Functions of Vitamin D, The Kidney Hormone

There are two nutritionally important forms of vitamin D in men and they are calciferol or vitamin D3 and cholecalciferol or vitamin D3. Calciferol is derived by irradiation of plant sterol, ergosterol. Cholecalciferol is naturally occurring vitamin D and is found in fish liver oil and animal fats. Cholecalciferol is also derived from exposure to sunlight or ultra violet rays of sun light which can convert 7-dehydrocholesterol in skin to vitamin D. Vitamin D is a fat soluble vitamin and is stored in fat depots.

Vitamin D: The Kidney Hormone

Vitamin D is proposed to be named as kidney hormone as it does not meet the criteria to be defined as vitamin i.e. a substance that has to be obtained by dietary means because of lack of capacity of human body to synthesize it. But vitamin D3 is not a dietary requirement in presence of adequate sunlight. Human body can synthesize adequate amount of vitamin D in the skin by exposure of as little as 5 minutes per day. No more than 5 minutes exposure to sunlight is required to synthesize enough quantity of vitamin D for one day.

Vitamin D itself is metabolically inactive unless it undergoes endogenous transformations to several active metabolites like 25, hydroxy cholecalciferol and 1, 25, dihydroxy cholecalciferol, at first in liver and than in kidney. 25, hydroxy cholecalciferol and 1, 25, dihydroxy cholecalciferol are bound to transport proteins and delivered to target tissues, bone and intestines.

Functions of Vitamin D and its Metabolites:

  1. In bone: Vitamin D stimulates normal mineralization of bone. It also increases normal resorption of bone. Vitamin D improves collagen maturation.
  2. In Intestine: In the intestine vitamin increase absorption of calcium and phosphorus.
  3.  In the Kidney: Vitamin D increase re absorption of phosphates in the kidney. The re absorption of calcium in the kidney is variable.
  4. Other Functions: Among the other functions of vitamin D, other than the functions mentioned above is its role in the normal growth of human body.
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