Vitamin D
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Introduction
Vitamin D is the key vitamin for calcium absorption.
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).
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
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.
Deficiency
Vitamin D deficiency results in rickets in children and osteomalacia in adults.
Hypocalcemia, hypophosphatemia, muscle weakness, and bone pan can also occur.
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.
Resources and References
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