Medically reviewed by Drugs.com. Last updated on Jan 18, 2021.
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 defect in the gene that helps create the enzyme needed to break down phenylalanine.
Without the enzyme necessary to process 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.
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.
Newborns with PKU initially don't have any symptoms. However, without treatment, babies usually develop signs of PKU within a few months.
PKU signs and symptoms can be mild or severe and may include:
- A musty odor in the breath, skin or urine, caused by too much phenylalanine in the body
- Neurological problems that may include seizures
- Skin rashes (eczema)
- Fair skin and blue eyes, because phenylalanine can't transform into melanin — the pigment responsible for hair and skin tone
- Abnormally small head (microcephaly)
- Intellectual disability
- Delayed development
- Behavioral, emotional and social problems
- Psychiatric disorders
The severity of PKU depends on the type.
- Classic PKU. The most severe form of the disorder is called classic PKU. The enzyme needed to convert phenylalanine is missing or severely reduced, resulting in high levels of phenylalanine and severe brain damage.
- Less severe forms of PKU. In mild or moderate forms, the enzyme retains some function, so phenylalanine levels are not as high, resulting in a smaller risk of significant brain damage.
But most children with the disorder still require a special PKU diet to prevent intellectual disability and other complications.
Pregnancy and PKU
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 fetus or cause a miscarriage.
Even women with less severe forms of PKU may place their unborn children at risk by not following the PKU diet.
Babies born to mothers with high phenylalanine levels don't often inherit PKU. But they can have serious consequences if the level of phenylalanine is high in the mother's blood during pregnancy. Complications at birth may include:
- Low birth weight
- Delayed development
- Facial abnormalities
- Abnormally small head
- Heart defects and other heart problems
- Intellectual disability
- Behavioral problems
When to see a doctor
Seek medical advice in these situations:
- Newborns. If routine newborn screening tests show that your baby may have PKU, your child's doctor will want to start dietary treatment right away to prevent long-term problems.
- Women of childbearing years. It's especially important for women with a history of PKU to see a doctor and maintain the PKU diet before becoming pregnant and during pregnancy to reduce the risk of high blood phenylalanine levels harming their unborn babies.
- Adults. People with PKU continue to receive care across the life span. Adults with PKU who have stopped the PKU diet in their teens may benefit from a visit with their doctors. Returning to the diet may improve mental functioning and behavior and slow damage to the central nervous system that can result from high phenylalanine levels.
A defective gene (genetic mutation) causes PKU, which can be mild, moderate or severe. In a person with PKU, this defective gene causes a lack of or deficiency 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, and even grains such as bread and pasta, or eats aspartame, an artificial sweetener. This buildup of phenylalanine results in damage to nerve cells in the brain.
For a child to inherit PKU, both the mother and father must have and pass on the defective gene. This pattern of inheritance is called autosomal recessive.
It's possible for a parent to be a carrier — to have the defective gene that causes PKU, but not have the disease. If only one parent has the defective 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 carriers of the disorder, but don't know it.
To have an autosomal recessive disorder, you inherit two mutated genes, one from each parent. These disorders are usually passed on by two carriers. Their health is rarely affected, but they have one mutated gene (recessive gene) and one normal gene (dominant gene) for the condition. Two carriers have a 25% chance of having an unaffected child with two normal genes (left), a 50% chance of having an unaffected child who also is a carrier (middle), and a 25% chance of having an affected child with two recessive genes (right).
Risk factors for inheriting PKU include:
- Having both parents with a defective gene that causes PKU. Two parents must pass along a copy of the defective gene for their child to develop the condition.
- Being of certain ethnic descent. The gene defect that causes PKU varies by ethnic groups and it's less common in African-Americans than in other ethnic groups.
Untreated PKU can lead to complications in infants, children and adults with the disorder. When mothers with PKU have high blood phenylalanine levels during pregnancy, fetal birth defects or miscarriage can occur.
Untreated PKU can lead to:
- Irreversible brain damage and marked intellectual disability beginning within 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 problems
If you have PKU and are considering getting 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 you start trying to conceive.
- Consider genetic counseling. If you have PKU, a close relative with PKU or a child with PKU, you may also benefit from genetic counseling before becoming pregnant. A doctor who specializes in medical genetics (geneticist) can help you better understand how PKU is passed through your family tree. He or she can also help determine your risk of having a child with PKU and assist with family planning.
Newborn blood testing 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 doctor may recommend screening tests before pregnancy or birth. It's possible to identify PKU carriers through a blood test.
Testing your baby after birth
A PKU test is done a day or two after your baby's birth. The test is done after your baby is 24 hours old and after your baby has ingested some protein in the diet to ensure accurate results.
- A nurse or lab technician collects a few drops of blood from your baby's heel or the bend in your baby's arm.
- A laboratory tests the blood sample for certain metabolic disorders, including PKU.
- If you don't deliver your baby in a hospital or are discharged soon after the birth, you may need to schedule a newborn screening with your pediatrician or family doctor.
If this test indicates your baby may have PKU:
- Your baby may have additional tests to confirm the diagnosis, including more blood tests and urine tests
- You and your baby may have genetic testing to identify gene mutations
The main treatment for PKU includes:
- A lifetime diet with very limited intake of protein, because foods with protein contain phenylalanine
- Taking a PKU formula — a special nutritional supplement — for life to make sure you get enough essential protein (without phenylalanine) and nutrients that are crucial for growth and general health
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 normal growth and body processes, but no more. Your doctor can determine a safe amount through:
- Regular review of diet records, growth charts and blood levels of phenylalanine
- Frequent blood tests that monitor phenylalanine levels as they change over time, especially during childhood growth spurts and pregnancy
- Other tests that assess growth, development and health
Your doctor 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.
Which foods to avoid
Because the amount of phenylalanine that a person with PKU can safely eat is so low, it's crucial to avoid all high-protein foods, such as:
Potatoes, grains and other vegetables that have protein 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 over-the-counter products or prescription medications.
Talk with your doctor or registered dietitian to learn more about your specific dietary needs.
Formula for people with PKU
Because of the restricted diet, people with PKU need to get essential nutrients through a special nutritional supplement. The phenylalanine-free formula provides protein and other essential nutrients in a form that's safe for people with PKU.
Your doctor and dietitian can help you find the right type of formula.
- Formula for babies and toddlers. Because regular infant formula and breast milk contain phenylalanine, babies with PKU instead need to consume a phenylalanine-free infant formula. A dietitian can carefully calculate the amount of breast milk or regular formula to be added to the phenylalanine-free formula. Parents introduce solid foods with low levels of phenylalanine to children with PKU on the same schedule used for other infants.
- Formula for older children and adults. Older children and adults continue to drink or eat a protein substitute formula daily, as directed by a doctor or dietitian. Your daily dose of formula is divided between your meals and snacks, instead of consumed all at once. The formula for older children and adults is not the same as the one used for infants, but it works on the same principle. It provides essential protein (amino acids) without phenylalanine and is continued for life.
The need for a nutritional supplement, especially if your child doesn't find it appealing, and the limited food choices can make the PKU diet challenging. But families need to make a firm commitment to this lifestyle change because it's the only way to prevent the serious health problems that people with PKU can develop.
Neutral amino acid therapy
Another possible addition to the PKU diet is a supplement called neutral amino acid therapy in powder or tablet form. This supplement may block some absorption of phenylalanine. This may be a treatment option for adults with PKU. Ask your doctor or dietitian if this supplement is appropriate for your diet.
The Food and Drug Administration (FDA) has approved the drug sapropterin (Kuvan) for the treatment of PKU. It works by increasing your tolerance to phenylalanine. The drug is for use in combination with a PKU diet. But it doesn't work for everyone with PKU.
In approving sapropterin, the FDA directed that studies continue because there are no long-term studies on the drug's efficacy and long-term safety.
Lifestyle and home remedies
Some strategies to help manage PKU may include the following.
Keep track and measure correctly
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 sure you're sticking to the specific, individualized dietary guidelines recommended by your dietitian.
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 or computer programs are available that list the amount of phenylalanine in baby foods, solid foods, PKU formulas, and common baking and cooking ingredients.
Buy low-protein products
To add variety to your diet, buy some of the many low-protein products, such as low-protein pasta, rice, flour and bread, which are available through specialty food retailers.
These products allow people with PKU to eat some meals that more closely resemble what everyone else is eating. Like the PKU formulas, these products can be expensive, but you might consider splurging on a few favorites with the money you save on dairy and meat products.
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 pack a flavorful punch. Just remember to measure and count every ingredient and adjust recipes to your individualized diet.
If you have any other health conditions, you may also need to consider those when you plan your diet. Talk with your doctor or dietitian if you have any questions.
Coping and support
Living with PKU can be challenging. These strategies may help:
- Stay informed. Knowing the facts about PKU can help you take charge of the situation. Discuss any questions with your pediatrician, family doctor, geneticist or dietitian. Read books and cookbooks specifically written for people with PKU.
- Learn from other families. Ask your doctor about local or online support groups for people dealing with PKU. Talking with others who have mastered similar challenges can be very helpful. The National PKU Alliance is an online support group for adults with PKU.
- Get help with menu planning. A registered dietitian with experience in PKU can help you devise delicious low-phenylalanine dinners. He or she may also have great ideas for holiday meals and birthdays.
- Plan ahead when you eat out. A meal at the local restaurant gives you a break from the kitchen and can be fun for the whole family. Most places offer something that fits into the PKU diet. But you may want to call ahead and ask about the menu or bring food from home.
- Find sources of financial aid. Ask your doctor or dietitian if there are programs or insurance plans that help cover the high costs of formula and low-protein foods. Also, see if your local school lunch program will accommodate special dietary needs.
- Don't focus on food. Encourage children with PKU to focus on sports, music or favorite hobbies, not on just what they can and can't eat. Also consider creating holiday traditions that center on special projects and activities, not just food.
- Let your child help manage his or her diet as early as possible. Toddlers can make choices about which cereal, fruit or vegetable they'd like to eat and help measure out portions. They can also help themselves to pre-measured snacks. Older children can help with menu planning, pack their own lunches and keep their own food records.
- Make your grocery list and meals with the whole family in mind. A cupboard full of restricted foods can be tempting to a child or adult with PKU, so try to focus on foods that everyone can eat. Serve stir-fried vegetables that are lower in protein. If the other family members wish, they can add peas, corn, meat and rice. Or set up a salad bar with low-protein and moderate-protein options. You can also serve the whole family a delicious low-phenylalanine soup or curry.
- Be prepared for potlucks, picnics and car trips. Plan ahead, so there's always a PKU-friendly food option. Pack dehydrated fruit snacks, raisins and lower protein crackers for the car. Take fruit kebabs or vegetable skewers to a cookout, and make a low-phenylalanine salad for the neighborhood potluck. Other parents, friends and family members will likely be accommodating and helpful if you explain the dietary restrictions.
- Talk to teachers and other staff in your child's school. Your child's teachers and cafeteria staff can be a big help with the PKU diet if you take the time to explain its importance and how it works. By working with your child's teachers, you can also plan ahead for special school events and parties so that your child always has a treat to eat.
- Maintain a positive food attitude. When children know nothing but the foods they are given, they can be surprisingly accepting of the PKU diet — especially when their parents are positive problem-solvers.
Preparing for an appointment
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 doctor who specializes in treating PKU and a dietitian with expertise in the PKU diet.
The following information will help you prepare for your appointment, and understand what to expect from your doctor.
What you can do
Before your appointment:
- Ask a family member or friend to come with you — sometimes it can be difficult to remember all of the information provided during an appointment.
- Make a list of questions to ask your doctor and dietitian to help you make the most of your time together.
Some basic questions to ask may include:
- How did my child get PKU?
- How can we manage PKU?
- Are there any medications to treat this disease?
- What foods are completely off-limits?
- What is the recommended diet?
- Will my child have to stay on this special diet for life?
- What kind of formula will my child need? Can my child have breastmilk?
- Are there any other supplements needed?
- What happens if my child eats a food that he or she is not supposed to eat?
- If I have another child, will he or she have PKU?
- Are there brochures or other printed material that I can have? What websites do you recommend?
What to expect from your doctor
Your doctor is likely to ask you several questions. Be ready to answer them to reserve time to go over points you want to focus on. For example, your doctor may ask:
- Has your child had any symptoms that concern you?
- Do you have any questions about your child's diet?
- Are you having any difficulty following the diet?
- Has the growth and development of your child been typical of other children the same age?
- Have you ever had genetic testing?
- Do any other relatives have PKU?