The production of 25-hydroxycholecalciferol (25(OH)D) in the liver is dependent on vitamin D obtained from the diet and from exposure to UV light. UV rays stimulate the conversion of provitamin D in the skin to vitamin D, making it available to the liver for hydroxylation to 25(OH)D. The circulating concentrations of 25(OH)D are considered to be reflective of the individual’s total vitamin D ex...