Phenylketonuria (PKU)

What is phenylketonuria?

Phenylketonuria (PKU) is a rare birth defect. PKU means that the body is not able to break down an amino acid called phenylalanine. Amino acids are the building blocks for protein. Too much of this amino acid can be dangerous.

What is the cause?

Inside each cell of your body are genes. Genes contain the information that tells your body how to develop and work. Changes in the genes can be passed from parents to children.

For a child to be born with PKU, both parents must have the PKU gene. If only one parent has the gene, the child will not have PKU. A parent can have the PKU gene, but not have the disease. This is called being a “carrier.”

If a pregnant woman with PKU does not follow a PKU diet during pregnancy, the baby may be born with

  • An intellectual disability, which used to be called mental retardation
  • An abnormally small head
  • Heart defects

PKU is more common in white people than in black or Asian people.

What are the symptoms?

Newborns who have PKU do not have symptoms. Without treatment, babies usually show signs of PKU within a few months. Symptoms can be mild or severe and may include:

  • A musty odor in the child’s breath, skin or urine
  • Lighter skin and hair than other family members because the body produces less melanin, which is the chemical that controls hair and skin color
  • Rocking the body back and forth, shaking, or having seizures
  • Skin rashes
  • Small head size and slow growth
  • Vomiting

Untreated PKU leads to brain damage within the first few months of life. A child with untreated PKU may be irritable, restless, and have behavioral problems.

How is it diagnosed?

Newborn screening finds almost all cases of PKU. All 50 states test newborns for PKU.

Tests for PKU are done between 24 hours and 14 days after birth. The first test is usually done before a new baby leaves the hospital. If you don’t deliver your baby in a hospital or go home before your baby is tested, schedule a newborn screening test with your healthcare provider.

How is it treated?

The main treatment for PKU is a strict diet that limits phenylalanine, which is mostly found in protein. People with PKU need to follow the diet for life.

Because regular infant formula and breast milk contain phenylalanine, babies with PKU are put on a special infant formula. A small amount of breast milk may be OK for some babies. Talk with your healthcare provider about this.

You can get a nutritional drink for children with PKU. This drink has protein substitutes and nutrients but little or no phenylalanine. Your healthcare provider or dietitian will tell you how much of this drink your child should have each day.

How can I help my child?

Keep records of what your child eats every day. It helps to use a food diary or computer program that lists how much phenylalanine is in baby foods, PKU formulas, and other foods.

Milk, eggs, and other common foods contain phenylalanine. Give your child low-protein rice, pasta, pizza crusts, tortillas, bagels, breads, cookie dough, and baking mixes, as well as egg substitutes and imitation cheeses. These products let your child eat meals that are similar to what others are eating.

Teach your child to avoid foods and medicines made with aspartame, such as NutraSweet and Equal. Aspartame, found in many products, releases phenylalanine when digested.

A safe amount of phenylalanine differs for each child. Your healthcare provider can tell you what is safe for your child. Talk to your healthcare provider or dietitian if you have any questions.

How can I help prevent PKU?

If either parent has a family history of PKU, you can have a blood test before pregnancy or birth that can tell if you are a PKU carrier.

If you have PKU, you can prevent birth defects by sticking to a PKU diet. Even if you have mild PKU, you should follow the diet while you are pregnant.

Developed by RelayHealth.
Pediatric Advisor 2015.1 published by RelayHealth.
Last modified: 2012-12-17
Last reviewed: 2014-11-18
This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.
Copyright ©1986-2015 McKesson Corporation and/or one of its subsidiaries. All rights reserved.

Patient Portal

Our Patient Portal provides safe and secure online access to better communicate with your Tufts Medical Center Community Care doctor. This easy-to-use web tool is a convenient way to book appointments, request referrals, renew prescriptions, view medical records/test results and communicate with your healthcare provider from the privacy of your own computer.


Your privacy is important to us. Learn more about ourwebsite privacy policy. X