Nutrition Science and Everyday Application - beta v 0.1

Vitamin D: Important to Bone Health and
Beyond
Vitamin D is unique among the vitamins because we can synthesize most of what we need in
our skin. Sunlight is an essential ingredient in this process, so vitamin D is sometimes called
the “sunshine vitamin.” However, the amount of vitamin D synthesized in the body is often
not enough to meet our needs, so many people also need to consume dietary sources.
METABOLISM AND FUNCTIONS OF VITAMIN D
Vitamin D synthesis in the skin begins with the conversion of cholesterol to
7-dehydrocholesterol. Then, in the presence of ultraviolet (UV) rays from sunlight,
7-dehydrocholesterol is converted to vitamin D 3 (also called cholecalciferol), which is
transported to the liver by a binding protein.
In dietary sources, vitamin D may be present in the form of vitamin D 3 from animal
products or vitamin D 2 (also called ergocalciferol), made by plants, mushrooms, and yeast.
Dietary vitamin D 2 and vitamin D 3 are transported to the liver via chylomicrons and then
taken up in chylomicron remnants.
Vitamins D 2 and D 3 are both inactive until they undergo two hydroxylations—chemical
reactions that add a hydroxyl (-OH) group. The first hydroxylation occurs in the liver, creating
calcidiol. This is the circulating form of vitamin D and the form measured in blood to assess
a person’s vitamin D status. The second hydroxylation occurs in the kidneys and forms
calcitriol, the biologically active form of vitamin D.
Recall from our discussion of regulation of blood calcium that one of the actions of
parathyroid hormone (PTH) is to stimulate enzymes in the kidney that perform this last step
in the activation of vitamin D. Active vitamin D increases the absorption of both calcium and
phosphorus in the intestine, as well as working with PTH to reduce calcium loss in the urine
and stimulate release of calcium and phosphorus from the bone. In these ways, vitamin D
plays a critical role in both maintaining blood calcium homeostasis and enhancing the supply
of calcium and phosphorus for bone mineralization. Vitamin D deficiency results in poor
bone mineralization, with serious consequences in both children and adults, as we’ll discuss
later on this page.
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