Vital function in Ca and P homeostasis.
Works in concert with PTH to increase blood calcium levels by promoting the resorption
of Ca from the renal tubule and by the resorption of Ca from bone stores.
Increases the enteric absorption of Calcium through increased synthesis of Calcium
Binding Protein and through an unspecified “nongenomic” process.
Increases enteric phosphate absorption also with some unknown mechanism…Process
appears to be independent of Calcium concentration.
Plays a critical role in the formation bone formation (mineralization) as well as
mobilization (demineralization) of bone – Calcium homeostasis)
Mineralization thought to be mediated through 1,25 dihydroxy Vit D via the
osteoblasts. Other Vit D metabolites may be involved in this process (24,25
dihydroxy Vit D).
Demineralization (Ca. Homeostasis) 1,25 dihydroxy Vit D affects the osteoclasts
through the differentiation of macrophages
Osteoporosis: Loss of bone mass starting in the third to forth decade in females and forth
to fifth decade in males.
Postmenopausal women usually have lower circulating levels of Vit D which has led to
the theory that the hydroxylation reaction that occurs in the kidney may affected by the
loss of estrogen.
Osteoporosis is also observed in laying hens (cage layer fatigue)
Calcium in laying hen rations is exceedingly high because of this.
Does not, not, not “come from the sun”. even though its called the sunshine vitamin
Sunlight catalyzes the opening up of the cholesterol precursor (lenosterol). The
first hydroxylation occurs in the liver (25-hydroxycholicalciferol (25-
hydroxyvitD3 is same)) and the 2 on the kidney (1,25 dihydroxy vit D)
(turned on by PTH ).
Most species can utilize either D (2erived from plants) or D . Ho3ever, birds utilize D 2
with 1/7 the efficiency as D .3 Routinely added to wh