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Until fairly recently, vitamin D was regarded as a rather insignificant nutrient, whose benefits pertained almost exclusively to bone health and little else. In the past decade, however, vitamin D (a.k.a. calcitriol) has received an enormous increase in attention as researchers and medical doctors recognize its crucial, complex role in supporting overall health, especially with respect to the immune system.1
Some of the confusion and prior lack of interest in vitamin D stemmed from the fact that although classified as a fat-soluble vitamin, vitamin D functions as a hormone,2 which means it initiates and regulates critical functions in target tissues throughout the body. Also unlike other vitamins, our primary source of vitamin D is not diet; vitamin D is instead synthesized in the skin from cholesterol upon exposure to sunlight. But because vitamin D can also be obtained through supplements and the diet, vitamin D has been regarded as a fat-soluble nutrient like vitamin A, vitamin E and vitamin K. All types of vitamin D - that which is made in the skin, that which comes from animal-source vitamin D supplements (D3 or cholecalciferol), or fungal source-supplements (D2 or ergocalciferol) - each must first be converted into the active form of vitamin D (25-hydroxyvitamin D) by the liver.
Small amounts of vitamin D precursors are found in cod liver oil, fatty fish, egg yolks and fortified food products. Optimal production of vitamin D can only occur with supplementation or regular and significant exposure of unprotected skin to sunlight of fairly high intensity and for a sufficient duration.3 Since many people cannot receive this kind of regular sun exposure, suboptimal levels and vitamin D deficiencies may be much more common than previously thought, and medical experts have increased the recommended amounts of vitamin D substantially.4
Vitamin D protects health in many ways. In the nucleus of cells, vitamin D regulates the transcription of genetic code so cell growth, function and maintenance are carried out properly and unchecked cell proliferation is controlled; these functions have prompted many studies of vitamin D with respect to cancer.5 Vitamin D also helps regulate calcium metabolism by affecting absorption in the intestines, re-absorption from the kidneys and the mobilization of calcium from bone when insufficient amounts of vitamins D are present.6 Vitamin D modulates the immune system, enhancing function and inhibiting the development of autoimmune reactions.7 Vitamin D is also thought to play a role in insulin metabolism8 and regulating blood pressure,9 both of which can be adversely affected by insufficient level. Finally, research shows that vitamin D may be beneficial for seasonal affective disorder (SAD).10
The ability to synthesize and absorb vitamin D diminishes with age, and is impaired in overweight individuals, putting the obese and elderly populations at a greater risk for deficiency. Strict vegetarians who do not consume fish, eggs or dairy products are also at risk for deficiency. In children, deficiency of vitamin D can cause rickets but this is rarely seen anymore because of vitamin-D fortified foods. In adults, vitamin D levels can be only determined by a doctor. Most health experts agree that vitamin D supplementation is essential for all adults who do not receive regular sun exposure.
Adults usually supplement 400-2000 IU of vitamin D per day, some take more or less on the advice of their doctor.
Vitamin D supplements come in two forms. Animal-derived vitamin D (D3) is called cholecalciferol while fungal-derived vitamin D (D2) is called ergocalciferol. There is mixed evidence that the D3 form is more potent.11 Pregnant women should not take more than the recommended dose, as high doses of vitamin D may be unhealthy for the developing fetus. Strict vegetarians who do not consume animal products should consider a vitamin D supplement.
Consult your physician before using any dietary supplements if you are currently taking any medications.Use as directed.
1. Curr Opin Investig Drugs. 2008 May;9(5):485-90.
2. Clin J Am Soc Nephrol. 2008 Jun 4.
3. Am J Clin Nutr. 2008 Apr;87(4):1080S-6S.
4. Am J Clin Nutr. 2008 Apr;87(4):1080S-6S.
5. Ann Epidemiol. 2008 May 24
6. Linus Pauling Institute Micronutrient Information Center
7. Scand J Immunol. 2008 May 29.
8. Diabetes Obes Metab. 2008 Mar;10(3):185-97.
9. Nutr Rev. 2008 May;66(5):291-7.
10. J Nutr Health Aging. 1999;3(1):5-7.
11. J Clin Endocrinol Metab. 2008 Mar;93(3):677-81. Epub 2007 Dec 18.