The Hip: Preservation, Replacement and Revision

Vit D: Discussion



- 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.



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Tuesday, September 18, 2012 9:27 pm