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It is a fat soluble vitamin.
Resembles sterols in structure and functions
like a hormone.
Synthesized by the body after exposure to UV
rays or obtained from food sources
Milk, cheese, yogurt, orange juice
Ready to eat cereals
Rays from the sun trigger synthesis in the skin
Ergocalciferol (vitamin D2) is formed from
ergosterol and present in plants.
Cholecalciferol (vitamin D3) is found in
Ergocalciferol and cholecalciferol are sources
for vitamin D activity and are referred as
Both ergocalciferol and cholecalciferol are
similar in structure expect that ergocalciferol
has an additional methyl group and double
25 hydroxy vit D3 (calcidiol)
1,25 dihydroxy vit D3 (calcitriol).
◦ Vitamin D is absorbed from the small intestine
for which bile is essential.
◦ Stored in adipose tissue
◦ Transported to the liver, then kidneys, where
it is activated
Excreted in bile
Metabolized to water soluble metabolites
and then excreted in urine as well.
Calcitriol is biologically active form of
It regulates the plasma levels of calcium and
It acts at 3 different levels:
1. Action of calcitriol on intestine.
2. Action of calcitriol on bone.
3. Action of calcitriol on kidney.
It is an another metabolite of vitamin D.
Synthesized in kidney by 24 hydroxylase.
Exact function is not known.
It is believed that when calcitriol
concentration is adequate 24 hydroxylase
leading to formation of less imp compound
In this way to maintain homeostasis of
calcium synthesis of 24,25 DHCC is also
inhibits proliferation and stimulates the
normal differentiation of cells
The inactive form of vitamin D aids in the
formation of active vitamin D which enhances
immunity and inhibits the development of
◦ With decreased dietary calcium, PTH is
secreted and causes increased production of
calcitriol, a hormone
◦ Calcitriol restores normal calcium levels by
increased absorption of dietary calcium,
increased mobilization of calcium from bone,
and increased reabsorption from kidneys
◦ Renin, important in regulation of blood
pressure and heart health, is associated with
vitamin D levels
Maintaining normal phosphorus levels
◦ Bone cannot be properly mineralized without
During vitamin D deficiency, calcium
absorption cannot be increased to meet
Therefore, calcium is taken from bones
to maintain normal serum levels