What is Phenylketonuria (PKU)?
Phenylketonuria (PKU) is a genetic disorder characterized by the inability of the body to utilize the essential amino acid phenylalanine (Phe). Amino acids are the building blocks of body proteins, and essential amino acids can only be obtained from the food we eat as the body does not generally produce them.
With Classic PKU the enzyme that breaks down Phe is deficient. This enzyme is meant to convert Phe into another amino acid called tyrosine. Without the enzyme, Phe accumulates in the blood and body tissues. A high level of Phe is called hyperphenylalaninemia, meaning an elevated blood phenylalanine, and usually refers to a group of disorders other than Classic PKU. If the Phe level rises too high it can damage the brain, causing severe mental retardation.
PKU affects about one in 15,000 Caucasian or Oriental births, but is much less prevalent in African American births.
Causes of Phenylketonuria:
PKU is caused by a genetic mutation. For a child to inherit PKU both biological parents must have and pass on the defective gene, which is a pattern of inheritance called autosomal recessive. It is possible for one parent to have the defective gene and not have the disease, which is called being a carrier. Most often, PKU is passed on to children by parents who are carriers of the disorder without knowing it. PKU carriers can be identified through a simple blood test.
Types of Phenylketonuria:
Classic PKU is the most severe form of the condition. Children with untreated Classic PKU usually develop an obvious and permanent form of mental retardation.
Mild or moderate PKU has a smaller risk of significant brain damage, but most children with the moderate disorder still require a special diet to prevent mental retardation among other complications.
Risk Factors of Phenylketonuria:
Pregnant women with PKU are at risk for Maternal PKU. Blood Phe levels can become out of range and harm the developing fetus, which is why it is important to maintain a strict PKU-friendly diet. Although babies born to mothers with high Phe levels may be born with complications, most do not inherit PKU and will not need to follow the diet after birth.
Some of the risk factors those babies are born with include:
- Mental retardation
- Low birth weight
- Behavioral problems
- Heart defects
- Abnormally small head (microcephaly)
Symptoms of Phenylketonuria:
Newborns with PKU initially will not show symptoms; however, without treatment babies can develop signs of PKU within a few months. Symptoms may be mild or severe and can include:
- Small head size (microcephaly)
- Stunted growth
- Seizures, tremors or jerking movements in the arms and legs
- Mental retardation
- Behavioral and social problems
- A musty odor in the breath, skin, or urine (caused by raised levels of Phe)
- Fair skin and blue eyes, because Phe cannot transform into melanin which is the pigment responsible for hair and skin tone
Diagnosis of Phenylketonuria:
In the U.S. and many other countries, all babies born in hospitals are screened for PKU in order to diagnose and treat the condition early in life. Although PKU is rare, recognizing it right away can help prevent serious health problems.
A newborn screening is conducted a few days after the baby’s birth. A few drops of blood are taken from the baby’s heel or elbow, and the blood sample is tested for various metabolic disorders including PKU. The test is specifically done after a few days so that the baby has had a chance to ingest protein in the diet to ensure accurate results.
If your baby is not born in a hospital, or you are discharged before the test is conducted, you may need to schedule a newborn screening with your pediatrician or family doctor.
Treatment of Phenylketonuria:
For newborns positive for PKU, the doctor will likely start dietary treatment immediately. Newborns with PKU are given a special formula, and parents may be referred to a registered dietitian who can help educate them about the PKU diet.
Adults with PKU who have strayed from the diet may benefit from a follow-up visit. In the past doctors advised that PKU patients could cease the diet once in the teens, but doctors now recommend the diet be used for life. Staying on the PKU diet has been shown to improve mental functioning and slow the damage to the central nervous system in adults with high Phe levels.
Women with PKU should consult their doctor about their diet during pregnancy. It is important that they return to the diet prior to becoming pregnant. Even the mildest of cases of Maternal PKU can pose serious risks to the unborn child. Pregnant women with PKU who don’t follow the diet have a higher risk of miscarriage, as well.
Additional Information About Phenylketonuria:
Helpful questions to ask your doctor if your newborn or child is diagnosed with PKU:
- Is this condition temporary or long-term?
- How did my child get PKU?
- How can we manage our child’s PKU?
- Are there medications we should consider?
- What foods are strictly off-limits?
- How much formula should my child need?
- What happens if my child eats an off-limits food item?
- If I have another child, what’s the likelihood they will have PKU?
- Are there brochures or books you would recommend I read about PKU?
- Is there something I could do differently during pregnancy if I get pregnant again?
Tips for Living with Phenylketonuria:
It is important to follow the special diet plan created for PKU. The diet consists of foods that are low in Phe, which is primarily found in protein. A safe amount of Phe will differ from person to person, so it is important to discuss with your doctor a specific diet plan for yourself.
Frequent blood tests may be run to track Phe levels and to adjust the diet as necessary. Supplements may be suggested to add neutral amino acids to the diet. Those supplements will block absorption of Phe; however this is a new treatment that hasn’t been studied in depth yet.
While each diet plan is customized to the PKU patient, high-protein foods are generally avoided. These include:
- Chicken, beef, pork, fish
- Milk, cheese
- Soybeans, beans, peas
It is also recommended by many doctors to avoid diet sodas and foods made with aspartame (found in sugar substitutes), as it releases Phe when digested.
Additional Phenylketonuria Resources:
PKU.com offers a wealth of information on managing life with Phenylketonuria.
The National PKU Alliance is an awareness and advocacy group that supports research for a PKU cure.
Mayo Clinic is a nonprofit worldwide leader in medical care and research for people from all walks of life. Their mission is to inspire hope and contribute to health and well-being by providing the best care to every patient through integrated clinical practice, education and research.
US National Library of Medicine is the world’s largest medical library on the National Institutes of Health campus in Bethesda, Maryland. The Library holds a collection of materials and research on all areas of biomedicine and health care.
National PKU News is a non-profit organization located in Seattle, WA dedicated to providing up-to-date and accurate news to families and professionals dealing with PKU.
The National Health Service is an organization in England launched in 1948 with the intention of providing good healthcare to all, regardless of wealth. The NHS remains free for residents of the UK and covers anything from coughs and colds to health screenings, surgeries, emergency treatments and more.