New approach to osteopenia in phenylketonuric patients

Aim: To study bone mineralization in a group of phenylketonuric patients and to search for a possible relationship between bone mineral density, dietary control, serum minerals and nutrition intake. The response to treatment with low-dose 1.25-(OH)₂ vitamin D in patients with osteopenia was evaluated. Methods: Twenty-eight phenylketonuric patients (age range: 10-33 y) on dietary treatment were investigated. Bone density at the lumbar spine (Dual Energy X-ray Absorptiometry), bone formation markers (osteocalcin and bone alkaline phosphatase), serum minerals, index of dietary control and protein, vitamin D and mineral intakes were determined. Results: Of the patients studied, 78.6% had good dietary compliance (462 ± 89 μmol/L). Mean protein, vitamin D and mineral intakes met the recommended dietary allowances (RDAs). Nevertheless, 8 patients had calcium intakes lower than 1000 g/d, and a positive correlation between Z-score and calcium (r = 0.585; p = 0.002) or phosphorus intake (r = 0.546; p = 0.005) was observed. Osteopenia was detected in 14 patients (50%). Moreover, bone alkaline phosphatase in phenylketonuric patients older than 18 y of age was significantly lower than that in controls (p < 0.0001). No correlation was found between bone mineral density, age, serum minerals, bone formation markers or index of dietary control. Treatment with 0.25 μg/d calcitriol significantly increased bone density in 6 patients. Conclusion: A defect in bone mineralization was detected in 50% of patients in our series. The correct amount of formula intake seems to be necessary for bone mineralization in phenylketonuric patients. Calcitriol can be a useful treatment for these patients, although more studies are needed to confirm these results. Hypercalcaemia and hypercalciuria need to be carefully monitored.