BH₄-sensitive phenylketonuria (PKU) patients relax their phenylalanine (Phe) restricted diet due to increased Phe tolerance, while keeping dried blood Phe concentrations with in the therapeutic range. We aimed to investigate metabolic control, eating habits and nutrient supply under long-term BH₄-therapy.

Retrospective analysis of mean dried blood Phe concentrations and their variability, food and nutrient intake in BH₄-sensitive patients (n = 8, 3f, age 6.0–16.6 y) under classical dietary treatment for one year and during the three years after initiation of BH₄.

Phe concentrations of BH₄-sensitve PKU patients remained within therapeutic range throughout the observation period, independent of therapeutic regime. Under BH₄, Phe tolerance increased significantly (493.2 ± 161.8 mg/d under classical diet vs 2021.93 ± 897.4 mg/d two years under BH₄; P = 0.004). Variability of Phe concentrations remained unchanged (mean SD; P = 1.000). Patients adjust their food choice and significantly increased their intake of cereals, potatoes, dairy products and meat (P = 0.019, P = 0.016, P = 0.016 and P = 0.016, respectively). Under diet changes after implementation of BH₄ a drop in micronutrient intake (vitamin D, folic acid, iron, calcium, iodine) could be revealed (P = 0.005, P < 0.001, P = 0.004, P = 0.001, P = 0.003, respectively).

BH₄-sensitive PKU patients can achieve good metabolic control under an adjuvant BH₄- or a BH₄ monotherapy. The liberalized diet under BH₄ seems to jeopardize the quality of patients' nutrition, and these patients require close follow-up and special nutrition education to minimize the risk for imbalanced diet and nutrient deficiencies.