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. It’s sometimes called
the “sunshine vitamin” because it’s produced in your skin in response to
sunlight.
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. Without
sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D
sufficiency prevents rickets in children and osteomalacia in adults. Together
with calcium, vitamin D also helps protect older adults from osteoporosis.
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