Vitamins

  • The steroid vitamin crosses the cell membrane and binds to cytosol receptor proteins
  • Activation of this complex within cytosol allows its entry into the nucleus and binding to chromatin
  • This results in increased formation and transfer into cytosol of mRNA for Ca and P transport proteins
  • This enhances rate of synthesis of proteins involved in Ca and P absorption or resorption
  • Receptors for 1,25(OH)2D are found in cells of other tissue (including pancreas and skin) so vitamin D may have other, as yet undiscovered functions
  • Because kidney activation of the vitamin-hormone is central to its action and regulation of blood Ca, loss of kidney function will produce hypocalcemia and bone disease

    • VI.  Dietary Sources of Vitamin D

    1. Fish oils and egg yolks
    2. Milk and other supplemented processed foods
    3. Raw foodstuffs contain little or no vitamin D

    VII.  Deficiency

    1. Deficiency results from:
      1. Insufficient vitamin production
      2. Dietary lack
      3. Malabsorption of fats
    2. Levels of PTH are increased, which enhance urinary excretion of phosphorus and further reduce bone calcification
    3. Deficiency symptoms
      1. Rickets
        1. Appears during growing period (children)
        2. Poor calcification of bones
        3. Poor cartilage cell growth
        4. Deformities of bones
      2. Blood changes
        1. Serum 25(OH)D3 below 5 ng/ml (normal levels are 30 mg/ml in US and 20 ng/ml in Europe)
        2. Decreased serum Ca and P
        3. Increased serum alkaline phosphatic
      3. Osteomalacia
        1. Occurs in adults
        2. Bone reabsorption weakens bone
        3. May occur with failure to absorb fat and fat soluble vitamins
        4. Chronic renal failure causes a problem

    VIII.  Toxicity

    1. 10 times the RDA
    2. Symptoms
      1. Hypercalcemia (Ca> 12mg/dl) (also caused by hyperparathyroidism)
      2. Deposition of Ca in soft tissues (kidney, heart, lung, vasculature)
        1. Hypercalcuria, possibly stones
        2. Cardiac myopathy and afilerosclerotic lesions
        3. Melanin in the skin may protect man from vitamin D toxicity
        4. Exposure to equatorial versus Boston sunlight doubles the initial rate of pre-D3 production
        5. With increased exposure, previtamin D3 production decreased and increasing proportions of biologically inactive forms (lumisterol) were produced

    Vitamins




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