The diet for PKU patients must be individualized depending on each patient’s requirements . The amount of PHE allowed in the diet depends on the patient’s residual enzyme activity and growth velocity. Tolerance varies from one patient to another. It will be influenced by residual PAH activity and growth rate. Close monitoring is essential.

PHE is an essential amino acid, which cannot be synthesized by the body, and thus needs to be obtained from a food source. Some PHE in the diet is needed for optimal growth and development, but restricted to the level allowed to prevent excessive build-up of PHE in the blood, depending on the residual activity of PAH and growth velocity.1

Tyrosine becomes a semi-essential amino acid, since it is not or limited supplied through phenylalanine hydroxylation. It therefore needs to be supplemented in the diet. Tyrosine is added to the L-amino acid supplements to meet tyrosine requirements supply adequately. The PKU MRC working group recommends the PHE free protein substitutes to supply 100-120 mg TYR/kg/day2.

Diet principles1

Limit the intake of dietary PHE amounts to maintain blood PHE concentration levels within in the desired range
Ensure an adequate amount of dietary PHE, to support normal growth and development but also to maintain blood PHE concentration levels within the desired range.
Adequate amino acid supplementation by a PHE-free protein substitute, enriched with tyrosine, micronutrients, essential fatty acids (EFA) and long chain polyunsaturated fatty acids (LCPUFAs) up to a level recommended in the age range, in order to maintain adequate protein status for PKU patients.
Adequate energy intake to promote appropriate growth and weight maintenance and prevent catabolism
Monitor nutritional status and adjust diet as needed
Provision of a palatable, varied and ‘easy to maintain’ diet.

GMDI/SERC recommended intakes of Phe, TYR and protein for individuals with PAH deficience4
Age	PHE (mg/day)	TYR (mg/day)
0 to <3 months	130-430	1100-1300
3 to <6 months	135-400	1400-2100
6 to <9 months	145-370	2500-3000
9 to <12 months	135-330	2500-3000
1 to < 4 years	200-320	2800-3000
4 years to adult	200-1100	4000-6000

The diet can be calculated by counting either grams of total natural protein; mg PHE in natural food or exchanges (or equivalent).

On average the natural protein contains the following % of phenylalanine:

Calculation of the amount of PHE, using the percentage of Phe found in milk products, cereals, vegetables and fruits protein3
% of PHE in	Cereals and milk products	vegetables	fruits
	5%	3-4%	3%

References

Dixon MD, MacDonald A, White F, Stafford J. In: Clinical Paediatric Dietetics. 4th ed: West Sussex: John Wiley & Sons Limited, 2014:381-525.
Bross R, Ball RO, Clarke JT et al. Tyrosine requirements in children with classical PKU determined by indicator amino acid oxidation. Am J Physiol Endocrinol Metab, 2000, 278 E195−201.
Paul AA, Southgate DAT, Russell J First Supplement to McCance and Widdowson’s the Composition of Foods. London: HMSO, 1980.
Vockley J, Andersson HC, Antshel KM et al. Phenylalanine hydroxylase deficiency: diagnosis and management guideline. Genet Med 2014;16(2):188-200

PKU: Diet principles