vit. D

It is now well established that vitamin D has direct action on bone, intestine, kidney, and clinicians would also include muscle. The function of vitamin D in bone is two‐fold. It is required for the mineralisation of bone, that is, in calcium acquisition, and also for the mobilisation of calcium into the blood. The balance of calcium between the plasma and bone is maintained by vitamin D, calcitonin and parathyroid hormone. This second hormone is able to act only in the presence of vitamin D. These two effects — mobilisation of calcium from bone tissue, and absorption of the mineral from ingested food, are quite different. This is clearly shown in histological lesions produced by calcium deficiency which are distinct from those of rickets. Conversely, calcium infusions to rachitic animals and osteomalacic patients do not fully lead to normal calcification of osteoid tissue.