Establish your blood Phenylalanine (Phe) range

Diagram showing the levels of PKU

Up until 1970, clinicians believed that dietary treatment was only necessary until the end of childhood. It was thought that the brain was only vulnerable to the toxic effects of elevated Phenylalanine (Phe) blood levels during the period of accelerated development. However with research came concerns about the safety of abandoning the diet at all ages.

Blood Phenylalanine (Phe) levels, which are a marker of the severity of the disorder, are usually stable by the age of five years. There are different classifications of Phenylketonuria (PKU).  Generally: 

  • Patients with blood Phe levels of between 360–1200 μmol/L (6–20 mg/dL) are classified as having mild to moderate PKU.
  • Those with blood Phe levels >1200 μmol/L (>20 mg/dL) are classified as having classic PKU.

There are no international guidelines as to what constitutes a safe level of Phe in the blood. However PKU specialists do agree that low Phe blood levels must be maintained throughout life.

Ask your PKU specialist what represents a ‘safe’ blood Phe control target for your child.

Phenylalanine (Phe) control options for your child  

Children with Phenylketonuria (PKU) are managed with a strict diet with very low Phenylalanine (Phe) content to maintain blood Phe levels within a safe range as advised by their PKU team.

In addition to the Phe-restricted diet, the remaining nutrition needed by children with PKU is provided by special medical food and supplements or substitutes. These are made especially to contain a balance of amino acids (excluding Phe), minerals, vitamins and other nutrients.

Some children with PKU from the age of four years may also benefit from Sapropterin Dihydrochloride, (also known as BH4) a prescription drug approved in the European Union, Liechtenstein, Norway, Iceland, Switzerland, the United States and Japan.
The medication functions exactly like BH4, a substance found naturally in the body, which ultimately helps the body break down Phe by making the defective Phenylalanine Hydroxylase (PAH) enzyme more active.

Owing to the many different defects possible in the PAH enzyme, not all children with PKU will be helped by medication.

The full range of treatment options include:

  • Phe free food.
  • Supplements or substitutes (including LNAA).
  • Medication.

Everyone has their own way of managing PKU. Your PKU specialist will help advise on the best approach to Phenylalanine (Phe) management, balancing available treatment options, as your child makes the transition from toddler to pre-schooler to pre-teen.

Preparing to see PKU specialists for children

Download a list of useful questions to ask at your next visit.


About PKU for children

Learn more about PKU in children and how to help them cope.


PKU directories

PKU specialist, dietitian -or nutritionist, a supplier, or PKU association in your area.