Phenylketonuria : tyrosine supplementation in phenylalanine-restricted diets

• Published in: The American journal of clinical nutrition Vol. 73; no. 2; pp. 153 - 157
• Main Authors: VAN SPRONSEN, Francjan J, VAN RIJN, Margreet, BEKHOF, Jolita, KOCH, Richard, SMIT, Peter G. A
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Clinical Nutrition 01.02.2001; American Society for Clinical Nutrition, Inc
• Subjects: Amino acids; Amino Acids - therapeutic use; Biological and medical sciences; Circadian Rhythm - physiology; Diet; Dietary Supplements; Dose-Response Relationship, Drug; Female; Food, Fortified; Humans; Maternal-Fetal Exchange; Medical sciences; Miscellaneous; Nutrition; Phenylalanine - administration & dosage; Phenylalanine - adverse effects; Phenylketonurias - blood; Phenylketonurias - diet therapy; Phenylketonurias - physiopathology; Pregnancy; Pregnancy Complications - blood; Pregnancy Complications - diet therapy; Pregnancy Complications - physiopathology; Proteins; Radiotherapy. Instrumental treatment. Physiotherapy. Reeducation. Rehabilitation, orthophony, crenotherapy. Diet therapy and various other treatments (general aspects); Taste; Tyrosine - administration & dosage; Tyrosine - blood; Tyrosine - metabolism; Human; Tyrosine; Nervous system diseases; Phenylketonuria; Phenylalanine; Metabolic diseases; Maternal diseases; Enzymopathy; Feeding; Genetic disease; Diet therapy; Aminoacid; Supplementation; Restricted diet; Aminoacid disorder
• ISSN: 0002-9165; 1938-3207
• DOI: 10.1093/ajcn/73.2.153

Treatment of phenylketonuria (PKU) consists of restriction of natural protein and provision of a protein substitute that lacks phenylalanine but is enriched in tyrosine. Large and unexplained differences exist, however, in the tyrosine enrichment of the protein substitutes. Furthermore, some investigators advise providing extra free tyrosine in addition to the tyrosine-enriched protein substitute, especially in the treatment of maternal PKU. In this article, we discuss tyrosine concentrations in blood during low-phenylalanine, tyrosine-enriched diets and the implications of these blood tyrosine concentrations for supplementation with tyrosine. We conclude that the present method of tyrosine supplementation during the day is far from optimal because it does not prevent low blood tyrosine concentrations, especially after an overnight fast, and may result in largely increased blood tyrosine concentrations during the rest of the day. Both high tyrosine enrichment of protein substitutes and extra free tyrosine supplementation may not be as safe as considered at present, especially to the fetus of a woman with PKU. The development of dietary compounds that release tyrosine more slowly could be beneficial. We advocate decreasing the tyrosine content of protein substitutes to approximately 6% by wt (6 g/100 g protein equivalent) at most and not giving extra free tyrosine without knowing the diurnal variations in the blood tyrosine concentration and having biochemical evidence of a tyrosine deficiency. We further advocate that a better daily distribution of the protein substitute be achieved by improving the palatability of these products.