Phenylketonuria (fen-ul-ke-toe-NU-re-uh) is a birth defect in which a mutation occurs in a gene containing instructions for making the enzyme needed to break down the amino acid phenylalanine.
Amino acids are the building blocks for protein, but too much phenylalanine can cause a variety of health problems. People with phenylketonuria (PKU) — babies, children and adults — need to follow a diet that limits phenylalanine, which is found mostly in high-protein foods.
Babies in the United States and many other countries are screened for phenylketonuria soon after birth. Although phenylketonuria is rare, recognizing phenylketonuria right away can help prevent serious health problems.
Newborns with phenylketonuria initially don't have any symptoms. Without treatment, though, babies usually develop signs of PKU within a few months. Phenylketonuria symptoms can be mild or severe and may include:
Less severe forms of PKU — sometimes called mild or moderate PKU — have a smaller risk of significant brain damage, but most children with these forms of the disorder still require a special diet to prevent mental retardation and other complications.
Pregnancy and PKU
Although babies born to mothers with high phenylalanine levels may have complications at birth, most don't actually inherit PKU and won't need to follow a PKU diet after birth. However, these babies are at risk of being born with:
When to see a doctor
A genetic mutation causes PKU. The defective gene contains the instructions for making an enzyme needed to process the amino acid called phenylalanine. Amino acids are the building blocks for protein. In a person with PKU, this gene is defective, causing a complete or near-complete deficiency of the enzyme. Without the enzyme necessary to process phenylalanine, a dangerous buildup of this amino acid can develop when a person with PKU eats foods that are high in protein, such as milk, cheese, nuts or meats. This can eventually lead to serious health problems.
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 have the defective gene, but not have the disease. This is called being a carrier. Most often, PKU is passed down the family tree by parents who are carriers of the disorder, but don't know it.
Autosomal recessive inheritance pattern
In an autosomal recessive disorder, the mutated gene is recessive, which means it won't express its trait when paired with a normal, dominant gene. To have an autosomal recessive disorder, you ...
Both parents must pass along a copy of the mutated PKU gene for their child to develop the condition. If only one parent has the PKU gene, there's no risk of passing PKU to a child. The gene defect occurs mainly in people of Northern European and Native American ancestry. It's much less common in blacks, Asians and Hispanics.
Children of mothers who have PKU but who didn't follow the PKU diet during pregnancy also may be affected. Although these children don't often have PKU, they do have consequences of the high level of phenylalanine in the mother's blood.
Preparing for your appointment
PKU 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 experience in managing PKU. In addition to a doctor who specializes in treating PKU, you'll also need the assistance of a dietitian with expertise in the PKU diet.
Because appointments can be brief, and there's often a lot of ground to cover, it's a good idea to arrive well prepared. Here's some information to help you get ready for your appointment.
What you can do
Your appointment time may be limited, so preparing a list of questions can help you make the most of your time. List your questions from most important to least important in case time runs out. For PKU, some basic questions to ask include:
Tests and diagnosis
Newborn screening identifies almost all cases of PKU. All 50 states in the United States require newborns to be tested for PKU. Many other countries also routinely screen infants for PKU.
If you have PKU or a family history of PKU, 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
Your baby may also have additional tests to confirm the diagnosis, including more blood tests and urine tests. You and your baby may also undergo genetic testing to identify gene mutations.
If your baby's blood sample shows a high level of phenylalanine, he or she will need to be fed a special formula. Your doctor can help you find the right type of formula to feed your baby.
Screening during pregnancy
Untreated PKU can lead to:
Treatments and drugs
The main phenylketonuria treatment is a strict diet with very limited intake of phenylalanine, which is mostly found in protein-rich foods. Doctors used to believe it was OK for a person with PKU to stop the diet in adolescence, but today, doctors recommend sticking to the diet for life.
A safe amount of phenylalanine differs for each person. Your doctor will determine a safe amount through regular review of diet records, growth charts and blood levels of phenylalanine. Frequent blood tests will monitor PKU levels as they change over time, especially during childhood growth spurts and pregnancy. In general, the idea is to consume only the amount of phenylalanine that's necessary for normal growth and body processes, but no more.
Which foods to avoid
Children and adults should also avoid foods, including many diet sodas, and medications made with aspartame (NutraSweet, Equal). Aspartame, found in many artificial sweeteners, releases phenylalanine when digested.
Adults and children with PKU also have to limit foods, such as:
Too much of a good thing can sometimes be harmful. Even if you're eating approved foods, eating too many at one time can be dangerous. Consider the total amount of phenylalanine in all the foods you eat when planning your diet.
Formula for babies
Because regular infant formula and breast milk contain phenylalanine, babies with PKU need to have their diets substituted with a phenylalanine-free infant formula. A dietitian will carefully calculate a sparing amount of breast milk or regular formula to be mixed with 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. These foods replace the phenylalanine a baby has been receiving from breast milk or formula and need to be precisely figured into the daily phenylalanine intake.
Formula for older children and adults
The need for a nutritional supplement, especially if your child doesn't find it palatable, and the limited food choices can make the PKU diet challenging. But, it's the only way to prevent the serious health problems people with PKU can develop. Families need to commit to this lifestyle change wholeheartedly, realizing that it may be difficult but not impossible.
Lifestyle and home remedies
Keeping track, and measuring correctly
To do so, use a food diary or computer program that lists the amount of phenylalanine in baby foods, solid foods, PKU formulas, and common baking and cooking ingredients. To be as accurate as possible, measure portions, using standard measuring cups and spoons and a kitchen scale that reads in grams.
These products provide some dietary variety, and they allow people with PKU to eat lunches and dinners 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.
Food preparation ideas
You can transform the same basic lower phenylalanine vegetables into a whole menu of different dishes, using a little creativity — and a lot of seasonings. Herbs and flavorings low in phenylalanine can pack a flavorful punch. Just remember to measure and count every ingredient and adjust ideas to your individualized diet. Talk to your doctor or dietitian if you have any questions. If you have any other health conditions, you may need to consider those too when you plan your diet.
Coping and support
Living with PKU can be difficult. The combination of a limited diet, expensive groceries, regular blood tests, detailed food records and frequent visits to the doctor can make for trying and frustrating times. If you have a child with PKU, it can be difficult to explain why he or she can't eat "normal" foods. And, mealtimes and snack times can be a battle. The following strategies may help:
Last Updated: 2009-10-20
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