Introduction To Vitamin D

Definition of  Vitamin D

Vitamin D is a fat-soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement. It is also produced endogenously when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol.



  • Vitamin D regulates the calcium and phosphorus levels in the blood by promoting their absorption from food in the intestines, and by promoting re-absorption of calcium in the kidneys.
  • It promotes bone formation and mineralization and is essential in the development of an intact and strong skeleton.
  • It inhibits parathyroid hormone secretion from the parathyroid gland.

Vitamin D affects the immune system by promoting immunosuppression, phagocytosis, and anti-tumor activity.

Hormone for Calcium and Phosphate regulation

  • Calcium is vital for nerve cell transmissions and muscle fiber contractions.
  • Calcitriol functions in concert with parathyroid hormone (PTH) and calcitonin to regulate serum calcium and phosphorous levels by:

1.Increasing dietary calcium absorption from the small intestine.

2.Decreasing the urinary calcium excretion (increasing renal reabsorption).

3.Stimulating resorption of calcium from bone

  • Instrumental in the growth, hardening and repair of bones.
  • Too much vitamin D, however, can increase calcium losses from bone
  • Vitamin D is essential for normal insulin secretion by the pancreas and therefore control of blood sugar levels.

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When vitamin D goes abnormal

  • Too little leads to Vit D deficiency:

–Rickets (bone deformities in children)


–Osteomalacia (weak bones)

–Demineralization of bones

  • Too much (5 times the RDA, chronically): It is the most toxic of the vitamins.

–Nausea, thirst, loss of appetite, stupor(অসাড়তা)

–Hypocalcemia: calcium gets deposited in soft tissues, arteries and kidneys.


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