Carnitine, Amino Acids, Vitamins, and Hematological Status in Children with Classical Phenylketonuria: A Case–Control Study

Background: Classical phenylketonuria (PKU) requires the lifelong dietary restriction of phenylalanine-containing foods. In this study, we aimed to evaluate the effects of a phenylalanine-restricted diet on carnitine, amino acid, vitamin, and mineral status in children with classical PKU. Methods: This case–control study included 30 children with classical PKU and 30 age- and sex-matched healthy controls. Dietary adherence was categorized as good, moderate, or poor according to blood phenylalanine concentrations during the preceding year. Dried blood spot amino acid and acylcarnitine profiles combined with serum micronutrient and hematological parameters were analyzed. Results: No significant differences were observed between the PKU and control groups regarding tyrosine, free carnitine (C0), acetyl carnitine (C2), valine, or methionine concentrations (p > 0.05). Arginine levels were significantly lower in the PKU group than in controls (40.14 ± 29.89 vs. 48.25 ± 16.08 µmol/L, p = 0.008). Vitamin B12, folate, 25-hydroxyvitamin D, and mean corpuscular volume were significantly higher in patients with PKU (all p < 0.05). Dietary adherence exhibited a strong negative correlation with phenylalanine concentrations (r = −0.86, p < 0.001), a moderate negative correlation with the phenylalanine-to-tyrosine ratio (r = −0.56, p = 0.001), and a moderate negative correlation with body mass index (r = −0.55, p = 0.002). Conclusions: Children with classical PKU receiving long-term dietary treatment maintained adequate carnitine and micronutrient status. Reduced arginine concentrations observed in treated children with classical PKU warrant further investigation, although their clinical significance remains uncertain.