Vitamin D and skin physiology: A D-lightful story

Throughout evolution, exposure to sunlight and the photosynthesis of vitamin D-3 in the skin has been critically important for the evolution of land vertebrates. During exposure to sunlight, the solar UVB photons with energies 290-315 nm are absorbed by 7-dehydrocholesterol in the skin and converted to previtamin D-3. Previtamin D-3 undergoes a rapid transformation within the plasma membrane to vitamin D-3. Excessive exposure to sunlight will not result in vitamin D intoxication because both previtamin D-3 and vitamin D-3 are photolyzed to several noncalcemic photoproducts. During the winter at latitudes above similar to 35 degrees, there is minimal, if any, previtamin D3 production in the skin. Altitude also has a significant effect on vitamin D-3 production. At 27 degrees N in November, very little (similar to 0.5%) previtamin D-3 synthesis was detected in Agra (169 in) and Katmandu (1400 in). There was an similar to 2- and 4-fold increase in previtamin D-3 production at similar to 3400 m and at Everest base camp (5300 in), respectively. Increased skin pigmentation, application of a sunscreen, aging, and clothing have a dramatic effect on previtamin D-3 production in the skin. It is estimated that exposure in a bathing suit to 1 minimal erythemal dose (MED) is equivalent to ingesting between 10,000 and 25,000 IU of vitamin D-2. The importance of sunlight for providing most humans with their vitamin D requirement is well documented by the seasonal variation in circulating levels of 25-hydroxyvitamin D [25(OH)D]. Vitamin D deficiency [i.e., 25(OH)D < 20 ng/ml] is common in both children and adults worldwide. Exposure to lamps that produce UVB radiation is an excellent source for producing vitamin D-3 in the skin and is especially efficacious in patients with fat malabsorption syndromes. The major cause of vitamin D deficiency globally is an underappreciation of sunlight's role in providing humans with their vitamin D-3 requirement. Very few foods naturally contain vitamin D, and those that do have a very variable vitamin D content. Recently it was observed that wild caught salmon had between 75% and 90% more vitamin D-3 compared with farmed salmon. The associations regarding increased risk of common deadly cancers, autoimmune diseases, infectious diseases, and cardiovascular disease with living at higher latitudes and being prone to vitamin D deficiency should alert all health care professionals about the importance of vitamin D for overall health and well being.