Phenylketonuria (PKU) – The Dietary Management on Different Levels.



Frequency of PKU

PKU has a frequency of roughly 1 in every 10,000 births and is one of the “rare” genetic diseases.


Treatment – Nutritional Therapy

Optimal treatment requires diagnosis to be done as soon as possible, ideally in the second week of life (before 1 month of life), strict monitoring throughout life and maintaining a PHE-restricted diet. Additionally, dietary intake must be adjusted to the PHE value that is considered safe for maintaining normal growth and development. Good metabolic control means maintaining blood levels between 2 and 6 mg/dL (120-360 µmol/L) throughout life.


Dietary Management in Newborns

  • When PHE value is between 6 and 10 mg/dL (360-600 µmol/L), treatment should begin in the form of a PHE-restricted diet.
  • As PHE value is not extremely high, 50% of the total volume should be PHE-free formula and 50% breast milk or infant formula. This requires estimating the total volume of liquids, considering the recommendation of 150 mL/kg/d (recommended dietary intake [RDI]).
  • Blood PHE value should be measured weekly, and according to the results obtained, PHE intake should be adjusted. If values remain high, the contribution of breast-feeding should be reduced and the volume of PHE-free formula increased. The recommendation is always to provide PHE-free formula first and then place the baby to the breast.
  • Mother’s milk, as well as providing amino acids and essential fatty acids (mainly Omega 3 with neuroprotective effects), also contains small amounts of phenylalanine. Immediately following diagnosis, the daily levels of Phe must first be reduced to within the safe range. Breast-feeding for babies affected by PKU is therefore encouraged and helps create a strong tie between mother and child. Breast feeding must subsequently be supplemented with appropriate amino acid mixtures. Finally, the levels of phenylalanine in the baby’s blood must be regularly monitored.


Dietary Management in Children

  • Children are growing and have high calorie and protein requirements. There must therefore be a balance between the minimum possible amount of phenylalanine while meeting their protein and energy requirements.
  • The first step is to prevent the accumulation of phenylalanine. This does not mean a total absence of phenylalanine. It is unnecessary to exclude all foods containing protein; it is advisable to provide foods with a moderate protein content that are however low in phenylalanine. This dietary approach depends on the child’s tolerance (i.e. the form of Phenylketonuria present).
  • A diet totally lacking in protein would result in inadequate growth. There must be adequate provision of the right amino acids to keep the child healthy and ensure growth. This can be achieved through supplementary mixtures of amino acids that do not contain phenylalanine and are age appropriate.
  • It is also important to pay attention to a balanced intake of vitamins (above all vitamin D, vitamin A, and vitamin B12), mineral salts (calcium), essential fatty acids (Omega 3) and micronutrients (selenium, zinc, iron). Depending on the phenylalanine content tolerated by the child, the protein requirements can be met by means of mixtures of amino acids and natural proteins (foods).
  • For Classical Phenylketonuria, for example, 350 mg of phenylalanine a day corresponds to approximately 6 g of protein from foods. The caloric requirements must be age appropriate. However, because of the dietary restrictions, may be insufficient. This could compromise growth while prompting the catabolism of proteins from muscle and cause the release of phenylalanine.
  • The diet adherence and effectiveness should be monitored by taking a blood sample each week to check the levels of phenylalanine.


Dietary Management in Adults

  • The right diet permits the regular development of adolescents, both mentally and physically, allowing them to lead normal lives.
  • Adult requirements for amino acids are not significantly different from those of children, but there are some other, equally important, nutrients. These include:
    • Calcium and vitamin D to prevent osteoporosis (loss of bone mass)
    • Folic acid to prevent arteriosclerosis (hardening of arterial walls)
    • Fiber, for normal digestive health
    • Omega 3 essential fatty acids which strengthen immunity and have neuroprotective effects
  • The amino acid mixtures for young people and adults are available in practical packaging that allows for an active lifestyle.
  • Testing of phenylalanine in the blood is less frequent than for infants, but regular monthly monitoring is still necessary.


Dietary Prescription

  1. Estimate the recommended intake of protein, energy, and phenylalanine.
  2. Evaluate the appropriateness (based on child’s age) of formula. Phenyl-Free 1* is appropriate for infants and toddlers. Phenyl-Free 2* is appropriate for children and adults.
  3. Calculate the amount of formula to be given; provide 85-90 percent of the protein required for age from formula.
  4. Calculate the phenylalanine, tyrosine, protein, and energy provided by the prescribed amount of formula.
  5. Determine the remaining amount of phenylalanine needed to fulfill the daily phenylalanine prescription and maintain a stable blood phenylalanine level. Negotiate the source of additional phenylalanine (infant formula, e.g., Enfamil, Similac; cow’s milk e.g., whole, low-fat; phenylalanine from food).
  6. Consider the amount of food consumed in one day. How much food is it reasonable to expect the toddler to eat? Will foods with high to moderate amounts of phenylalanine need to be used? Will this lead to confusion about “yes” and “no” foods for the toddler? Consider juices as an extra source of energy.
  7. Using milk or infant formula as a source of phenylalanine has several effects on PKU management for infants and young children.
  8. Allows adjustments of dietary phenylalanine intake without changing the “message” about food. This can simplify many of the issues described above.
  9. Provides a source of protein
  10. Stabilizes blood phenylalanine levels
  11. Determine the amount of water to mix with the formula. This will vary according to the child’s age and fluid requirements. For an older child drinking other fluids, the amount of water added to the formula is relatively arbitrary. Ask about other fluids the child is consuming and how the child prefers the formula. Prescribe water for formula with above information considered. The child may prefer a more concentrated formula if consuming enough fluid from other sources.
  12. Calculate nutrients provided by formula and food prescription to ensure the pattern provides an adequate amount of protein and energy.


PKU affecting Pregnancy

The desire to have children is the most natural impulse in the world. For women with PKU, constant and rigorous attention is required from the initial “decision” to have a child.

  • A strict diet is necessary even before conception in order to keep the concentration of phenylalanine within the desired limits. Ideally, in the six months before conception, a level of phenylalanine in the blood of 120 µmol/l should be achieved and maintained.
  • Foods that should be cut out completely are all products of animal origin (meat, fish, eggs, milk and dairy products in general) and beans.
  • Grain-based foods (bread, pasta and bakery products) as well as eggs and milk can be substituted with protein- free dietary products.
  • Women with PKU whose children are born without the disease can breastfeed their babies, even though their milk is richer in phenylalanine than milk from a healthy mother.
  • The diet must be maintained for life and routine blood testing will be required to establish that nutritional intake is balanced.
  • The greater the motivation to follow a strict, rigorous diet, the greater its success in ensuring that a child will not face any consequences (e.g. mental retardation) of the mother’s Phenylketonuria.
  • The levels of phenylalanine in the blood must be reduced before pregnancy and held stable at 120 µmol/l of blood. If prior to conception, the blood level of phenylalanine in the mother-to-be is fully controlled, i.e. less than 240 µmol per liter of blood, the child has optimum conditions for normal development and, in most cases (as the probability is minimal), the unborn baby will not be affected Phenylketonuria even if it is a carrier of the altered gene.



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