Tyrosine supplementation for phenylketonuria (Cochrane Review)

A substantive amendment to this systematic review was last made on 23 February 1999. Cochrane reviews are regularly checked and updated if necessary.

Background: Phenylketonuria is an inherited disease for which the main treatment is the dietary restriction of the amino acid phenylalanine. The diet has to be initiated in the neonatal period to prevent or reduce mental handicap however the diet is very restrictive and unpalatable and can be difficult to follow. A deficiency of the amino acid tyrosine has been suggested as a cause of some of the neuropsychological problems exhibited in PKU. Therefore, this review aims to assess the efficacy of tyrosine supplementation for phenylketonuria.

Objectives: To assess the effects of tyrosine supplementation alongside or instead of phenylalanine restricted diet for people with phenylketonuria who commenced on diet at diagnosis and either continued on the diet or relaxed the diet later in life. To assess the evidence that tyrosine supplementation alongside, or instead of phenylalanine restricted diet improves intelligence, neuropsychological performance, growth and nutritional status, mortality rate and quality of life.

Search strategy: We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group's trials register which is a specialist trials register which comprises references identified from comprehensive electronic database searches, handsearching relevant journals and handsearching abstract books of conference proceedings. Additional studies were identified from handsearching the Journal of Inherited Metabolic Disease (from inception, 1978, to 1998). The manufacturers of prescribable dietary products used in the treatment of phenylketonuria were also contacted for further references.Date of the most recent search of the Group's register: April 2002.

Selection criteria: All randomised or pseudo-randomised trials investigating the use of tyrosine supplementation versus placebo in people with phenylketonuria in addition to, or instead of, a phenylalanine restricted diet. People treated for maternal phenylketonuria were excluded.

Data collection and analysis: Two reviewers independently assessed the trial eligibility, methodological quality and extracted the data.

Main results: Six trials were found, of which three trials, which reported the results of a total of 56 participants, were suitable for inclusion in the review. The blood tyrosine concentrations were significantly higher in the participants receiving tyrosine supplements than those in the placebo group, weighted mean difference 23.00 (95% Confidence interval (CI) 12.90 to 33.10). No significant differences were found between any of the other outcomes measured.

Reviewers' conclusions: From the available evidence no recommendations can be made about whether tyrosine supplementation should be introduced into routine clinical practice. Further randomised controlled studies are required to provide further evidence.

Citation: Poustie VJ, Rutherford P. Tyrosine supplementation for phenylketonuria (Cochrane Review). In: The Cochrane Library, Issue 3, 2003. Oxford: Update Software.

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This is an abstract of a regularly updated, systematic review prepared and maintained by the Cochrane Collaboration. The full text of the review is available in The Cochrane Library (ISSN 1464-780X).

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