Bone metabolism is significantly influenced by the activity of osteoblasts and osteoclasts, the calciotropic hormones 1,25-dihydroxy vitamin D hormone), parathyroid hormone and calcitonin, and the extracellular calcium concentration.

Vitamin D are a group of structurally related hydrophobic vitamins, of which the isoforms vitamin D3 from animals and vitamin D2 from plants and fungi are the physiologically most important representatives. Vitamin D is hydroxylated in the liver to 25-hydroxy vitamin D (25-OH vitamin D) and in another hydroxylation step in the kidneys 1,25-dihydroxy vitamin D is produced. The vitamin D circulating in the blood is mainly bound to proteins. Up to 90% is present bound to the vitamin D binding protein, and approximately 10% bound to albumin. Less than 1% of the vitamin D is free. Free and albumin-bound vitamin D are considered as biologically available.