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Protein substitute dosage in PKU: how much do young patients need?
  1. A MacDonald1,
  2. A Chakrapani1,
  3. C Hendriksz1,
  4. A Daly1,
  5. P Davies2,
  6. D Asplin1,
  7. K Hall1,
  8. I W Booth2
  1. 1Birmingham Children’s Hospital, Birmingham, UK
  2. 2Institute of Child Health, University of Birmingham, Birmingham, UK
  1. Correspondence to:
    Dr A MacDonald
    Consultant Dietitian, The Children’s Hospital, Steelhouse Lane, Birmingham B4 6NH, UK; anita.macdonald{at}


Background: The optimal dose of protein substitute has not been determined in children with phenylketonuria (PKU).

Aim: To determine if a lower dose of protein substitute could achieve the same or better degree of blood phenylalanine control when compared to the dosage recommended by the UK MRC.1

Methods: In a six week randomised, crossover study, two doses of protein substitute (Protocol A: 2 g/kg/day of protein equivalent; Protocol B: 1.2 g/kg/day protein equivalent) were compared in 25 children with well controlled PKU aged 2–10 years (median 6 years). Each dose of protein substitute was taken for 14 days, with a 14 day washout period in between. Twice daily blood samples (fasting pre-breakfast and evening, at standard times) for plasma phenylalanine were taken on day 8–14 of each protocol. The median usual dose of protein substitute was 2.2 g/kg/day (range 1.5–3.1 g/kg/day).

Results: When compared with control values, median plasma phenylalanine on the low dose of protein substitute increased at pre-breakfast by 301 μmol/l (95% CI 215 to 386) and in the evening by 337 μmol/l (95% CI 248 to 431). On the high dose of protein substitute, plasma phenylalanine concentrations remained unchanged when compared to control values. However, wide variability was seen between subjects.

Conclusions: A higher dosage of protein substitute appeared to contribute to lower blood phenylalanine concentrations in PKU, but it did have a variable and individual impact and may have been influenced by the carbohydrate (+/− fat) content of the protein substitute.

  • phenylketonuria
  • phenylalanine
  • protein substitute
  • energy intake
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  • * Children 0–5 years maintained blood phenylalanine concentrations at 120−360 µmol/l; school age children 120−480 µmol/l.

  • Published Online First 17 March 2006

  • Funding: Birmingham Children’s Hospital Research Foundation

  • Competing interests: The principle author receives a grant from the BCH Research Foundation to pay for blood sampling and materials investigating alternative methods of estimating phenylalanine exchanges in PKU. The principle author also receives research grants from SHS International to pay for blood analysis for a project on the efficacy of a new vitamin and mineral tablet for use in PKU and from Vitaflo International to pay for blood analysis for a project in very long chain fatty acid disorders.

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