When ultraviolet rays from the sun strike the skin of our bodies, the skin cells in our body called keratinocytes trigger vitamin D synthesis. The endogenously made as well as vitamin D obtained from food are biologically inert (called vitamin D3). D3 is the major circulating form of Vitamin D and is what gets measured in the lab tests. D3 must be metabolized further in the liver and in the kidney to yield activated forms of D called calcidiol and calcitriol, respectively. It is this activated form that can form calcium-binding protein needed for the absorption of calcium from the intestines.
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