid of it.

• If PKU isn't treated, phenylalanine builds up in the blood and causes brain problems (intellectual disability)

• Children with PKU must stay on a lifelong special diet that has very little phenylalanine in it

Having 2 defective PKU genes causes PKU. If you have 2 defective PKU genes, you can't make an enzyme that breaks down phenylalanine.

• PKU happens when both parents pass the defective gene that causes PKU to a child

• If only one parent has the defective PKU gene and the other parent doesn't, the child can't have PKU

• If both parents have don't have PKU but have one copy of the defective PKU gene (are carriers), the child has a 1 in 4 chance of having PKU 

Babies don't usually have symptoms of PKU right away, although sometimes they are sleepy or don't eat well. Symptoms in untreated children usually develop over a few months after birth and include:

• Seizures

• Throwing up

• Eczema-like rash

• Sometimes, a musty body smell

As untreated children get older, their brain doesn't develop as it should (they have intellectual disability). They may become aggressive or hyperactive.

Children who are treated in the first few days of life do not develop the severe symptoms of PKU. A phenylalanine-restricted diet, if started early and maintained well, allows for mostly normal development.

Doctors test all newborn babies for PKU as part of normal newborn screening. This includes a blood test using a few drops of blood from the baby's heel.

If PKU runs in your family, doctors can do testing before the baby is born (prenatal testing, such as amniocentesis) to see if the baby has PKU.

You can get a blood test to find out if you're a carrier of the PKU gene.

Starting in the first few weeks of a baby's life, the baby must go on a diet nearly free of phenylalanine. Doctors treat PKU with:

• A strict diet that limits phenylalanine

• Sometimes, medicine to increase tolerance of phenylalanine

People with PKU can't eat any milk, meat, or other foods with protein. They can have some fruits, vegetables, and grain cereals. They also eat special food, such as phenylalanine-free formula and nutritional products. Your child's doctor will tell you what your child can eat.