Functions and Mechanisms

1. A "prohormone": the active metabolite, calcitriol, is a steroid hormone.
2. similar in base structure to Testosterone
3. Sunlight on our skin is converted into Vitamin D
4. required for bone and teeth development
5. involved in the absorption and metabolism of phosphorus and calcium.
6. fat-soluble (toxic in large amounts).
7. Absorption may be decreased during iron deficiency.
8. Promotes calcium absorption while utilizing phosphorus in the intestines
9. interacts with vitamin K during bone metabolism.
10. Vitamin D metabolism depends on P450 enzymes. Moreover, one of these enzymes also catalyzes the breakdown of cholesterol: Usui E, Noshiro M, Ohyama Y, Okuda K.
11. Helps decrease systemic inflammation.
    • "[A] study evaluated inflammatory markers in patients with congestive heart failure. Patients took 500 mg of calcium, plus either a placebo or 2000 IU of vitamin D, daily for nine months. Researchers measured cytokines (chemical messengers of the immune system), such as pro-inflammatory TNF (tumour necrosis factor) and anti-inflammatory IL-10 (interleukin 10). It was found that people who took the vitamin D supplement showed a 43% increase in interleukin 10, whereas the placebo group showed no change. The vitamin D group also maintained levels of the inflammatory TNF cytokine, where the placebo group had a 12% increase in the inflammatory marker.
    • "Previous research suggests that vitamin D may also improve muscular function, control blood pressure, and improve glucose tolerance. Inflammation is an underlying cause in those conditions, and in cardiovascular diseases, such as congestive heart failure. In the recent past, physicians recommended 400 IUs of vitamin D daily. Evidence is mounting that supplementing 2000 IUs of vitamin D is necessary for optimal health."
    • From "High-Dose Vitamin D Supplements May Reduce Inflammation" in the July 2006 newsletter of N.E.E.D.S; they refer to the The American Journal of Clinical Nutrition, 83 (4), 2006; pp 754 -759.

Treats

• Osteoporosis
• Eczema
• Cancer
  • For example, see Cholecalciferol (vitamin D3) inhibits growth and invasion by up-regulating nuclear receptors and 25-hydroxylase (CYP27A1) in human prostate cancer cells. Tokar EJ, Webber MM. Clin Exp Metastasis. 2005;22(3):275-84. PubMed ID 16158255
• Inflammation - decreases the cytokine tumor necrosis factor (TNF)

Testing:

• 25-Hydroxy Vitamin D for deficiency
  • Blood should be spun down, serum frozen. Transport preferably on dry ice - or on ice pack if this is impractical. 
  • LabSafe lists it at $148
• Vitamin D 1,25 Dihydroxy is the form produced by the kidney from the 25-HD form; testing for it is useful for thyroid disease and ricketts ($199 at Labsafe)

Supplementation:

• Forms available:
  • Vitamin D2 (ergocalciferol) - most supplements are made of this.
  • Vitamin D3 - more effective than D2 at increasing the levels of vitamin D hormone in circulation (see Laura A. G. Armas, Bruce W. Hollis and Robert P. Heaney (2004). "Vitamin D2 Is Much Less Effective than Vitamin D3 in Humans". The Journal of Clinical Endocrinology & Metabolism 89 (11): 5387–5391.
    • calcitriol (the result of several steps of Vitamin D metabolism, produced by the kidneys) - extremely powerful, prescription only
    • cholecalciferol - the form to take for regular supplementation
• 2000 IU is now common. BUT "except in those with conditions causing hypersensitivity, there is no evidence of adverse effects with serum 25(OH)D concentrations <140 nmol/L, which require a total vitamin D supply of 250 µg (10000 IU)/d to attain. Published cases of vitamin D toxicity with hypercalcemia, for which the 25(OH)D concentration and vitamin D dose are known, all involve intake of 1000 µg (40000 IU)/d. Because vitamin D is potentially toxic, intake of >25 µg (1000 IU)/d has been avoided even though the weight of evidence shows that the currently accepted, no observed adverse effect limit of 50 µg (2000 IU)/d is too low by at least 5-fold." Reinhold Vieth, Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. American Journal of Clinical Nutrition, Vol. 69, No. 5, 842-856, May 1999
• Overdose: Symptoms of intoxication include nausea, vomiting, diarrhea, headache, polydipsia, and polyuria, all associated with hypercalcemia. Prolonged increased vitamin D levels may lead to hypertension, nephrolithiasis, and metastatic calcifications.

For More information

• Vitamin D pharmacology - A useful analysis of the confusion of some of substances called "vitamin D." Does not mention D2
• Vitamin D pysiology - starts with basic info almost the same as on the page mentioned just above, then gets into detail on Vitamin D metabolism.
• Vitamin D (calcitriol) This description is easy to follow. Unfortunately it contains at least one inaccuracy: ergosterol, found in plants, is not Vitamin D2 but provitamin D2. It is converted to ergocalciferol (vitamin D2) upon irradiation by ultraviolet light or electronic bombardment.
• Vitamin D3: Higher Doses Reduce Risk of Common Health Concerns, by Chris D. Meletis, ND. On the site of Vitamin Research Products. A good general summary of the state of the research.
• Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Reinhold Vieth. American Journal of Clinical Nutrition, Vol. 69, No. 5, 842-856, May 1999. A more technical presentation.
• Vitamin D3 — Essential for Health, by Chaya Venkat, on the CLL topics site. Rambling, chatty, wordy -- but includes several interesting abstracts that are fairly recent.
• The World's Healthiest Foods: Vitamin D
• The Nutrition Farm / Vitamin D
• Unraveling the Enigma of Vitamin D — on the National Academy of Sciences' Beyond Discovery site.
• Take ten minutes of sunshine — about the research of Michael D. Holick.
• Review of Vitamin D by the Expert Group on Vitamins and Minerals, part of the Food Standards Agency of the UK. Revised August 2002.