Vitamin D, the sunshine vitamin, is unique as the body can manufacture it in skin that is exposed to the sun. UVB is the only form of sunlight that stimulates this action. A molecule found in the skin cells absorbs UVB and produces cholecalciferol, a form of Vitamin D (also called Vitamin D3). Ergocalciferol or Vitamin D2 is another form of Vitamin D that is found in some plants. Both these forms of Vitamin D are inactive.
In order for Vitamin D to become effective it must go through two steps of activation in the body – one in the liver and another in the kidneys. The liver transforms Vitamin D to
25-hydroxyvitamin D but this form is still not active until the kidneys activate it to
1, 25-dihydroxyvitamin D.
Vitamin D acts differently to most vitamins and scientists today believe that it acts more like a hormone than a vitamin. Hormones can bind to a receptor on the cell and transmit a signal to the cell to influence its activity. A Vitamin D receptor is found in almost every cell in the body and this is why Vitamin D has such a wide variety of actions.
Recent research on Vitamin D has revealed that it has benefits beyond helping to build strong muscles and bones. Vitamin D made Time magazine's list of "Top 10 Medical Breakthroughs" in 2007!
You can actually store Vitamin D in body fat as Vitamin D is soluble in fat. It can then be released during winter if the skin is not exposed to sun.
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