Phenylketonuria (PKU) is an inherited metabolic disorder in which a person does not have the liver enzyme needed to break down phenylalanine, an amino acid found in proteins and some artificial sweeteners. Without this enzyme, phenylalanine can build up in the blood and become toxic to the brain. Approximately 1 in 15,000 infants in the US is born with PKU.
PKU is caused by a mutation of the gene responsible for making the enzyme that breaks down phenylalanine.
The only way to get PKU is to inherit the defective gene from both parents. PKU appears to be more common in babies born to parents of Turkish, Yemenite Jewish, northern and eastern European, Italian, and Chinese ancestry.
If detected and treated early, PKU symptoms may never appear. If untreated, one or more of the following symptoms of PKU may appear by the first birthday:
- Intellectual disability
- Hyperactivity and behavioral problems
- Small head size
or seborrhea rash
- Musty odor
- Light coloring
- Progressive development delay
PKU is diagnosed at birth by a routine newborn screening test which is required by law in all states. The hospital draws blood from the heel between 1-7 days after birth. If a high level of phenylalanine is detected in the blood, additional tests of the blood and urine may be done to confirm the diagnosis of PKU. If you have your baby at home, arrange for a PKU test and other standard screening tests to be done by a pediatrician after birth.
A baby with PKU will need to see a doctor who specializes in genetic disorders and a registered dietitian (RD) who can advise on the dietary treatment for the condition.
PKU is not curable, but it is treatable with a special diet beginning at birth.
Women with PKU must follow the diet strictly both before and during pregnancy, maintaining a phenylalanine level <6 milligrams/deciliter (mg/dL) [<363 micromole/liter (µmol/L)], to prevent brain damage in the unborn baby.
Women with PKU need to follow a strict diet while pregnant to avoid serious birth defects.
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This strict diet significantly reduces the intake of phenylalanine (also referred to as “phe”) to a level that a person with PKU can handle without seeing harmful effects. Newborns require 40-60 mg/kg/day of phenylalanine to maintain adequate growth. For most older children and adults with PKU the safe range is 200-400 mg of phenylalanine per day. The diet consists of:
- Drinking phenylalanine-free medical formula, which provides calories, protein, vitamins, and minerals without phenylalanine. Phenylalanine-free medical formulas are manufactured for infants, children, and adults and are the most important feature of the diet for PKU.
- Starches, including bread, potatoes, corn, and beans must be restricted.
- Eliminating high-protein foods such as milk, meat, fish, chicken, pork, eggs, beans, and nuts.
- Eliminating foods and beverages made with aspartame, an artificial sweetener that contains phenylalanine.
Part of the treatment plan for PKU is regular monitoring of the level of phenylalanine in the blood. During infancy, the pediatrician may monitor the blood very frequently, as often as once or twice a week. Over time, blood monitoring may not be needed as frequently. The treatment goal is to keep the level of phenylalanine in the blood in the safe range between 2-6 mg/dL (121-363 µmol/L). The doctor or registered dietitian may also perform an occasional diet analysis to calculate the amount of phenylalanine being consumed and to check for potential nutrient deficiencies.
There is no way to prevent PKU, but proper management can prevent complications.
Behrman RE, Kliegman RM, Jenson HB.
Nelson Textbook of Pediatrics. 17th ed. Philadelphia, PA: Saunders; 2004.
The essentials of PKU. University of Washington PKU Clinic website. Available at
. Accessed on September 13, 2005.
Phenylketonuria. Mayo Foundation for Medical Education and Research website. Available at
. Accessed on September 13, 2005.
Phenylketonuria: screening and management [National Institutes of Health Consensus Statement, October 16-18, 2000]. National Institutes of Health website. Available at
. Accessed on September 22, 2005.
RM Kleigman, RE Behrman, HB Jenson, BF Stanton.
Nelson Textbook of Pediatrics. 18th Edition. Eds. Saunders Publishers, Philadelphia PA, 2007.
Last reviewed November 2012 by Michael Woods, MD
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.
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