- See:
-
l,25-Vitamin D3
-
Abnormalities Associated With Vitamin D.
- Discussion:
- primary function of vitamin D is to maintain skeletal calcium homeostasis;
- promotes gut absorption of calcium;
- promotes bone absorption;
- important for maintaining adequate quantities of Ca & Phos for bone formation thru
its effects on the kidney & gut;
- in addition, it may have a direct effect on bone formation;
- proper functioning of the vitamin D system is necessary for
PTH to
maintain plasma calcium effectively, although drops in the plasma
Ca occur only with severe Vit D depletion;
- it appears that only minute amounts of Vit D are necessary for
PTH
to carry out its actions on the bone and kidney;
- biosynthesis:
- provitamin D2 (ergocalciferol from certain plants and animal fat) and 7-dehydrocholesterol (endogenous)
to form cholecalciferol (vit D3);
- provitamin D3 (7-dehydrocholesterol synthesized in
liver & stored in skin);
- hepatic hydroxylation to 25-OH vit D3;
- renal hydroxylation to
1,25 di OH D3;
- proximal tubule of kidney to
1,25-dihydroxyvitamin D,
24,25-dihydroxyvitamin D, or 25,26-dihydroxyvitamin D);
-
1,25-dihydroxyvitamin D: is the most active of these metabolites;
- Vit D Deficiency:
- primary effect of lack of Vit D is decrease in miceralization of newly formed bone matrix (osteoid);
- rate of bone formation is decreased;
- net result is less total bone & marked change in quality of bone;
-
osteomalacia, group of dz processes characterized by defective mineralizaiton can be
caused by any problem resulting in inadequate amounts of Vit D or low plasma PO4;
- in osteomalacia defective mineralizaiton results in relative increase in amount of osteoid
(unmineralized bone matrix) in bone;
- in skeletally immature individuals, mineralization in the growth plates is also affected;
- osteomalacia in this setting is called
rickets;
- in rickets, impaired mineralization of cartilage in zone of calcification arrests
enchondral ossification & prevents formation of the primary spongiosa;
- persons who do not receive adequate daily sunlight exposure, such as those confined to home or
nursing facility, may be at special risk for vitamin D deficiency;
- Vit D for Osteoporosis (see
osteoporosis)
-
l,25-Vitamin D3
- recommended daily allowance for Vit D is 400 U/day;
- supplementation with calcium and at least 800 U daily of vitamin D is treatment of choice;
- among elderly patients with hip fractures, 10 to 20 percent have impaired bone mineralization
because of vitamin D deficiency;
- Vit D Toxicity:
- high doses of vitamin D may be toxic;
- toxicity has occurred at levels as low as 2,000 to 5,000 IU / day;
- it is not recommended that anyone consume more than 600 to 800 IU/day
without a doctor's recommendation.
