Naturally occurring form vitamin D3
The naturally occurring form is vitamin D3 (cholecalciferol) which is formed in the skin by the action of ultraviolet (VV) irradiation on 7-de hydrocholesterol. However, ergocaleiferol (vitamin D 2), which is derived from UV irradiation of ergosterol in plants, is a food additive, is consumed from the food and can be used by the body as an altemative to vitamin D3. When there is adequate exposure of the skin to sunlight, oral intake of ergosterol is unnecessary, indeed it may lead to toxic manifestations.
It comes into its own when sunlight is scarce or the skin is heavily shrouded, as for example in certain ethnic communities. Vitamin D is converted to 25-hydroxycalciferol in the liver and subsequently to 1,25 dihydroxycalciferol (1,25-DHCC) in the kidney under the control of parathyroid hormone.This1,25-DHCC (calcitriol) is the active form of vitamin D,as a hormone in calcium homeostasis. Calcitriol is transported to distal target tissues. Here it enters the cell, binds to a specific receptor and is taken up by the nucleus. Within the nucleus, it acts like any steroid hormone stimulating the production of protein via transcription of messenger RNA. it is a calcium-binding. protein which is formed by this process which probably facilitates the transport of calcium ions across membranes. However, a different mechanism may be responsible. Whatever the mechanism, the hormone increases calcium absorption (and indirectly phosphate absorption) from the intestine. The effect within bone is complicated. By increasing circulating calcium ions it assists calcification but the direct effect is to increase the release of calcium and phosphate from bone, probably by activating osteoclasts.
Workplace of calcitriol
This evidence that calcitriol functions in the kidney too, increasing renal absorption of calcium. it may also feedback on the parathyroid glands, regulating the secretion of parathyroid hormone. it is not clear whether the muscle weakness which is a feature of low vitamin D levels is the result of a low circulating level of calcium ions or whether the muscle membrane movement of calcium ions is also calcitriol dependent.
Vitamin D in baby children
In infants and children, vitamin D deficiency gives rise to rickets, with skeletal deformity, bone pain and muscle and weakness. Onceskeletalformation is complete, vitamin D deficiency in adult life leads to osteomalacia with bone demineralisation, bone tenderness and muscle weakness.