Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link
Quick Facts

Phenylketonuria (PKU)


The Manual's Editorial Staff

Last full review/revision Apr 2021| Content last modified Apr 2021
Click here for the Professional Version
Get the full details

What is 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

What causes PKU?

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 gene that causes PKU to a child

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

  • If both parents have the PKU gene, the child has a 1 in 4 chance of having PKU 

What are the symptoms of PKU?

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

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

How can doctors tell if my child has PKU?

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.

How do doctors treat PKU?

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.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version


Others also read

Test your knowledge

Motion Sickness
Motion sickness includes symptoms, particularly nausea, that affect people while they ride in a moving object. The object can be a transport vehicle such as a car, boat, train, or airplane, or an amusement park ride. A contributing factor is over-stimulation of a part of the body involved in balance control. Which of the following is that body part?
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID