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Randomised controlled trial of tyrosine supplementation on neuropsychological performance in phenylketonuria
  1. Mary Lou Smitha,
  2. William B Hanleyb,
  3. Joe T R Clarkeb,
  4. Paula Klima,
  5. Wanda Schoonheytb,
  6. Valerie Austinb,
  7. Denis C Lehotayc
  1. aUniversity of Toronto and Hospital for Sick Children, Toronto, Canada: Department of Psychology, bDepartment of Pediatrics and Division of Clinical Genetics, cClinical Biochemistry
  1. Dr Mary Lou Smith, Department of Psychology, Erindale College, University of Toronto, Mississauga, Ontario L5L 1C6, Canada. e-mail:smithml{at}


OBJECTIVE To test the efficacy of tyrosine supplementation, as an adjunct to dietary treatment, on neuropsychological test performance in individuals with phenylketonuria.

DESIGN A randomised controlled trial of tyrosine supplementation using a double blind crossover procedure with three four week phases.

SETTING The Hospital for Sick Children, Toronto.

PARTICIPANTS 21 individuals with phenylketonuria (ages 6 to 28 years, mean 11.3).

INTERVENTION Participants were given 100 mg/kg body weight/d of l-tyrosine or l-alanine (placebo).

RESULTS At baseline, performance on several of the neuropsychological test measures was correlated with tyrosine levels. Dietary supplements of tyrosine increased plasma tyrosine concentrations; however, no change in test performance was found across the tyrosine and placebo phases of the study.

CONCLUSIONS Tyrosine supplementation of this type does not appear to alter neuropsychological performance in individuals with phenylketonuria.

  • phenylketonuria
  • tyrosine
  • phenylalanine
  • frontal lobe function

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