Simplifying the diet for patients with phenylketonuria (PKU): unrestricted consumption of fruit and vegetables

Phenylketonuria (PKU) (OMIM1 261600) is one of the most common congenital metabolic disorders. The phenylalanine (Phe) hydroxylase enzyme needed to break down phenylalanine exhibits significantly reduced activity as a result of mutations in the encoding gene. Untreated, it causes severe psychomotor retardation. Only thanks to early diagnosis as part of newborn screening and immediate commencement of treatment by means of a strict, protein-restricted diet will patients develop normally [1, 2]. Dietary treatment is based on the principle of substrate reduction (Phe) and product supplementation (tyrosine, other amino acids, micronutrients). Metabolic control is monitored by regular determination of the Phe concentration in plasma or dried blood. Depending on the residual activity of phenylalanine hydroxylase, each PKU patient has an individual Phe tolerance level. This is the amount of Phe, which can be introduced with food, without Phe plasma concentrations exceeding the therapeutic range. In patients with classical PKU, the average Phe tolerance is only approx. 200–400 mg/ day, which corresponds to approx. 5–10 g protein/day. PKU patients now have access to quite a wide range of low-protein speciality foods. Patients are able to satisfy their protein requirement only up to approx. 20 % from natural foods. Precise calculation of Phe content in foods by means of nutritional tables is essential. As a result of the very limited food choice, nutrient intake in these patients will be inadequate. They therefore depend on a nutrient supplement composed of amino acids and micro-nutrients, which must be taken several times a day. Patients are recommended to follow the diet throughout their lives [3], but it is of crucial importance primarily during the growth of the central nervous system, i.e. up to the age of 10 years [2].