Vitamin D in its hormonal active form, 1,25-dihydroxyvitamin D (calcitriol), has a major impact on bone turnover by regulating calcium and phosphate homoeostasis. By binding the active vitamin D hormone to the vitamin D receptor (VDR), it acts as a nuclear transcription factor (Bouillon et al., Endocr Rev 29(6):726–776, 2008). The discovery that almost all tissues and cells in the body express the VDR and that several tissues possess the enzymatic capability to convert 25-hydroxyvitamin D (25(OH)-D3; cholecalciferol) to the active form, suggests that vitamin D fulfills various extra-osseous functions (Bouillon et al., Endocr Rev 29(6):726–776, 2008; Holick, N Engl J Med 357(3):266–281, 2007). For example, VDR ensures adequate intestinal calcium absorption by regulating the synthesis of several calcium transport proteins in the duodenum (Bouillon et al., Endocr Rev 29(6):726–776, 2008). Additionally, vitamin D is important for proper muscle function, and some studies suggest it may contribute to prevent type 1 diabetes mellitus, certain autoimmune diseases, hypertension, and several types of cancer (Holick, N Engl J Med 357(3):266–281, 2007).