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Fact Sheet
Phenylketonuria (PKU)
An experienced healthcare professional should be
consulted for the management of PKU
What is PKU?
PKU is a genetic disorder that a child inherits from both parents. For a
child to be born with PKU both parents must either be carriers of PKU or
have PKU, which means they carry at least one PKU gene.
PKU occurs due to a deficiency or absence of the enzyme phenylalanine
hydroxylase (PAH), which interferes with a person’s ability to metabolize
or process the amino acid phenylalanine (PHE) into tyrosine, another
amino acid. Amino acids, like PHE, are the building blocks of protein
found in food and the human body. Consuming too much PHE from
food can lead to a toxic build-up of PHE in the blood and brain of a
person with PKU. Without treatment, this buildup of PHE will cause
irreversible brain damage. Presently, there is no cure for PKU, but it can
be managed effectively with proper diet and special nutritional formulas
designed for persons with PKU.
Hyperphenylalaninemia
The Body Is Similar to a Factory…
When the body has enough PAH it converts Phe to Tyr.
TYR
TYR
TYR
TYR
TYR
PHE
PHE
PHE
PHE
PHE
PAH
If the body does not have enough PAH,
the Phe is not converted to Tyr.
TYR
TYR
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PHE
PAH
The Result: Too much Phe and not enough Tyr.
PAH: Phenylalanine Hydroxylase Phe: Phenylalanine Tyr: Tyrosine
Bernstein, L, Freehauf, C: Eat Right Stay Bright:
Guide for Hyperphenylalaninemia. 1997-2001
Autosomal Recessive Inheritance
Normal Gene
PKU Gene
Carrier
Non Carrier
Carrier
Carrier
Carrier
PKU
WHAT DOES AUTOSOMAL
RECESSIVE INHERITANCE REALLY MEAN?
Autosomal recessive disorders are passed to a child through both
parents’ genes. If both parents carry the PKU gene, there is a:
25% chance that the PKU gene will not be passed; the
child will NOT be a carrier and will NOT have PKU.
There are two
50%
chance that the PKU gene will be passed from
classifications of PKU:
one parent; the child will be a PKU carrier.
classic PKU and
25% chance that the PKU gene will be passed from
hyperphenylalaninemia
both parents, and the child will have PKU.
Bernstein, L, Freehauf, C: Eat Right Stay Bright:
(hyperphe). Both classic
Guide for Hyperphenylalaninemia. 1997-2001
PKU and hyperphe
Available at NutriciaLearningCenter.com
are characterized by a
mutation in the phenylalanine hydroxylase (PAH) gene which encodes
for the working enzyme. Classic PKU is the result of a severe defect
of this enzyme. In the case of hyperphe, the defective enzyme is less
severe or “leaky” and the management of diet and maintaining blood
PHE levels within an appropriate range are usually less restrictive than
with classic PKU. Blood PHE levels however may still be above the
normal range which may require some dietary restriction or therapy.
It is important for a person with PKU, whether classic or hyperphe, to
continue following the dietary recommendations made specifically for
them by their healthcare professional.
PAH deficiency most accurately describes the range of clinical
phenotypes* ranging from Classic PKU to hyperphenylalaninemia
and based on the new American College of Medical Genetics and
Genomics (ACMG) guidelines is the term that will be most likely used.
Incidence:
In order for an individual to have PKU, both parents must pass along their PKU gene to their child. When both
parents are carriers of PKU, there is 1 in 4 chance with each pregnancy that the child will have PKU, 1 in 2 chance
of the child being a carrier, and 1 in 4 chance that the child will neither have PKU nor be a carrier. It is estimated that
one out of 10,000 live births has decreased PAH activity1.
Nutricia North America provides a range of medical formulas as well as low protein foods. Please contact us for more
information. Your dietitian will help you decide which products are best.
*Phenotype- observable traits, outward appearances, development or behavior
Newborn Screening:
Newborn screening for PKU began in the US and Canada in the 1960s. It is important that an infant is diagnosed and
managed with diet within days after birth to prevent permanent negative effects from PKU.
Nutrition Management:
Since all foods with protein contain PHE, a person with PKU must restrict the amount and types of food containing
protein. For this reason, PKU is managed with a low PHE diet consisting of foods low in protein and a PHE-free
metabolic formula. The main sources of foods with protein allowed in the low-PHE diet are fruits and vegetables,
and a limited amount of cereals and grains. In addition to foods, individuals with PKU must drink a special PHEfree metabolic formula to supply the body with the necessary protein requirements the body needs for growth and
maintenance, without the addition of the offending amino acid, PHE. There is also a wide range of commerciallyavailable low protein food products available for today’s PKU population.
Nutrition management of PKU includes frequent monitoring of blood PHE levels and regular clinic visits. Blood PHE
levels can be measured either at the clinic or hospital, or collected at home and mailed to the clinic for analysis.
Managed blood PHE levels in persons with PKU are between 120-360 μmol/L (2-6 mg/dL)2.
Symptoms of Elevated Blood PHE Levels:
All patients with PKU, who are not started on diet immediately, will begin to build-up PHE in the blood and brain. In an
infant with classic PKU, poorly controlled PHE levels will result in irreversible brain damage. Therefore, it is essential
that each infant who is diagnosed with PKU be started on a low-PHE diet to lower and maintain PHE levels within an
appropriate range as soon after birth as possible.
If a child or adult previously diagnosed with PKU, and on
diet, decides to discontinue the diet, he or she will begin to
build up PHE in the blood, which may increase PHE levels
in the brain. Over time, toxic levels of PHE in the brain
can result in a change in their brain executive functions.
Executive function involves thinking skills, memory, and
concentration, all of which can decline with high PHE
levels. Also, an adult might feel shaky and clumsy. If
an adult is exposed to high brain PHE levels for a long
enough time, IQ losses may occur. For these reasons,
it is important that persons with PKU stay on the low-PHE
diet for life2,3.
References
1. Vockley J, Andersson HC, Antshel KM, et al. Phenylalanine hydroxylase deficiency: diagnosis and management guideline. Genet Med. 2014;16(2):188-200.
2. Singh RH, Rohr F, Frazier D, et al. Recommendations for the nutrition management of phenylalanine hydroxylase deficiency. Genet Med. 2014;16(2):121-31.
3. Thompson AJ, Smith T, Brenton D et al.: Neurological deterioration in young adults with phenylketonuria. Lancet 1990; 336:602-605.
Nutricia North America would like to thank Laurie Bernstein, MS RD FADA, Children’s Hospital Colorado, for her help and consultation.
FOR MORE INFORMATION:
WWW.MEDICALFOOD.COM
Phenylketonuria (PKU)
United States:
(800) 605-0410
Canada:
(877) 636-2283
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