In addition, its physiological actions are attributable to the active metabolite, 1,25-dihydroxyvitamin D which, because it is synthesised in the kidneys and acts elsewhere, is often called a hormone.ġ µg cholecalciferol is equal to 0.2 µg 25(OH)D. Thus, vitamin D is not a nutrient in the usual sense, since under normal conditions it is supplied mainly by the skin. In skin, 7-dehydrocholesterol is converted to pre-vitamin D3 by a narrow band of solar ultraviolet radiation (290-320 nm) which undergoes isomerisation in a temperature-dependent manner to vitamin D 3. If sunlight exposure is adequate, dietary vitamin D can be considered unnecessary (Holick 2001). Vitamin D status is generally maintained in the population by exposure to sunlight (Glerup et al 2000, Holick 1996, Rasmussen et al 2000). In the circulation, vitamin D appears as 25-hydroxyvitamin D (25(OH)D) which is five times more potent than cholecalciferol.

Its metabolite, 1.25-dihydroxyvitamin D (1,25(OH 2)D, or calcitriol) is the biologically active hormone responsible for its physiological actions. Vitamin D in foods is fat soluble and is biologically less active. With current food supplies and patterns of eating, it is almost impossible to obtain sufficient vitamin D from the diet alone (Fuller & Casparian 2001). One is produced by the action of sunlight on skin (D 3 or cholecalciferol) and the other is found in a limited range of foods (D 2 or ergocalciferol). However, there is a limited capacity to mobilise sufficient calcium from bone to have a significant effect on blood calcium levels. When dietary calcium is inadequate for the body's needs, 1,25-dihydroxyvitamin D - the active form of vitamin D - together with parathyroid hormone, can mobilise stem cells in bone marrow to become mature osteoclasts which in turn increase the mobilisation of calcium stores from bone.

Vitamin D maintains the blood calcium at supersaturating levels such that it is deposited in the bone as calcium hydroxyapatite. Vitamin D also plays a role in enhancing absorption of phosphorus from the diet, but the blood concentration of phosphorus is not well regulated and varies according to supply and the renal excretory threshold. The major function of Vitamin D in humans is to maintain appropriate serum calcium concentrations by enhancing the ability of the small intestine to absorb calcium from the diet.

Download vitamin d breastfed