Sunday, November 21, 2010

Long known Vitamin D ..and physiology

Of all the vitamins..Vitamin D has recently featured in almost all medical journals..given the huge prevalance(57% of hospitalised pts in US) of Vitamin D deficiency and the association of it to many important disease processes known to man! 
Vitamin D has properties of both being a hormone..and a vitamin. Lets get a quick recap of the physiology.

There are 2 main forms of vitamin D: vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D3 is formed in the skin, after exposure to ultraviolet B radiation, from 7-dehydrocholesterol that resides in cell membranes.Vitamin D2 is obtained through irradiation of ergosterol in plants from sunlight, and enters our circulation through diet. Vitamin D3 also is available from animal sources, such as cod liver oil, salmon, mackerel, and herring. 
After vitamin D is ingested from the diet or synthesized in the skin, it is metabolized in the liver to a biologically
inactive 25-hydroxyvitamin D, 25(OH)D, which is the major circulating form of vitamin D. It is this form( 25-hydroxyvitamin D) of vitamin D that is used to determine a person’s vitamin D status. 25(OH)D is then converted into its active form 1,25 (OH)D by the kidneys. This then interacts with vitamin D receptors in the intestine and bone to augment intestinal calcium absorption and mobilize osteoclast activity. Non-calcium regulating tissues, such as the prostate,colon, and breast, locally convert 25(OH)D to 1,25(OH)2D....and this forms the basis of Vitamin D being involved in prevention of malignancy in these tissues,aas it facilitates programmed cell growth.

Although controversies exist regarding the cut off for calling deficiency( due to summer Vs winter, morning Vs evening etc)......widely accepted value of < 20ng/ml of 25(OH)D is considered deficient ....21 - 29 is considered Vit D insufficiency......and > 30 ng/ml signifies sufficiency.  Without vitamin D, only 10% to 15% of dietary calcium and approximately 60% of phosphorus will be absorbed. The efficiency of calcium absorption is increased by 30% to 40%, and phosphorus by 80%, with the interaction of 1,25(OH)2D to its receptor.
I frequently got confused with the different names vit D has! 25(OH)D is an inactive form..but this is what we measure to diagnose deficiency. 1,25(OH)D is the active form..and we dont measure it. D2 & D3 are just numbers to describe the source of VitD....D2 (cholecalciferol) from human skin, and D3(ergocalciferol) from food products. 


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