See QuickFacts for easy-to-read facts about Vitamin D. Vitamin D
is a fat-soluble vitamin that is naturally present in very few
foods, added to others, and available as a dietary supplement.
It is also produced endogenously when ultraviolet rays from
sunlight strike the skin and trigger vitamin D synthesis.
Vitamin D obtained from sun exposure, food, and supplements is
biologically inert and must undergo two hydroxylations in the
body for activation. The first occurs in the liver and converts
vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as
calcidiol. The second occurs primarily in the kidney and forms
the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D],
also known as calcitriol . Vitamin D promotes calcium absorption
in the gut and maintains adequate serum calcium and phosphate
concentrations to enable normal mineralization of bone and to
prevent hypocalcemic tetany. It is also needed for bone growth
and bone remodeling by osteoblasts and osteoclasts [1,2].
Without sufficient vitamin D,