Patients will come for an inclusion visit and will receive treatment with 0.5 µg/day alfacalcidol for 4 weeks (28±3 days, run-in). They will be instructed to maintain dietary calcium intakes (1 g/day) for the duration of the study and will be supplemented throughout the study with native vitamin D in order to maintain the concentration of 25OH vitamin D ≥ 40 ng/L. Magnesium supplementation (100 mg/day) will be maintained throughout the study.