Vitamin D3: a Vital Prohormone

– Essential for Maintaining Internal Health and Preserving Normal Bones


Vitamin D3 a fat-soluble vitamin, known as a prohormone since its action is similar to that of a hormone. It is formed in human skin by exposure to UV sunlight. In foods, it is found mainly in cod-liver or other fish liver oil, egg yolks and milk.

Vitamin D is important for the health of bones and the immune system. It is essential in the synthesis of bones, maintaining their normal density and integrity. Bones also host the red bone marrow essential for the creation and proliferation of blood and immune cells.

D2 and D3

Vitamin D comes in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). Of these, Vitamin D3 is the preferred form because it’s the type our skin produce upon exposure to sunlight, and thus it’s easier and faster for the body system to utilize and break into its active ingredients.

In the liver, the photosynthesized “previtamin” D3 is then converted to 25(OH)D3. In the kidneys, this is in turn converted to its active form, (1,25(OH)2D3).

Causes of Vitamin D Deficiency

  • Inadequate dietary intake of vitamin D
  • Decreased absorption due to liver disease or intestinal malabsorption
  • Hypoparathyroidism – decreased levels parathyroid hormone (PTH)
  • Use of certain medications, such as Phenobarbital, phenytoin, or rifampin
  • Lack of skin exposure to sunlight for example in northern climates where sunny days are rare occasions
  • Life habits (such as wearing more clothing)
  • Genetic disorders such as Type I and Type II vitamin D–dependent rickets.


In children, Vitamin D deficiency can result in:

  • Defective calcification of growing bone.
  • Hypertrophy of the epiphyseal cartilages. (In other words, an irregular increased in width of articular cartilages – widened joints, wide hyaline cartilage on the articular surfaces of bones. This occurs since the cells of epiphyseal cartilage cease to degenerate normally, whilst new cartilage continues to form.)
  • The accumulation of reduced calcification and osteoid material around the capillaries of the diaphysis. The cancellous bone of the diaphysis and cortical bone may be resorbed in chronic deficiency.

In adults, Vitamin D deficiency can cause similar changes, affecting the whole skeleton.

The Role of Vitamin D in The Body

  • Controls the process of providing sufficient amounts of Calcium (Ca) and Phosphate (PO4) necessary for building healthy bones by regulating the balance of both Ca and PO4 in collaboration with parathyroid hormone (PTH) and Calcitonin. (Calcitonin is a hormone secreted by the thyroid C cells to reduce plasma Ca concentration by renal excretion, bone formation and enhancing its cellular uptake.) This is important because increased concentrations of PO4 & Ca may increase the risk of atherosclerotic vascular disease.
  • Regulates the level of Ca in the body by increasing the intestinal absorption of Ca from food. The mechanism is that Ca concentration in blood is regulated by PTH, released into blood after the parathyroid cells’ Ca receptors notice an inadequate level of plasma calcium. PTH then fixes this low blood calcium (hypocalcemia) by increasing formation of 1,25(OH)2D3, thus in turn increasing the intestinal absorption and renal absorption of dietary Ca. It will also allow resorption (a catabolic reaction that results in the breaking down of composition ingredients and products) of Ca and PO4 from the bones where required.
  • Promotes normal bone formation and mineralization by stimulating a type of bone stem cell (osteoblast) to produce less collagen and more alkaline phosphatase and osteocalcin (a vitamin K dependent protein).
  • Maintenance of Ca homeostasis to provide sufficient amounts of activated vitamin D3 by inhibiting the synthesis of 1a-hydroxylase and stimulating synthesis of 24-hydroxylase in the kidneys.
  • Controlling the level of Ca in blood, hormonal regulation through the mechanism of inhibiting PTH secretion.
  • Helps fight cancer. Here, Vitamin D3 stimulates both immunogenic and antitumor activity in the immune system (the lymphomedullary system made of lymphatic vessels and lymph nodes). The role of the defense system is to protect our bodies from foreign material by working as a filter against antigens and their toxins. Vitamin D3 activates and promotes the proliferation of immune cells, especially the T cell natural killers (NK), known to fight tumor specific target cells.



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