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Vitamin D - Calciferol

Vitamin D is produced in the body in the presence of sunlight. Because it is fat soluble it is stored in the body.

Calciferol is encountered in the primary form known as cholecalciferol (D3) and also as ergocalciferol (D2).

Sun exposure

Sun exposure is perhaps the most important source of vitamin D because exposure to sunlight provides most humans with their vitamin D requirement. Ultraviolet (UV) rays from the sun trigger vitamin D synthesis in skin. The seasons, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis.

For example, sunlight exposure from November through February in Boston is insufficient to produce significant vitamin D synthesis in the skin. Complete cloud cover halves the energy of UV rays, and shade reduces it by 60%. Industrial pollution, which increases shade, also decreases sun exposure and may contribute to the development of rickets in children with insufficient dietary intake of vitamin D.

It is important for individuals with limited sun exposure to include good sources of vitamin D in their diet.

The functions of vitamin D

Vitamin D is involved in:

In the absence of vitamin D, dietary calcium is not absorbed efficiently. Vitamin D stimulates a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood. The best-studied of these calcium transporters is calbindin, an intracellular protein that ferries calcium across the intestinal epithelial cell.

Vitamin D receptors are present in most if not all cells in the body and vitamin D has a potent effects on the growth and differentiation of many types of cells. This suggests that vitamin D has physiologic effects much broader than a role in mineral homeostasis and bone formation. There is research being conducted on the functions of vitamin D and more information about its functions will be available in the near future.

The symptoms and signs of vitamin D deficiency

The classical manifestation of vitamin D deficiency is rickets, which is seen in children and results in bony deformities including bowed long bones. Deficiency in adults leads to the disease osteomalacia. Both rickets and osteomalacia reflect impaired mineralization of newly synthesized bone matrix, and usually result from a combination of inadequate exposure to sunlight and decreased dietary intake of vitamin D.

Vitamin D deficiencies are also associated with tooth decay and osteoporosis.

Vitamin D deficiency or insufficiency occurs in several other situations. These include:

Food sources of vitamin D

Vitamin D is contained in fish (such as mackerel and herring), egg yolks and fish liver oils and some fortified foods.

The absorption of vitamin D supplements will be encouraged by taking the supplements at the same time as eating a meal which contains some fat.

How much supplement to take

Most capsules contain approximately 400 IU (in the form of natural fish liver oil) and doses between 400 and 1,000 IU can be taken daily. Dosages above 1,000 IU are not recommended.

Special notes

The elderly are less able to efficiently synthesize vitamin D and may require vitamin D supplements.

Vegans, especially those who live in cold climates are advised to take vitamin D supplements.

Vitamin D is fat soluble and large or repeated doses can build up to toxic levels in the body. The symptoms of toxicity include:

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Bland, J. 1996, Contemporary Nutrition. J & B Associates.

Davies, S. and A. Stewart., 1997, Nutritional Medicine. Pan.

Holden, S., Hudson, K., Tilman, J. & D. Wolf, 2003, The Ultimate Guide to Health from Nature. Asrolog Publication.

Pressman, A. and S. Buff, 2000, The Complete Idiot's Guide to Vitamins and Minerals. (2nd Ed.) Alpha Books.

Soothill, R. 1996, The Choice Guide to Vitamins and Minerals. A Choice Book Publication.

Sullivan, K. 2002, Vitamins and Minerals: A Practical Approach to a Health Diet and Safe Supplementation. Harper Collins.

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Sunscreens with a sun protection factor (SPF) of 8 or greater will block UV rays that produce vitamin D, but it is still important to routinely use sunscreen to help prevent skin cancer and other negative consequences of excessive sun exposure in regions and seasons where sunshine is plentiful. An initial exposure to sunlight (10 -15 minutes) allows adequate time for Vitamin D synthesis and should be followed by application of a sunscreen with an SPF of at least 15 to protect the skin. Ten to fifteen minutes of sun exposure at least twice a week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D.