Phenylketonuria: tyrosine supplementation in phenylalanine-restricted diets

Treatment of phenylketonuria (PKU) consists of restriction of natural prote in and provision of a protein substitute that lacks phenylalanine but is en riched in tyrosine. Large and unexplained differences exist, however, in th e tyrosine enrichment of the protein substitutes. Furthermore, some investi gators advise providing extra free tyrosine in addition to the tyrosine-enr iched protein substitute, especially in the treatment of maternal PKU. In t his article, we discuss tyrosine concentrations in blood during low-phenyla lanine, tyrosine-enriched diets and the implications of these blood tyrosin e concentrations for supplementation with tyrosine. We conclude that the pr esent method of tyrosine supplementation during the day is far from optimal because it dues 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 o f 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 co uld be beneficial. We advocate decreasing the tyrosine content of protein s ubstitutes to approximate to6% 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 ty rosine deficiency. We further advocate that a better daily distribution of the protein substitute be achieved by improving the palatability of these p roducts.