Vitamin D can be obtained from two principal sources: diet and sunlight exposure. Dietary vitamin D can be acquired as ergocalciferol, or vitamin D-2, from plants or as cholecalciferol, or vitamin D-3, from animals. Vitamin D-3 is synthesized in your skin whenever you are exposed to ultraviolet light, specifically that in the ultraviolet-B, or UVB, spectrum. According to the August 2010 issue of "BMC Public Health," vitamin D acquired from UVB exposure accounts for 90 percent of your body's vitamin D requirement, while your diet provides a relatively small percentage.
Functions
One of vitamin D's primary roles is to regulate calcium metabolism in your body. Vitamin D increases the absorption of calcium from your intestine, enhances retention of calcium by your kidneys and mobilizes calcium from your bones when your dietary calcium intake is inadequate. In addition, vitamin D modulates a variety of physiologic functions, such as cell growth, hormone secretion, blood pressure and immunity. Increased frequencies of autoimmune diseases, infections, cardiovascular disease and cancer have been noted among populations whose vitamin D levels are low due to limited UVB exposure.
Synthesis
According to experts at Brown University in Providence, Rhode Island, your skin cells normally produce large amounts of a cholesterol-like molecule--7-dehydrocholesterol--which they incorporate into their membranes. When 7-dehydrocholesterol interacts with UVB light, it is converted to cholecalciferol, or vitamin D-3, which enters your bloodstream and travels to your liver. In your liver, cholecalciferol is enzymatically converted to 25-hydroxyvitamin D-3, or calcidiol, which re-enters your circulation and travels to your kidneys, where yet another enzyme converts it to 1,25-dihydroxyvitamin D-3, or calcitriol, the biologically active form of vitamin D.
Regulation
Large doses of vitamin D can be toxic, leading to high calcium levels, loss of appetite, nausea, weakness, irritability and, in severe cases, kidney failure. Your body has developed a mechanism to prevent overproduction of vitamin D during sun exposure. Once cholecalciferol forms in your skin, it continues to absorb ultraviolet light, which eventually causes its degradation into biologically inactive byproducts. The March 2011 issue of "Acta Dermato-Venereologica" reports that long-term exposure to UV light leads to a "steady state" where only 10 to 15 percent of your skin's 7-dehydrocholesterol is effectively converted to cholecalciferol. This "photoregulation" prevents the synthesis of toxic amounts of vitamin D when you are in the sun for long periods of time.
Considerations
UVB light is required for production of vitamin D-3 in your skin. Your liver and kidneys then convert vitamin D-3 to calcitriol, the biologically active form of vitamin D. Dr. Michael Holick at Boston University Medical Center recommends 5 to 15 minutes of daily sun exposure to your arms and legs or arms, hands and face during the spring, summer and fall to stimulate vitamin D-3 synthesis. During the winter, vitamin D supplementation might be necessary to maintain healthy blood levels. Sun exposure has been linked to the development of some skin cancers, so ask your physician if exposure to UVB light is appropriate for you.
References
- "BMC Public Health"; The Effects of Air Pollution on Vitamin D Status in Healthy Women: A Cross-Sectional Study; F. Hosseinpanah, et al.; August 2010
- "Acta Dermato-Venereologica"; Factors Influencing Vitamin D Status; W.G. Tsiaris, M.A. Weinstock; March 2011
- "The Journal of Nutrition"; The Vitamin D Epidemic and Its Health Consequences; M.F. Holick; November 2005
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