Answer to Question N-03 about Hypervitaminosis D


 

 

Answer to Biochemistry Question N-03 : ( c ) Vitamin D

 

 

Vitamin D is synthesized from a derivative of cholesterol, the 7 dehydrocholesterol.  By the action of hv light (290-315 nm) in the skin, it becomes cholecalciferol or Vitamin D3 (inactive).

 

                                   Cholecalciferol structure

 

Cholecalciferol is hydroxylated in the liver by  25 hydroxylase, and in the kidney by a 1 hydroxylase,  becoming 1,25 dihydroxycholecalciferol or calcitriol, the active form of vitamin D3.

 

                                    Calcitriol structure

  

Vitamin D participates in the regulation of multiple processes, being the more obvious the regulation, together with Parathormone, of Ca++ homeostasis, regulating Ca++ in blood and the balance between Ca++ deposition and Ca++ mobilization from bone.

 

Hypocalcemia stimulates releasing of PTH, that through cAMP activates 1 hydroxylase  in the conversion of VitD to calcitriol.

 

Calcitriol interacts with a vitamin D receptor (VDR)  and initiates a cascade of molecular changes that stimulates the transcription of specific genes, including  the synthesis of an intestinal Ca++ binding protein, essential for uptake of dietary Ca++. Together with PTH, Vitamin D  facilitates Calcium reabsortion by the kidneys and mobilizes calcium from bone.

 

All these actions help bring back blood calcium levels within the normal range.

 

Intoxication with Vitamin D is usually a consequence of a prolonged ingestion of an overdose of supplements or as a consequence of accidents in the formulation of pills or in the misuse of some oils or containers.

 

The intoxication requires the prolonged ingestion of very excessive amounts of Vitamin D (around 100 times the Recommended Diary allowance) during a period of months, or an acute overdose of 10 000 times the RDA.

 

Vitamin D intoxication is characterized by hypercalcemia, hypercalciuria, gastrointestinal disorders, weak bones, deposits of calcium in soft tissue that can be revealed by imagen studies, renal calcium stones and kidney failure.

25 hydroxycolecalciferol is tipically elevated in serum.

 

General information about Vitamin D can be found in:

 

Linus Pauling Institute. Micronutrient Information Center. Vitamin D

 

 

Dusso, A.S. et al: Vitamin D

Am J Physiol Renal Physiol 289:F8-F28, 2005

 

http://en.wikipedia.org/wiki/Vitamin_D

 

 

Information about Vitamin D requirements and safety levels:

 

Vieth, R.:

Vitamin D Supplementation, 25-hydroxyvitamin D concentrations and safety

Amer J Clin Nut 69:5, 842-856, 1999

 

Muskiet, F.A.J. et al:

Do we really need > 100 ug vitamin D/d and is it safe for all of us?

Amer J Clin Nutr 74: 6, 862-863, 2001

 

More about Vitamin D intoxication:

 

Klontz, K.C. and Acheson, D.W.: Dietary Supplement-induced vitamin D intoxication

NEJM, 357(3) 308-309,  2007

 

Vitamin D Intoxication Associated with an Over-the-Counter Supplement

NEJM, 345 (1)  66-67, 2001

 

Marriot, B.M.: Vitamin D supplementation: a word of caution.

Annals of Internal Medicine, 127(3)  231-233

 

http://en.wikipedia.org/wiki/Hypervitaminosis_D

 

 

 

One thought on “Answer to Question N-03 about Hypervitaminosis D

  1. Pingback: Too Much Vitamins (Q: N-03) « The Biochemistry Questions Site

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