PKU is caused by a change in the gene that helps create an enzyme needed to break down phenylalanine. Treatment includes a special diet and medication.
Phenylketonuria (fen-ul-key-toe-NU-ree-uh), also called PKU, is a rare inherited disorder that causes an amino acid called phenylalanine to build up in the body. PKU is caused by a change in the phenylalanine hydroxylase (PAH) gene. This gene helps create the enzyme needed to break down phenylalanine.
Without the enzyme necessary to break down phenylalanine, a dangerous buildup can develop when a person with PKU eats foods that contain protein or eats aspartame, an artificial sweetener. This can eventually lead to serious health problems.
For the rest of their lives, people with PKU — babies, children and adults — need to follow a diet that limits phenylalanine, which is found mostly in foods that contain protein. Newer medications may allow some people with PKU to eat a diet that has a higher or an unrestricted amount of phenylalanine.
Babies in the United States and many other countries are screened for PKU soon after birth. Although there is no cure for PKU, recognizing PKU and starting treatment right away can help prevent limitations in areas of thinking, understanding and communicating (intellectual disability) and major health problems.
Newborns with PKU initially don't have any symptoms. However, without treatment, babies usually develop signs of PKU within a few months.
Signs and symptoms of untreated PKU can be mild or severe and may include:
The severity of PKU depends on the type.
Regardless of the form, most infants, children and adults with the disorder still require a special PKU diet to prevent intellectual disability and other complications.
Women who have PKU and become pregnant are at risk of another form of the condition called maternal PKU. If women don't follow the special PKU diet before and during pregnancy, blood phenylalanine levels can become high and harm the developing baby.
Even women with less severe forms of PKU may place their unborn children at risk by not following the PKU diet.
Babies born to women with high phenylalanine levels don't often inherit PKU. But a child can have serious problems if the level of phenylalanine is high in the mother's blood during pregnancy. At birth, the baby may have:
In addition, maternal PKU can cause the child to have delayed development, intellectual disability and problems with behavior.
Talk to your health care provider in these situations:
A gene change (genetic mutation) causes PKU, which can be mild, moderate or severe. In a person with PKU, a change in the phenylalanine hydroxylase (PAH) gene causes a lack of or reduced amount of the enzyme that's needed to process phenylalanine, an amino acid.
A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts or meat, or grains such as bread and pasta, or aspartame, an artificial sweetener.
For a child to inherit PKU, both the mother and father must have and pass on the changed gene. This pattern of inheritance is called autosomal recessive.
It's possible for a parent to be a carrier — to have the changed gene that causes PKU, but not have the disease. If only one parent has the changed gene, there's no risk of passing PKU to a child, but it's possible for the child to be a carrier.
Most often, PKU is passed to children by two parents who are both carriers of the changed gene, but don't know it.
To have an autosomal recessive disorder, you inherit two changed genes, sometimes called mutations. You get one from each parent. Their health is rarely affected because they have only one changed gene. Two carriers have a 25% chance of having an unaffected child with two unaffected genes. They have a 50% chance of having an unaffected child who also is a carrier. They have a 25% chance of having an affected child with two changed genes.
Risk factors for inheriting PKU include:
Untreated PKU can lead to complications in infants, children and adults with the disorder. When women with PKU have high blood phenylalanine levels during pregnancy, it can harm their unborn baby.
Untreated PKU can lead to:
If you have PKU and are considering getting pregnant:
Newborn screening identifies almost all cases of phenylketonuria. All 50 states in the United States require newborns to be screened for PKU. Many other countries also routinely screen infants for PKU.
If you have PKU or a family history of it, your health care provider may recommend screening tests before pregnancy or birth. It's possible to identify PKU carriers through a blood test.
A PKU test is done a day or two after your baby's birth. For accurate results, the test is done after your baby is 24 hours old and after your baby has had some protein in the diet.
If this test indicates that your baby may have PKU:
Starting treatment early and continuing treatment throughout life can help prevent intellectual disability and major health problems.
The main treatments for PKU include:
A safe amount of phenylalanine differs for each person with PKU and can vary over time. In general, the idea is to consume only the amount of phenylalanine that's necessary for healthy growth and body processes, but no more. Your health care provider can determine a safe amount through:
Your health care provider may refer you to a registered dietitian who can help you learn about the PKU diet, make adjustments to your diet when needed, and offer suggestions on ways to manage PKU diet challenges.
Because the amount of phenylalanine that a person with PKU can safely eat is so low, it's important to avoid all high-protein foods, such as:
Potatoes, grains and other vegetables will likely be limited.
Children and adults also need to avoid certain other foods and beverages, including many diet sodas and other drinks that contain aspartame (NutraSweet, Equal). Aspartame is an artificial sweetener made with phenylalanine.
Some medications may contain aspartame and some vitamins or other supplements may contain amino acids or skim milk powder. Check with your pharmacist about the contents of nonprescription products and prescription medications.
Talk with your health care provider or registered dietitian to learn more about your specific dietary needs.
Because of the restricted diet, people with PKU need to get essential nutrients through a special nutritional supplement. The phenylalanine-free formula provides essential protein (amino acids) and other nutrients in a form that's safe for people with PKU.
Your health care provider and dietitian can help you find the right type of formula.
The need for a nutritional supplement, especially if you or your child doesn't find it appealing, and the limited food choices can make the PKU diet challenging. But making a firm commitment to this lifestyle change is the only way to prevent the serious health problems that people with PKU can develop.
The Food and Drug Administration (FDA) approved the drug sapropterin (Kuvan) for the treatment of PKU. The drug may be used in combination with a PKU diet. Some people with PKU who are taking the medication may not need to follow a PKU diet. But the drug doesn't work for everyone with PKU.
The FDA also approved a novel enzyme therapy, pegvaliase-pqpz (Palynziq), for adults with PKU when current therapy does not adequately reduce the level of phenylalanine. But because of frequent side effects, which can be severe, this treatment is only available as part of a restricted program under the supervision of certified health care providers.
Strategies to help manage PKU include keeping track of foods eaten, measuring correctly, and being creative. Like anything, the more these strategies are practiced, the greater the comfort and confidence you can develop.
If you or your child is following a low-phenylalanine diet, you'll need to keep records of the food eaten every day.
To be as accurate as possible, measure food portions using standard measuring cups and spoons and a kitchen scale that reads in grams. The food amounts are compared with a food list or are used to calculate the amount of phenylalanine eaten every day. Each meal and snack includes the appropriately divided portion of your daily PKU formula.
Food diaries, computer programs and smartphone apps are available that list the amount of phenylalanine in baby foods, solid foods, PKU formulas, and common baking and cooking ingredients.
Meal planning or meal rotations of known foods may help reduce some of the daily tracking.
Talk with your dietitian to find out how you can be creative with foods to help you stay on track. For example, use seasonings and a variety of cooking methods to transform lower phenylalanine vegetables into a whole menu of different dishes. Herbs and flavorings low in phenylalanine can have lots of flavor. Just remember to measure and count every ingredient and adjust recipes to your specific diet.
If you have any other health conditions, you may also need to consider those when you plan your diet. Talk with your health care provider or dietitian if you have any questions.
Living with PKU can be challenging. These strategies may help:
Phenylketonuria is generally diagnosed through newborn screening. Once your child is diagnosed with PKU, you'll likely be referred to a medical center or specialty clinic with a specialist who treats PKU and a dietitian with expertise in the PKU diet.
Here's some information to help you get ready for your appointment and know what to expect.
Before your appointment:
Some questions to ask may include:
Your health care provider is likely to ask you several questions. For example:
Your health care provider will ask additional questions based on your responses, symptoms and needs. Preparing and anticipating questions will help you make the most of your appointment time.