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Vitamin D

Vitamin D is the key vitamin for calcium absorption.

 

• daily requirements
• sources
• biological roles
• deficiency
• overdose

 

daily requirements

The RDA for adults is 5mg cholecalciferol, or 200 IU of vitamin D. Higher doses may be required to prevent osteoporotic fractures.

 

AI:
< 50y               5 μg/d (200 IU)
 50-70y            10 μg/d (400 IU)
 71y+               15 μg/d (600 IU)

 

UL = 50 μg/d (2000 IU/d)

DRI values based on the absence of adequate exposure to sunlight.
Canada’s Food Guide recommends all adults over the age of 50 should take a daily supplement of 10 ug (400 IU).

 

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sources

 

The skin can produce D₃ from cholesterol and sunlight. Factors influencing D₃ production include age, skin colour, season, geographic latitude, time of day, cloud cover, smog, sunscreen.

 

Ergocalciferol (D₂) is found in plants, while cholecalciferol (D₃) is found in animal sources.

• fatty fish (ie salmon, herring)
• fortified milk (and some milk products)
• egg yolk
• some fortified cereal
  

 

 

 

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biological roles

D₃ is hydroxylated in the liver to produce 25-OH D₃, the predominant form in the plasma and the major storage form. Liver disease can result in vitamin D deficiency.

Kidney enzyme 1α hydroxylase, regulated by parathyroid hormone, creates the active molecule 1,25-dihydroxycholecalciferol (1,25 diOH D₃) under conditions of low serum calcium.

 

1,25 diOH D₃ maintains levels of plasma calcium by:

• increasing calcium uptake in the intestine by increasing transcription of calcium-binding proteins (acting like a typical steroid hormone)
• minimizing loss from the kidney
• increases osteoblastic activity and bone formation
• also stimulates calcium and phosphate mobilization from bone in a process requiring protein synthesis and PTH

I've heard it also increases phosphate absorption in the intestine.

 

 

 

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deficiency

Vitamin D deficiency results in rickets in children and osteomalacia in adults.

Hypocalcemia, hypophosphatemia, muscle weakness, and bone pan can also occur.

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overdose

Vitamin D is the most toxic of all vitamins. High doses (100,000 IU for weeks or months) can result in anorexia, nausea, thirst, and stupor.

 

Over-absorption of calcium (hypercalcemia) can lead to excess bone and soft tissue calcification, kidney stones, and renal failure.

 

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