Dietary Therapy

The treatment of phenylketonuria (PKU) is well standardized and aims to prevent toxically elevated phenylalanine (Phe) levels in order to avoid neurological damage. Therapy is required lifelong and consists of several components:

Low-phenylalanine diet

  • Strict restriction of natural proteins (e.g., meat, fish, eggs, dairy products, nuts, legumes).
  • Individually calculated Phe tolerance, depending on age, residual enzyme activity, and metabolic control.
  • Avoidance of aspartame (contains phenylalanine).

Amino acid mixtures (Phe-free)

  • Specialized medical protein substitutes without phenylalanine, enriched with vitamins, minerals, and trace elements.
  • Ensure adequate protein intake and normal development.

Initiation

  • As early as possible within the first days of life following newborn screening.
  • Early initiation of therapy virtually completely prevents severe intellectual disability.
  1. Therapy by Age Group
  2. Infants and young children
  • Therapy: diet plus Phe-free amino acids only.
  • Target Phe range: approximately 120–360 µmol/L.
  • Goal: normal brain development.
  • Medications: sapropterin only in confirmed BH4 responsiveness.
  1. Children and adolescents
  • Continuation of dietary therapy as standard.
  • Close metabolic monitoring due to growth and puberty.
  • Adherence problems are common → psychosocial support is essential.
  • Sapropterin may be used; pegvaliase is generally not indicated.
  1. Adults
  • Lifelong therapy recommended (no “diet breaks”).
  • Higher tolerance limits may be acceptable depending on guidelines.
  • Sapropterin for responders.
  • Pegvaliase in cases of:
    • persistently elevated Phe levels,
    • failure of dietary therapy,
    • neurocognitive or psychosocial symptoms.
  1. Pregnancy (maternal PKU)
  • Strictest indication for dietary therapy.
  • Target Phe levels: usually 120–240 µmol/L.
  • Sapropterin may be permitted; pegvaliase is contraindicated.
  • Preconceptional metabolic control is mandatory.

Therapy According to PKU Severity

PKU type

Phe (untreated)

Therapeutic approach

Classical PKU

> 1,200 µmol/L

Strict diet plus amino acids; pegvaliase often used in adulthood

Moderate PKU

600–1,200 µmol/L

Dietary therapy; BH4 testing often useful

Mild PKU

360–600 µmol/L

Partial dietary relaxation; often BH4-responsive

Hyperphenylalaninemia

< 360 µmol/L

In some cases monitoring only; therapy may not be required

Key Therapeutic Messages

  • Diet remains the foundation of all PKU treatment; medications are adjuncts, not substitutes.
  • “Lifelong” therapy is evidence-based, as adults with elevated Phe levels also show neurocognitive impairment.
  • Individualization of therapy (genotype, life stage, adherence, psychosocial factors) is now standard of care.
  • The transition from pediatric to adult care is a critical point at which treatment discontinuation frequently occurs.