A. Vitamin D is really a "conditional nutrient" in the same category as coenzyme Q-10, glutathione, and glucosamine. These are all small organic molecules that the body is able to make but cannot always make in sufficient amounts to maintain health. Although vitamin D clearly fits the definition of a hormone, it has long been classified as a vitamin. It is produced in the skin when a form of cholesterol is changed into a steroid hormone by ultraviolet light.
Compared to the current dietary intake recommendations of 200-400 IU per day for adults, 20 minutes of exposure to sunlight of the correct wavelength generates about 20,000 IU of vitamin D in a healthy adult. This is clearly enough to avoid deficiency and build up the body's stores. Additional exposure to sunlight does not lead to overproduction of vitamin D because continued sunlight exposure destroys vitamin D. Circumstances such as having dark skin or living in higher latitudes reduce the production of vitamin D.
The classic sign of vitamin-D deficiency is failure to maintain normal blood levels of calcium, resulting in the formation of weak bones. This condition is called rickets in children and osteomalacia (early osteoporosis) in adults. Vitamin D strengthens bone by stimulating the absorption, not only of calcium, but also of phosphorus and magnesium.
By taking calcium from bones, vitamin D can raise blood levels of calcium, causing calcium to deposit in the body's soft tissues. Because the consequences of these calcium deposits are so severe, the Food and Nutrition Board established a very conservative tolerable upper limit, 1000 IU/day for infants and 2000 IU/day for children and adults. However, recently published research suggests that these limits are overly conservative and that vitamin D toxicity in healthy adults may begin at 10,000 IU/day. Some nutritionists are pushing for a Daily Value recommendation of 1000 IU/day to replace the current 400 IU/day.
Vitamin D receptors are present in most, if not all, body cells, and research suggests that vitamin D is involved in regulation of cell growth and differentiation the process that determines what a cell will become. Vitamin D tends to inhibit cell growth and stimulate differentiation of cells. While cell growth is essential for wound healing, uncontrolled growth of cells with certain mutations may lead to cancer. The geographic distribution of several types of cancer (colon, breast, and prostate), high blood pressure, and several autoimmune diseases (multiple sclerosis, rheumatoid arthritis, and Type I diabetes) is similar to the historic geographic distribution of rickets, providing circumstantial evidence that decreased sunlight exposure may be related to an increased risk of developing these diseases.
The effect of vitamin D on psoriasis, a disfiguring skin disorder that affects some 50 million people worldwide, is well established. Psoriasis causes skin cells to multiply uncontrollably without differentiation so that they form an unsightly rash. Topical application of activated vitamin D can inhibit skin cell growth and is now approved by the U.S. Food and Drug Administration for treatment of psoriasis.
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