Sunlight and Vitamin D

Vitamin D is the sunshine vitamin that has been produced on this earth for more than 500 million years. During exposure to sunlight 7-dehydrocholesterol in the skin absorbs UV B radiation and is converted to previtamin D3 which in turn isomerizes into vitamin D3. Previtamin D3 and vitamin D3 also absorb UV B radiation and are converted into a variety of photoproducts some of which have unique biologic properties. Sun induced vitamin D synthesis is greatly influenced by season, time of day, latitude, altitude, air pollution, skin pigmentation, sunscreen use, passing through glass and plastic, and aging. Vitamin D is metabolized sequentially in the liver and kidneys into 25-hydroxyvitamin D which is a major circulating form and 1,25-dihydroxyvitamin D which is the biologically active form respectively. 1,25-dihydroxyvitamin D plays an important role in regulating calcium and phosphate metabolism for maintenance of metabolic functions and for skeletal health. Most cells and organs in the body have a vitamin D receptor and many cells and organs are able to produce 1,25-dihydroxyvitamin D. As a result 1,25-dihydroxyvitamin D influences a large number of biologic pathways which may help explain association studies relating vitamin D deficiency and living at higher latitudes with increased risk for many chronic diseases including autoimmune diseases, some cancers, cardiovascular disease, infectious disease, schizophrenia and type 2 diabetes. A three-part strategy of increasing food fortification programs with vitamin D, sensible sun exposure recommendations and encouraging ingestion of a vitamin D supplement when needed should be implemented to prevent global vitamin D deficiency and its negative health consequences.

Life forms began to evolve in the oceans over 1 billion years ago. They took advantage of sunlight and used it as an energy source to generate carbohydrates. Curiously some of the earliest phytoplankton including Emiliania huxleyi (which is a coccolithophore, i.e., has a calcium carbonate exoskeleton) which has existed unchanged in the Sargasso Sea (Atlantic Ocean) for more than 500 million years when exposed to sunlight not only photosynthesized glucose but also produced vitamin D2 (Fig. 1 ).1 This phytoplankton produces a large amount of ergosterol that when exposed to sunlight absorbs ultraviolet B (UVB) radiation and undergoes a photolysis reaction to form previtamin D2. Once formed this thermodynamically unstable isomer is transformed into vitamin D2. Similarly yeast and fungi also contain high amounts of ergosterol and when exposed to sunlight produce vitamin D2.1-4

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